Research Updates

Supreeth Gaddam, Ravi Sankar Haridas, Deepthi Tammana, Charlie Sanabria, Christopher J. Lammi, Diana Berman, & Rajiv S. Mishra

Nitrogen containing austenitic stainless steels (N-ASS) are widely utilized to fabricate various structural components in tokamak type fusion reactors owing to their suitable mechanical and functional properties. These components are exposed to a range of temperatures (4–500 K) and interact closely with the magnetic fields that are used to control and contain the plasma within the tokamak systems. Nitronic-40 (N40) or XM-11 stainless steel is one such N-ASS used for fabricating structural components in the magnetic and vacuum vessel systems in tokamak devices. Fabrication of most of the larger components in the magnetic and vacuum vessel systems typically involves some type of fusion-based welding process. This study presents a double-sided friction stir welding (FSW) approach as an alternative to fusion welding processes to join 12 mm thick N40 plates to obtain joints with a low fraction of δ ferrite (a detrimental ferromagnetic phase), high joint efficiency, no sensitization and loss of hardness in the heat affected zone, and minimal nitrogen desorption from the weld nugget. The double-sided FSW approach yielded superior weldments when compared to similar joints accomplished by fusion welding for application in tokamak devices.

Journal of Materials Science & Technology, 2023-10-01, ScienceDirect, ISSN 1005-0302
Volume 159, Pages 170-183

https://www.sciencedirect.com/science/article/pii/S1005030223003158

DOI 10.1016/j.jmst.2023.03.014

R. S. Mishra & H. Z. Yu

Additive manufacturing has captured the imagination of the research community and industries across various engineering disciplines because of the unprecedented design flexibility. The last 20 years have seen unprecedented growth in the number of process variants. It has also changed the manufacturing paradigm. For example, if we reflect on the 20th century, we see the push for “economy of scale.” The steel mills became bigger and bigger! High volume production in the integrated mills led to cost-effective availability of metals and alloys. Additive manufacturing has changed this to “print at the place of need.” The manufacturing is now distributed, and it has upended the conventional supply-chain. In the post-pandemic world, it also brings a possibility of developing a resilient supply chain. The solid-state additive manufacturing processes are the best way to replace large size castings and forgings. Friction stir additive deposition and friction stir additive manufacturing are two variants that have evolved from friction stir welding and processing. These processes result in very high deposition rates with wrought microstructure. The structural properties are generally far superior as compared to other additive manufacturing processes. This special topic covers recent advancements in science and technology of these relatively new processes. New applications such as recycling of metallic materials, gradient structures, and heterogeneous microstructures are also enabled by these processes.

JOM, 2023-09-11, Volume 75, Issue 10, Pages 4182-4184, Springer Link, ISSN 1543-1851

https://doi.org/10.1007/s11837-023-06138-1
DOI 10.1007/s11837-023-06138-1

Supreeth Gaddam, Amit Kishan Behera, Noriaki Arai, Qiaofu Zhang, & Rajiv S. Mishra

Niobium carbide (NbC) based cermets are emerging as strong contenders to replace conventional tungsten carbide (WC) based cermets for applications involving abrasion and wear resistance due to their superior mechanical, physical, and chemical properties. NbC cermets with various binders have been explored in literature and nickel has emerged as a promising candidate binder material. Mechanical mixing (MM) followed by spark plasma sintering (SPS) has been shown to be one of the effective methods to synthesize NbC-Ni cermets. However, the effect of certain crucial process parameters during the MM + SPS process on the end product have not been established. To address this, the present study reports the effect of ball milling time and the effect of sintering temperature on the microstructure, binder distribution, hardness, and indentation fracture toughness of NbC – 16 vol% Ni cermets synthesized using MM + SPS. Insufficient milling time resulted in inhomogeneous binder distribution in the consolidated cermets and sintering above a certain temperature resulted in complete loss of binder. Therefore, the combination of sufficient milling time and an appropriate sintering temperature was essential to obtain a consolidated cermet with homogeneous microstructure and uniform mechanical properties.

International Journal of Refractory Metals and Hard Materials, Volume 115, Pages 106323, 2023-09-01, ScienceDirect, ISSN 0263-4368

https://www.sciencedirect.com/science/article/pii/S0263436823002238
DOI 10.1016/j.ijrmhm.2023.106323

Anurag Gumaste, Abhijeet Dhal, Priyanshi Agrawal, Ravi Sankar Haridas, Vijay K. Vasudevan, David Weiss, & Rajiv S. Mishra

An innovative approach to build a high-performance, thermally stable Al-8Ce-10Mg (wt.%) alloy via friction-stir based solid-state additive manufacturing, called additive friction stir deposition, has been demonstrated in this study. The deposited material displayed 22% higher yield strength and 181% improvement in ductility as compared to the base material. The deposit also exhibited excellent tensile properties at elevated temperatures. The improved performance has been attributed to multiple strengthening mechanisms active in the built component. Al-Ce particle fragmentation, grain refinement, and retention of Mg in solid solution during the process synergistically resulted in the improved mechanical performance. The fragmentation of Al11Ce3 particles occurred due to intense frictional heating and shearing during the process. Scanning electron microscopy, nanoindentation, tensile testing, differential scanning calorimetry, and X-ray diffraction analysis were used to establish process–structure–property correlations at multiple length scales.

2023-08-24, Springer Link, ISSN 1543-1851

https://doi.org/10.1007/s11837-023-06044-6
Volume 75, Issue 10, Pages 4185-4198 Publication JOM

DOI 10.1007/s11837-023-06044-6

Ravi Sankar Haridas, Anurag Gumaste, Pranshul Varshney, Bodhi Ravindran Manu, Kumar Kandasamy, Nilesh Kumar & Rajiv S. Mishra

Deformation-based solid-state additive manufacturing techniques are known for high build rates, absence of process-induced defects that are detrimental to mechanical performance, refined equiaxed microstructure, large-scale customization, and manufacturing at ambient conditions. We introduce a novel severe plastic deformation-based SolidStir® additive manufacturing (SolidStir® AM) technique here. The process utilizes frictional heating between a non-consumable rotating tool and a feedstock to soften and plasticize the material inside a process chamber at high temperature, which is subsequently deposited in a layer-by-layer fashion under the coupled action of compressive and shear forces. In this work, aluminum 6061-T6 was used as the feedstock material for feasibility study of SolidStir® AM. The deposited material possessed a defect-free, refined, and wrought-like microstructure (~ 11 µm average grain size), enhanced ductility (~ 160% improvement), and better corrosion resistance than the base material. Based on preliminary results, SolidStir® AM appears to be a promising solid-state additive manufacturing tool that can be applied in many applications.

JOM, 2023-08-16, Volume 75, Issue 10, Pages 4231-4241, Springer Link, ISSN 1543-1851

https://doi.org/10.1007/s11837-023-06063-3
DOI    10.1007/s11837-023-06063-3

Anurag Gumaste, Ravi Sankar Haridas, Sanya Gupta, Supreeth Gaddam, Kumar Kandasamy, Brandon A. McWilliams, Kyu C. Cho & Rajiv S. Mishra

Solid Stir Extrusion (SSE) is an innovative friction-stir technology based process developed to obtain an extrudate of the preferred shape. As a solid-state manufacturing process, SSE embodies several benefits such as lower power consumption, continuity over a greater time frame, wide material range applicability, and flexibility to large-scale customization. This study demonstrates the feasibility of the SSE process using Al 6061-T6 as a feedstock material. Process outcomes like force and torque are correlated with comprehensive microstructural and mechanical characterization. The extrudate possessed a fine equiaxed grain structure (∼10–15 µm) with improved ductility as compared to the base material (∼100 µm). Post-process heat treatment additionally improved the mechanical properties of the extruded material. DSC analysis elucidates the effects of heat treatment. A conceptual model is proposed to correlate process parameters to process outcomes.

Journal of Materials Processing Technology, 2023-07-01, Volume 316, Pages 117952, ScienceDirect, ISSN 0924-0136

https://www.sciencedirect.com/science/article/pii/S0924013623000973
DOI 10.1016/j.jmatprotec.2023.117952

J. G. Lopes, Priyanka Agrawal, Jiajia Shen, N. Schell, Rajiv S. Mishra, & J. P. Oliveira

In recent years, high entropy alloys (HEAs) have been shown to be promising alternatives to common engineering alloys, depending on their composition and thermomechanical processing. Up to now, several works aimed at improving the mechanical properties and discovering different HEAs given the extremely large compositional possibilities made available by the multicomponent approach associated to these materials. Their processability, however, is an important topic that must be studied. Welding is a key manufacturing technique that will eventually be applied to HEAs. Thus, there is a need to evaluate the microstructure and property changes induced by the weld thermal cycles, to assess the suitability of certain welding process/HEAs combinations for possible industrial applications. In the present work, Gas Tungsten Arc Welding (GTAW) was used to achieve defect-free joints based on a novel transformation induced plasticity (TRIP) Fe50Mn30Co10Cr10 HEA. The microstructure and mechanical behavior of the joints were assessed by means of optical and electron microscopy, synchrotron X-ray diffraction, thermodynamical calculations, microhardness mapping and tensile testing. Overall, an excellent mechanical performance was obtained on the resulting joints, opening the door for their adoption in real-life applications.

Materials Science and Engineering: A, 2023-06-30, Volume 878, Pages 145233, ScienceDirect, ISSN 0921-5093

https://www.sciencedirect.com/science/article/pii/S0921509323006573

DOI 10.1016/j.msea.2023.145233

Rajiv S. Mishra & Sanya Gupta

Metallic materials derive their strength and ductility from their microstructural features. The general principle of alloying is not only to control the phases present in the alloys, but also how the alloying elements can help in microstructural control during the processing of materials. The overall thermodynamic framework of using free energy to explain the stability of an alloy has two significant terms, enthalpy, and entropy. Engineering alloys are processed by several far-from-equilibrium processes. During the processing, the materials are in metastable states. The metastability can also be purposely enhanced through alloy design. This paper provides a perspective on how “high enthalpy states” can be used to tailor the microstructure to overcome the conventional strength-ductility tradeoff. The emergence of new manufacturing processes also provides unique opportunities to design alloys to maximize the potential of such processes. A few illustrative examples are presented to tie the historical use of high enthalpy states and point to future opportunities. Co-development of advanced materials for disruptive new manufacturing processes can be enhanced through integrated computational materials engineering approaches.

Frontiers in Metals and Alloys, Volume 2, 2023-06-09, ISSN 2813-2459

https://www.frontiersin.org/articles/10.3389/ftmal.2023.1135481

Ravi Sankar Haridas, Anurag Gumaste, Priyanshi Agrawal, Surekha Yadav & Rajiv S. Mishra

The advent of high entropy alloys (HEAs) enabled fine-tuning of the alloy composition from the vast compositional space to develop solid-solution alloys having good physical and mechanical properties. Each HEA composition is the culmination of careful alloy design strategy aimed at a specific property intended for a specific application that is achieved by microstructural alteration and/or activation of deformation mechanisms. Methods to integrate the favorable properties pertaining to each HEA potentially benefit alloy selection for structural applications. In this study, dissimilar friction stir welding (FSW) was performed using Al0.3CoCrFeNi HEA and Fe38.5Co20Mn20Cr15Si5Cu1.5 TRIP HEA to integrate the beneficial mechanical properties of both HEAs. Unlike fusion welding methods that lead to a weaker weld zone, dissimilar FSW of the HEAs resulted in a refined microstructure in the stir zone (SZ) with enhanced mechanical performance compared to both base materials and enabled promising property integration. Improved and integrated mechanical property achieved in SZ was correlated to the microstructure, existing recrystallization mechanisms, and active deformation mechanisms in both alloys.

Materials Today Communications, Volume 35, Pages 105822, 2023-06-01, ScienceDirect, ISSN 2352-4928

https://www.sciencedirect.com/science/article/pii/S2352492823005135
DOI 10.1016/j.mtcomm.2023.105822

Jiajia Shen, Priyanka Agrawal, Tiago A. Rodrigues, J. G. Lopes, N. Schell, Jingjing He, Zhi Zeng, Rajiv S. Mishra, J. P. Oliveira

Weldability studies on high entropy alloys are still relatively scarce, delaying the deployment of these materials into real-life applications. Thus, there is an urgent need for in-depth studies of the weldability of these novel advanced engineering alloys. In the current work, an as-cast Fe42Mn28Co10Cr15Si5 metastable high entropy alloy was welded for the first time using gas tungsten arc welding. The weld thermal cycle effect on the microstructure evolution over the welded joint was examined using electron microscopy in combination with electron backscatter diffraction, synchrotron X-ray diffraction analysis, and thermodynamic calculations. Furthermore, tensile testing and hardness mapping were correlated with the microstructure evolution. The microstructure evolution across the joint is unveiled, including the origin of the ε-h.c.p. phase at different locations of the material. Different strengthening effects measured throughout the joint are associated with the weld thermal cycle and resulting microstructure. A synergistic effect of smaller grain size of the ε-h.c.p. phase in the fusion zone, overturns the reduced volume fraction of this phase, increasing the local strength of the material. Moreover, the brittle nanosized σ phase was also found to play a critical role in the joints’ premature failure during mechanical testing.

Materials Science and Engineering: A, 2023-03-03, Volume 867, Pages 144722, ScienceDirect, ISSN 0921-5093

https://www.sciencedirect.com/science/article/pii/S0921509323001466
DOI 10.1016/j.msea.2023.144722

Dwight A. Burford, Maurizio Manzo, Hector Siller, Supreeth Gaddam, Anurag Gumaste, James Koonce, Aleandro Saez, & Rajiv S. Mishra

Editors, Yuri Hovanski, Yutaka Sato, Piyush Upadhyay, Anton A. Naumov & Nilesh Kumar

Submerged bobbin toolBobbin Tools (SBT) tunneling is a new friction stir processing (FSP)Friction stir processing (FSP) technique for making integral channelsChannels within malleable materials. Like a conventional bobbin toolBobbin Tools (BT) for friction stir welding (FSW), an SBT toolset has two opposing shoulders spaced apart along the bobbin or probe section of the tool. Unlike a conventional BT, an SBT is used to form integral subsurface channelsChannels by passing the shoulder at the distal end of the probe through the workpiece during processing. Example uses of internal pathwaysInternal Pathways are found in heat exchangersHeat Exchangers, cooling platesCooling Plates, and vacuum toolsTools. Advance uses may include lessening weight and modifying the stiffness of structural components. A preliminary evaluation in AA6061-T6511 plates shows this special form of FSP has low process forcesProcess forces and is therefore capable of being deployed on CNC (computer numerical control) machiningMachining centers and friction stir-capable industrial robots as well as purpose-built FSP machines. Consequently, SBT tunneling holds potential use in a wide range of applications requiring curvilinear internal pathwaysInternal Pathways for wiring, gases, and fluids, as well as internal spaces for the placement of powders and solid materials like compositesComposite.

Friction Stir Welding and Processing XII, Conference Proceedings, 2023-02-12, The Minerals, Metals & Materials Series, Springer Nature Switzerland, ISBN 978-3-031-22661-8, Pages 249-267

DOI 10.1007/978-3-031-22661-8_23

Priyanka Agrawal, Sanya Gupta, Abhijeet Dhal, Ramprashad Prabhakaran, Lin Shao & Rajiv S. Mishra

Properties and radiation responses of a metastable high entropy alloy (HEA) exhibiting the transformation induced plasticity (TRIP) effect were studied. Innovative engineering used to manufacture this HEA has shown superior mechanical and corrosion properties in 3.5% NaCl than most advanced stainless steels. The microstructural evolution and corresponding mechanical response after irradiation have been evaluated using detailed transmission electron microscopy, and nanoindentation. The study shows a change in metastability of the alloy with irradiation via a recovery mechanism, where the irradiation-induced transformation is reversed by the temperature-induced transformation, thereby introducing the concept of self-healing, made possible due to the TRIP behavior of HEA.

Journal of Nuclear Materials, 2023-02-01, Volume 574, Pages 154217, ScienceDirect, ISSN 0022-3115

https://www.sciencedirect.com/science/article/pii/S0022311522006961
DOI 10.1016/j.jnucmat.2022.154217

Saket Thapliyal & Rajiv S. Mishra

Leveraging the structural fine-tuning capabilities of laser-powder bed fusion (L-PBF) additive manufacturing (AM) requires novel alloys. Furthermore, due to the complex multiphysics involved in L-PBF AM, variation in the attributes of microstructural and macrostructural features of a structural component occurs. This engenders the need for unconventional methods of part qualification. In this chapter, we consider the prospects of incorporating alloy chemistry into the traditional processing-microstructure-property paradigm as outlined by the materials systems approach. We discuss how the resulting modified systems approach may facilitate both alloy design and part qualification in L-PBF AM. The possible variations in several attributes of the structural hierarchy of an L-PBF AM-built component are described, including the variations at the microstructural and the macrostructural length scales. Following the chemistry-processing-structure-property relationships outlined in the modified systems approach, the implications of such variations for printability, mechanical properties, component performance, and part qualification are discussed. Subsequently, consideration of these variations is deemed important for both alloy design and part qualification. A comprehensive ICME-based approach is outlined for establishing the chemistry-processing-microstructure-property relationships. The ICME-anchored modified systems approach may guide the alloy design efforts and may also guide the application experts in various industries in optimizing L-PBF AM parameters and qualifying an L-PBF AM-built component.

2023-01-01, Pages 321-354, ScienceDirect, Elsevier, ISBN 978-0-323-88664-2

Book Title: Quality Analysis of Additively Manufactured Metals
Editors: Javad Kadkhodapour, Siegfried Schmauder & Felix Sajadi

https://www.sciencedirect.com/science/article/pii/B9780323886642000026

DOI: 10.1016/B978-0-323-88664-2.00002-6

Ravi Sankar Haridas, Priyanka Agrawal, Surekha Yadav, Priyanshi Agrawal, Anurag Gumaste, Rajiv S.Mishra

Excellent work hardening in transformation-induced plasticity (TRIP)-enabled metastable high entropy alloys (HEAs) owe to persistent austenite (γ) to martensite (ε) phase transformation; non-basal slip activity and deformation twinning in the transformed martensitic phase are additional deformation mechanisms that contribute to work hardening in selected TRIP HEAs. Mechanical response of TRIP HEAs under uniaxial tension is characterized by an intermediate stage distinguished by a gradual increase in work hardening rate preceded and succeeded by stages of rapid drop in the work hardening rate. A five-parameter empirical model that replicates the nature of the work hardening rate curve in TRIP HEAs has been developed. The ease of parametric identification of the model from the global stress-strain response simplifies implementation of the model over physically based models. As the propensity of stress induced transformation in TRIP HEAs is related to the stacking fault energy (SFE) of the parent austenite phase, an attempt is made to correlate the model parameters with SFE. Based on the trends indicated in the correlation of model parameters with SFE as well as the initial microstructure, a method is proposed to predict the tensile stress-strain response of TRIP HEAs.

Keywords: Work hardening Martensitic transformation High entropy alloys Stacking fault energy Deformation twinning

Journal of Materials Research and Technology
Volume 18, 2022, Pages 3358-3372, ISSN 2238-7854

https://doi.org/10.1016/j.jmrt.2022.04.016

(https://www.sciencedirect.com/science/article/pii/S2238785422005166)

Research Faculty: Dwight Burford, Maurizio Manzo, Hector Siller

Graduate Students: Supreeth Gaddam, Anurag Gumaste, James Koonce III, Aleandro Saez
SBT tunneling technology is a new solid-state friction stir processing (FSP) method for producing integral channels within structural components. Due to low out‐of‐plane process forces, this patent pending process is suited for robotic applications, opening possibilities for producing 3-D internal pathways for wiring, gases, fluids, powders, tubing, composites, etc. Example uses are in heat exchangers, cooling plates, vacuum tools, and structural components. Submerged bobbin tools, or SBTs, are specially designed to form integral sub‐surface channels (tunnels) within components. Like a conventional bobbin tool (BT) used in friction stir welding (FSW), an SBT has two opposing shoulders spaced apart along the bobbin or probe section of the tool. Unlike a conventional BT, however, an SBT is used to form enclosed internal channels by the distal shoulder – the one located at the terminal end of the bobbin – being submerged within the workpiece while the opposite shoulder rides along on an outer surface of the workpiece during processing.

Similar to other BT designs, the opposing shoulders of SBT designs serve to contain a substantial portion of stirred material generated throughout the progression of the process. As a result, process forces produced parallel to the tool’s axis of rotation are reacted between the opposing shoulders. Compared to single‐sided tool designs having one shoulder, SBT tools therefore produce relatively lower out‐of‐plane forces that must be supported by the fabrication equipment. In turn, fabrication equipment for internal channel production by SBTs have reduced force and stiffness requirements compared to equipment for single‐sided channeling methods.

Preliminary studies show that this innovation can be deployed on CNC machining centers, FSP purpose-built equipment and industrial robots to produce 3‐D curvilinear subsurface integral channels in complex‐shaped parts.

33rd AeroMat Conference and Exposition
Welding & Joining II, # 55159
Pasadena Convention Center Pasadena, CA
March 15, 2022

https://asm.confex.com/asm/aero22/webprogram/Paper55159.html

Priyanka Agrawal, Sanya Gupta, Shivakant Shukla, Saurabh S. Nene, Saket Thapliyal, Michael P. Toll, Rajiv S. Mishra

The mechanical properties of transformation induced plasticity high entropy alloys (TRIP HEAs) are affected by tailoring the metastability via alloying and processing. The current work focuses on as-cast and friction stir processed alloy where the initial microstructure is altered by Cu addition (a γ-fcc phase stabilizer) to the ε-hcp dominated HEA. With the Cu addition, the tensile stress-strain curves exhibited improved ductility and a delay in TRIP effect, suggesting an increased stacking fault energy, along with improved strength and work hardening. Improved properties with Cu addition are credited to almost 100% stabilized γ-fcc phase, increased type and number of interfaces: Cu-rich precipitates, shorter faults, phase separation, and increased grain boundary fraction. The new alloy was then friction stir processed (FSPed) to further improve the properties. An advanced TRIP effect is observed with FSP as compared to as-cast alloy, attributed to increased ε-hcp fraction and finer grain size. Based on microscopic observations, the improved strength is due to finer grain size, increased dislocation density, low density of faults, whereas reduced ductility is reasoned to be due to dissolution of Cu-rich precipitates and increased width of modulations from phase separation.

Keywords: High entropy alloys; Tensile properties; Stacking faults; Friction stir processing; Transmission electron microscopy; Phase separation

Materials & Design
Volume 215, 2022, 110487, ISSN 0264-1275

https://doi.org/10.1016/j.matdes.2022.110487.

(https://www.sciencedirect.com/science/article/pii/S0264127522001083)

Rajiv S. Mishra, Ravi Sankar Haridas & Priyanshi Agrawal

Additive manufacturing (AM) has completely altered the traditional component manufacturing and qualification paradigm. It provides unitisation and topological optimisation opportunities simultaneously. Broadly, the additive manufacturing processes are classified as fusion-based or solid-state. The solid-state additive manufacturing processes are relatively nascent. Among these, friction stir-based processes involve intense shear deformation of material while building. In this review, we focus on friction stir additive manufacturing (FSAM) and additive friction stir deposition (AFSD). These friction stir welding derived techniques have ability to produce microstructures that lead to better mechanical properties than the conventionally processed parent alloys; in many cases overcoming the traditional strength-ductility tradeoff paradigm. The best way to capture this advantage is to conduct materials selection for build which benefit from the attributes of these processes. This review provides a systems approach framework and a conceptual process model to guide researchers. A case is built that the best mechanical properties can be obtained by alloy design for such disruptive and innovative manufacturing processes. The intrinsic and extrinsic limitations are highlighted to guide researchers in the field of FSAM and AFSD. While AFSD is readily applicable to lower melting temperature materials currently, applying it to high-temperature materials requires significant research and development on tool materials. Examples of materials processed by FSAM/AFSD include aluminium alloys, magnesium alloys, titanium alloys, steels and nickel-base superalloy. A physics-based process modelling framework applicable to FSAM/AFSD is provided. To fully validate such models, it is imperative to use machines with appropriate sensors that capture the machine parameters, tool health, and workpiece temperature.

SCIENCE AND TECHNOLOGY OF WELDING AND JOINING
2022, VOL. 27, NO. 3, 141–165

https://doi.org/10.1080/13621718.2022.2027663

E. Polatidis, S. Shukla, J. Čapek, S. Van Petegem, N. Casati, R.S. Mishra

The mechanical behavior of a metastable high entropy alloy (HEA) with composition Fe39-Mn20-Co20-Cr15-Si5-1Al at% is investigated. The deformation mechanisms contributing to its mechanical properties are unveiled, by performing uniaxial tensile tests in situ with synchrotron X-ray diffraction. Three distinct deformation regimes are detected. The initial elastic and early dislocation-based plasticity deformation is followed by a moderate work hardening rate regime, which is associated with the deformation-induced phase formation of ε-martensite, with hexagonal close-packed (hcp) crystal structure as well as slip in the parent austenitic phase. During the third deformation regime, the phase transformation continues to occur in combination with 2 modes of hcp twinning as well as slip in the unfavorably oriented grains for twinning. The interplay of all these co-existing deformation mechanisms can be evidenced by synchrotron X-ray diffraction.

Keywords: High entropy alloy; Martensite; TRIP; Deformation; Work hardening; X-ray diffraction

Materials Today Communications
Volume 30, 2022, 103155, ISSN 2352-4928

https://doi.org/10.1016/j.mtcomm.2022.103155

(https://www.sciencedirect.com/science/article/pii/S2352492822000332)

Yuqi Jin, Xinyue Wang, Edward A. Fox, Zhiwu Xie, Arup Neogi, Rajiv S. Mishra, Tianhao Wang

Monitoring tool degradation during manufacturing can ensure product accuracy and reliability. However, due to variations in degradation conditions and complexity in signal analysis, effective and broadly applicable monitoring is still challenging to achieve. Herein, a novel monitoring method using ultrasound signals augmented with a numerically trained machine learning technique is reported to monitor the wear condition of friction stir welding and processing tools. Ultrasonic signals travel axially inside the tools, and even minor tool wear will change the time and amplitude of the reflected signal. An artificial intelligence (AI) algorithm is selected as a suitable referee to identify the small variations in the tool conditions based on the reflected ultrasound signals. To properly train the AI referee, a human-error-free data bank using numerical simulation is generated. The simulation models the experimental conditions with high fidelity and can provide comparable ultrasound signals. As a result, the trained AI model can recognize the tool wear from real experiments with subwavelength accuracy prediction of the worn amount on the tool pins.

Advanced Intelligent Systems published by Wiley-VCH GmbH
Adv. Intell. Syst. 2022, 2100215 (1 of 8)

https://doi.org/10.1002/aisy.202100215

(https://onlinelibrary.wiley.com/doi/full/10.1002/aisy.202100215)

Shubhrodev Bhowmik, Jianzhong Zhang, Sven C. Vogel, Saurabh S. Nene, Rajiv S. Mishra, Brandon A. McWilliams, Marko Knezevic

This paper describes the main results from an experimental investigation into tailoring the phase content and grain structure for high strength of a microstructurally flexible high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at%), using rolling, friction stir processing (FSP), and compression. Optical microscopy, neutron diffraction, and electron backscatter diffraction were employed to characterize microstructure and texture evolution. The material upon rolling was found to have triplex structure consisting of metastable gamma austenite (γ), stable sigma (σ), and stable epsilon martensite (ε) phases. The adaptive phase stability exhibited by the selected HEA of very low stacking fault energy with strain, strain rate, and temperature was used along with grain refinement to enhance the strength. To this end, the complex structure was refined by FSP at 350 revolutions per minute (RPM) tool rotation rate, while increasing the fraction of γ and decreasing the σ and ε content. The strength was further enhanced by FSP at 150 RPM by further refinement of the grain structure and increasing the fraction of ε phase. The peak ultimate tensile strength of ~1850 MPa was achieved by double pass FSP (350 RPM followed by 150 RPM), the sequence which even more refined the microstructure and increased the fraction of σ phase. While the content of diffusion created σ phase remains constant during subsequent compression, the fraction of ε increases due to the diffusionless strain induced γ→ε phase transformation. The transformation facilitates plastic strain accommodation and rapid strain hardening, which has been attributed to the increase in highly dislocated ε phase fraction and transformation induced dynamic Hall-Petch-type barrier effect. Interestingly, a great deal of hardening ability was exhibited by the HEA even at very high strength. Roles of texture, grain size, and phase content on the transformation during compression have been rationalized and discussed in this paper.

Keywords: High entropy alloys; Friction stir processing; Phase transformations; Grain structure; Texture

Journal of Alloys and Compounds
Volume 891, 2022, 161871, ISSN 0925-8388,

https://doi.org/10.1016/j.jallcom.2021.161871

(https://www.sciencedirect.com/science/article/pii/S0925838821032801)

Austin E. Mann, Andrew H. Baker, Rajiv Mishra, & Sivanesh Palanivel

A method for making an aluminum alloy includes steps of (1) weighing out starting materials to achieve a mass of material having a composition that includes aluminum, about 1.8 to about 5.6 percent by weight copper, about 0.6 to about 2.6 percent by weight lithium, and at least one of lanthanum up to about 1.5 percent by weight, strontium up to about 1.5 percent by weight, cerium up to about 1.5 percent by weight, and praseodymium up to about 1.5 percent by weight; (2) loading said starting materials into a crucible; (3) inserting said crucible into a chamber; (4) evacuating said chamber to a predetermined vacuum level; (5) melting said starting materials to form a molten mass; and (6) casting said molten mass into a mold.

US Patent App. 17/326,492, 2021

US Patent Application US20210277508A1

Saket Thapliyal, Priyanshi Agrawal, Priyanka Agrawal, Saurabh S. Nene, Rajiv S. Mishra, Brandon A. McWilliams, Kyu C. Cho

Laser-powder bed fusion (L-PBF) additive manufacturing offers unprecedented microstructural fine-tuning capabilities. Naturally, benefitting from such capability requires alloys that are amenable to microstructural heterogeneity and hierarchy (MHH) and that exhibit a low hot-cracking susceptibility (HCS). However, columnar growth, which is characterized by capillary effects and poor strain accommodation capabilities, is prevalent in L-PBF and increases the HCS of the processed alloys. Further, while solute segregation is prominent in cellular and dendritic growth modes during L-PBF, the effects of solute segregation on the alloy HCS and L-PBF processing window remain widely unexplored. Here, we demonstrate that solute segregation affects columnar growth, grain coalescence behavior during solidification, MHH and mechanical properties of a metastable Fe40Mn20Co20Cr15Si5 (at.%) high entropy alloy (CS-HEA) doped with 0.5 wt.% B4C (termed CS-BC). A theoretical framework is proposed, which reveals that a boundary-strengthening segregant may reduce the alloy HCS during L-PBF. In as-built CS-BC, boron, a boundary strengthener, segregated to the solidification cell boundaries, whereas carbon remained in the solid solution. The as-built CS-BC exhibited suppressed columnar growth, more random texture, smaller cell size and higher strength as compared to the as-built CS-HEA. Further, a wide crack-free L-PBF processing window of CS-BC allowed fine-tuning of its MHH and thus the mechanical properties. Upon annealing, as carbon-containing precipitates formed, CS-BC exhibited a metastable microstructure and transformation induced plasticity effect, which led to high synergistic strength-ductility. These findings will foster design of alloys that facilitate application-specific manufacture with L-PBF and thus, an extended outreach of L-PBF for structural applications.

Keywords: Grain boundary segregation, Additive manufacturing, Solidification; Alloy design, High entropy alloys, Transformation induced plasticity (TRIP)

Acta Materialia
Volume 219, 2021, 117271, ISSN 1359-6454

https://doi.org/10.1016/j.actamat.2021.117271

(https://www.sciencedirect.com/science/article/pii/S1359645421006510)

Priyanshi Agrawal, Ravi Sankar Haridas, Saket Thapliyal, Surekha Yadav, Rajiv S. Mishra, Brandon A. McWilliams, Kyu C. Cho

Fatigue failure is ubiquitous in structural components. In additively manufactured (AM) components, the processing induced defects limit the fatigue performance. Further, the stochastic nature of defects in laser-powder bed fusion (L-PBF) make it difficult to predict the fatigue life in these components. In this work, we explored exceptional work hardening (WH) of a metastable Fe40Mn20Co20Cr15Si5 high entropy alloy (CS-HEA) to obtain high fatigue-resistance with L-PBF. Further, a fatigue life estimation tool based on the statistical size distribution of microstructural entities such as grains, pores and solid-state inclusions and, their mutual interaction was used to estimate the fatigue life of as-printed material. Upon deformation, CS-HEA exhibited γ (f.c.c.) → ε (h.c.p.) martensitic transformation and subsequent twinning in ε (h.c.p.) phase. Such deformation behavior resulted in sustained WH and is specifically beneficial in the vicinity of critical pores. A high normalized fatigue strength of 0.65 with respect to the yield strength was thus obtained in as-printed condition. Further, the model accurately predicted extended crack initiation life for CS-HEA. The current work therefore provides guidance towards developing defect-tolerant alloys for L-PBF and presents a tool for estimation of fatigue life of AM alloys with unconventional WH behavior.

Materials Science and Engineering
A, Volume 826, 2021, 142005, ISSN 0921-5093

https://doi.org/10.1016/j.msea.2021.142005

(https://www.sciencedirect.com/science/article/pii/S0921509321012703)

M Frank, SS Nene, Y Chen, S Thapliyal, S Shukla, K Liu, S Sinha, T Wang, MJ Frost, K An, RS Mishra

Strain hardening in metallic materials delays catastrophic failure at stresses beyond the yield strength by the formation of obstacles to dislocation motion during plastic deformation. Conventional measurement of the instantaneous strain hardening rate originates from load–displacement data acquired during uniaxial mechanical testing, rather than the evolution of obstacles. In order to resolve hardening from the perspective of the very obstacles that cause strengthening, we used an in situ neutron diffraction experimental approach to determine the strain hardening rate based upon real-time measurement of stacking fault interspacing during plastic deformation. Results provide clear evidence of the evolving contribution of obstacles during plastic deformation. The collapse of interspacing between multiple obstacle types enabled immense strain hardening in a Fe38.5Mn20Cr15Co20Si5Cu1.5 high entropy alloy leading to a true tensile strength of ~1.7 GPa along with elongation of ~35% at room temperature.

Applied Physics Letters
Volume 119, Issue 8, Pages 081906-1 to -6,
AIP Publishing LLC

https://doi.org/10.1063/5.0062153

Holden Hyer, Le Zhou, Sharon Park, Thinh Huynh, Abhishek Mehta, Saket Thapliyal, Rajiv S. Mishra, Yongho Sohn

Using the calculation of phase diagrams approach and Scheil solidification modeling, the Al-2.5Mg-1.0Ni-0.4Sc-0.1Zr alloy was designed, intentionally with an extraordinarily high cracking susceptibility, making it prime for solidification cracking during laser powder bed fusion. This study demonstrates the ability to mitigate even the most extreme solidification cracking tendencies in aluminum alloys with only minor alloying additions of Sc and Zr, 0.5 wt.% max. Furthermore, by employing a simple direct ageing heat treatment, good tensile mechanical properties were observed with a yield strength of 308 MPa, an ultimate tensile strength of 390 MPa, and a total elongation of 11%.

Keywords: Additive manufacturing; Cracking susceptibility; Aluminum alloy; CALPHAD; Mechanical properties

Journal of Materials Science & Technology
Volume 103, 2022, Pages 50-58, ISSN 1005-0302

https://doi.org/10.1016/j.jmst.2021.06.023

(https://www.sciencedirect.com/science/article/pii/S1005030221006526)

Priyanshi Agrawal, Ravi Sankar Haridas, Surekha Yadav, Saket Thapliyal, Supreeth Gaddam, Ravi Verma, Rajiv S. Mishra

Additive friction stir deposition (AFSD) is a novel thermo-mechanical solid state additive manufacturing process. AFSD enables manufacturing of near net shape, fully dense builds with refined equiaxed grain structure resulting in excellent mechanical properties. AFSD has the potential to produce ingots and components/builds from recycled metals (chips/scraps/wastes from industrial machining processes or municipal recycling centers). In the present study, recycled Ti-6Al-4V alloy chips were deposited using additive friction stir deposition. Detailed microstructural and mechanical property investigation of the deposited material fosters understanding of the effect of deposition variables on the microstructure as well as mechanical properties. The as-deposited microstructure was characterized by lamellar α/β colonies inside fine equiaxed prior β grains containing α phase at the grain boundary. Additive friction stir deposited Ti-6Al-4V alloy using consolidated recycled metal chips as raw material exhibited tensile properties better than other additive processes; thus, AFSD of recycled metal provides opportunities to produce structurally sound components with reduced energy consumption as well as reducing environmental waste.

Mechanical properties Additive Manufacturing
Volume 47, 2021, 102259, ISSN 2214-8604

https://doi.org/10.1016/j.addma.2021.102259

(https://www.sciencedirect.com/science/article/pii/S221486042100419X)

Yuqi Jin, Tianhao Wang, Arkadii Krokhin, Tae-Youl Choi, Rajiv S. Mishra, Arup Neogi

Dissimilar material joints or multilayered metals have become inevitable in the manufacturing industry due to the increasing demand for multifunctional materials with variable mechanical, thermal, or electrical characteristics in a single assembly. Lattice mismatch of materials at the interface of dissimilar materials leads to inferior mechanical characteristics. In particular, the mismatch in elastic properties indicated by a large initial elastic deformation is critical to determine the extent of variation in stress. However, nanoindentation, the most common and accepted technique to measure elastic modulus, is destructive, time-consuming, and can only examine mechanical properties within a limited area. A non-invasive elastographic mapping technique evaluates the mechanical properties using ultrasonic elastography to study incompressibility. The dissimilar joint between steel and copper was obtained via friction stir welding. The variation of the stress developed at the welded joint of the two different metals was evaluated from the dynamic bulk modulus map. A tensile test of the involved workpiece confirmed a good agreement with our analysis based on the dynamic bulk modulus elastographic mapping results. This study provides a rapid and non-invasive technique for the bulk metallurgic elastic modulus inspection to overcome the limitations of conventional methods.

Keywords: Friction stir processing; Non-destructive evaluation; Elastography; Mechanical properties of metals; Friction stir welding; Elastic modulus; Ultrasonic inspection

Journal of Materials Processing Technology
Volume 299, 2022, 117301, ISSN 0924-0136

https://doi.org/10.1016/j.jmatprotec.2021.117301

(https://www.sciencedirect.com/science/article/pii/S0924013621002612)

Sanya Gupta, Priyanka Agrawal, Saurabh S. Nene, Rajiv S. Mishra

A systematic study of butt friction stir welding of a recently developed Cu-containing metastable high entropy alloy (HEA) was conducted. Different zones of the weld were evaluated using mechanical testing, microstructural characterization, and differential scanning calorimetry (DSC). The nugget exhibited high tensile strength as a result of the refined equiaxed microstructure. Interestingly, the heat affected zone, which is known to be the weakest region, exhibited an excellent combination of strength and ductility as compared to the base metal. DSC was explored as a novel and quick technique to obtain enthalpy information and thus understand the TRIP effect.

Keywords: Friction stir welding; Differential scanning calorimetry; Metastable high entropy alloys; Tensile properties; Hardness

Scripta Materialia
Volume 204, 2021, 114161, ISSN 1359-6462

https://doi.org/10.1016/j.scriptamat.2021.114161

(https://www.sciencedirect.com/science/article/pii/S1359646221004413)

Anumat Sittiho, Madhumanti Bhattacharyya, Jadzia Graves, Saurabh S. Nene, Rajiv S. Mishra, Indrajit Charit

High entropy alloys (HEA) are an emerging class of novel metallic alloys with the ability to develop unique microstructural modification by using thermomechanical processing techniques. In this study, friction sir processing (FSP), widely regarded as an effective thermomechanical processing technique, was applied to a rolled HEA of nominal composition Fe₄₂Co₁₀Cr₁₅Mn₂₈Si₅ (at%). The work involves examination and analysis of the microstructural characteristics by optical microscopy, transmission electron microscopy and electron backscatter diffraction. Also, X-ray diffraction was used to identify the different phases present in the HEA both before and after FSP. The mechanical behavior of the HEA was evaluated by compression testing at room temperature. The compressive yield stress of the base metal changed from 277 MPa to about 270 MPa although statistically no appreciable change was noted. The work hardenability of the SZ and the parent material was analyzed in terms of strain hardening exponents and strain hardening rates in the light of existing work hardening theories.

Keywords: Phase transformation; Friction stir processing; High entropy alloy; TRIP

Materials Today Communications
Volume 28, 2021, 102635, ISSN 2352-4928,

https://doi.org/10.1016/j.mtcomm.2021.102635

(https://www.sciencedirect.com/science/article/pii/S2352492821006279)

Ravi Sankar Haridas, Priyanshi Agrawal, Rajiv S. Mishra

Mechanical response of transformation-induced plasticity (TRIP)-enabled metastable high entropy alloys that display austenite (γ) to martensitic (ε) phase transformation under uniaxial tension is characterized by a constant work hardening segment preceded and succeeded by stages of gradual drop in work hardening. A four-parameter empirical model based on the nature of the work hardening curve is developed. The model enjoys ease of parametric identification from macroscopic mechanical response over physically-based models. Compared to tensile deformation of conventional alloys, some insights are drawn from the numerical value of the model parameters for TRIP HEAs when fitted to their tensile deformation response. Further, a method to predict the tensile mechanical response of TRIP HEAs/steels based on the trends of correlated parameters with stacking fault energy and microstructure is proposed.

Keywords: Martensitic transformation; High entropy alloys; Stacking fault energy; Plasticity; Work hardening

Materials Science and Engineering: A
Volume 823, 2021, 141778, ISSN 0921-5093

https://doi.org/10.1016/j.msea.2021.141778

(https://www.sciencedirect.com/science/article/pii/S0921509321010443)

S.S. Nene, P. Agrawal, M. Frank, A. Watts, S. Shukla, C. Morphew, A. Chesetti, J.S. Park, R.S. Mishra

Recent metastable alloy designs have demonstrated simultaneous attainment of high ultimate tensile strength (UTS) and ductility in high entropy alloys but with low yield strength. Here we present new strategy for improving the work hardenability and yield strength (YS) together in Fe38.5Mn20Co20Cr15Si5Cu1.5 high entropy alloy (Cu-HEA). Drastic increase in the YS (1.5 GPa) is attributed to the formation of γ/ε, ε/ε (twin type) and ε/ε (plate type) interfaces in the microstructure due to extreme grain refinement whereas high UTS-ductility synergy (2.2 GPa, 15%) is attained by dynamic Hall-Petch hardening across these interfaces (i.e. massive interface strengthening) and transformation induced plasticity in γ phase. Thus, this harmonious combination of YS and UTS-ductility synergy in Cu-HEA outperform all structural materials till date. Therefore, deformation-induced massive interface strengthening is a new, yet cost-effective pathway for synergizing the benefits of advanced steels and high entropy alloys together in a material by conventional processing route.

Keywords: Grain refinement; Dual-phase high entropy alloy; Metastability; Strength-ductility paradigm; ε-h.c.p. phase

Scripta Materialia
Volume 203, 2021, 114070, ISSN 1359-6462

https://doi.org/10.1016/j.scriptamat.2021.114070

(https://www.sciencedirect.com/science/article/pii/S135964622100350X)

Surekha Yadav, Qiaofu Zhang, Amit Behera, Ravi Sankar Haridas, Priyanshi Agrawal, Jiadong Gong, Rajiv S. Mishra

Mechanical behavior of the binder phase in a ceramic-metal (cermet) composite is critical for its overall properties, especially toughness and shock resistance. High entropy alloys (HEA) are promising binder phase materials owing to their excellent mechanical properties and impact on obtaining fine-grain cermet microstructure. In the current work, Co27.4Cr13.8Fe27.4Ni27.4Mo4 HEA designed via ICME approach was used as a binder to fabricate ultrafine WC-HEA composite by mechanical alloying and spark plasma sintering. The effect of an HEA binder on the microstructure and mechanical properties of WC-HEA composite are discussed in terms of experimental observation and Weibull modulus (9.7). Compared to a commercial WC-Co composite, a WC-10 wt.% HEA composite exhibits higher hardness and fracture toughness of 21 GPa and 10.5 MPa m1/2, respectively.

Keywords: Cemented carbide; High entropy alloys; Microstructure; Mechanical properties; Spark plasma sintering

Journal of Alloys and Compounds
Volume 877, 2021, 160265, ISSN 0925-8388

https://doi.org/10.1016/j.jallcom.2021.160265

(https://www.sciencedirect.com/science/article/pii/S0925838821016741)

Rajiv S. Mishra, Ravi Sankar Haridas, Priyanshi Agrawal

The paradigm shift of alloying approach that led to high entropy alloys (HEAs) is now well established. Although the initial years were dominated by equiatomic approach, recent years have seen expansion in non-equiatomic compositional space that can be termed as complex concentrated alloys (CCAs). These HEAs/CCAs provide opportunities for tunable performance by manipulating deformation mechanisms. Understanding has advanced to the point that certain aspects of core effects (entropy of mixing, lattice distortion, sluggish diffusion, and cocktail effect) can be critically examined. In addition, new aspects of metastability engineering and emergence of a wide range of processing strategies has put this field on an exponential growth path. In this review, we categorize the compositional and microstructural approaches that exhibit potential for a combination of shear induced phase transformation and twinning, thereby expanding beyond the slip based mechanisms. The emerging HEAs give greater flexibility for tailoring transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP), which have guided design of next-generation steels over the last 20 years to a new level. For TRIP HEAs, the ductility can be extended to as high as 50% while maintaining a strength exceeding 1 GPa. On the other hand, hierarchical microstructural engineering in AlxCoCrFeNi alloys can lead to over 2 GPa strength and >10% ductility. Observations of evolving c/a ratio in HCP phase of certain HEAs hint at possibility of new micromechanisms. While crack tip twin-bridging has been shown as a key mechanism to extend the toughness, concurrent phase transformation at the crack tip has been shown to push the fatigue endurance limit. Tunability of deformation mechanisms in HEAs is unprecedented as compared to the conventional metallic materials, particularly in compositions that exhibit shear induced transformation. The opportunities can be further enhanced by integrating the compositional and microstructure domains, and these aspects are highlighted in this review. The microstructural tailoring can take advantage of high enthalpy states in metastable HEAs with low stacking fault energy values of < 40 mJ m−2. The range of microstructural engineering in HEAs include, heterogeneous grain structure, duplex and triplex microstructures with intermetallic phases, twinning engineered microstructure, coherent boundary engineered microstructure, and dual-phase and triple-phase microstructure with solid solution phases.

Keywords: High entropy alloys; Deformation induced twinning; Deformation induced transformation; Mechanical behavior; Hierarchical micromechanisms; High enthalpy states

Materials Science and Engineering: A
Volume 812, 2021, 141085, ISSN 0921-5093

https://doi.org/10.1016/j.msea.2021.141085

(https://www.sciencedirect.com/science/article/pii/S0921509321003543)

Ravi Sankar Haridas, Priyanshi Agrawal, Saket Thapliyal, Surekha Yadav, Rajiv S. Mishra, Brandon A. McWilliams, Kyu C. Cho

Abstract: Compressive response of a novel Fe38·5Mn20Co20Cr15Si5Cu1.5 high entropy alloy with transformation induced plasticity made by laser powder bed fusion was studied at quasi-static, medium and high strain rates. Mechanical response and variation in work hardening rate with strain rate were correlated with γ (f.c.c.) → ε (h.c.p.) martensitic transformation, subsequent phase evolution and adiabatic heating. A strong near basal {0 0 0 1} texture observed in the transformed ε (h.c.p.) phase after deformation was correlated with the initial texture, γ (f.c.c.) → ε (h.c.p.) transformation orientation relationship, as well as the activated deformation mechanisms in ε (h.c.p.) phase. The initial c/a ratio of 1.612 for the ε (h.c.p.) phase evolved with deformation and this was quantified to understand the propensity of non-basal <c+a>slip activation. Metastable γ (f.c.c.) dominant microstructure in the as-built alloy enabled excellent hardening via γ (f.c.c.) → ε (h.c.p.) transformation accompanied by activation of non-basal <c+a>slip and twinning. Experimental results were correlated with existing empirical constitutive models such as Johnson-Cook, Modified Zerilli-Armstrong, Khan-Huang-Liang and Khan-Liu; the Khan-Liu model evidenced the best correlation with experimental results.</c+a></c+a>

Keywords: High entropy alloys; Transformation induced plasticity; Additive manufacturing; High strain rate loading; Microstructure

Mechanics of Materials
Volume 156, 2021, 103798, ISSN 0167-6636

https://doi.org/10.1016/j.mechmat.2021.103798

(https://www.sciencedirect.com/science/article/pii/S0167663621000533)

R.S. Mishra and S.S. Nene

This short review paper examines the uniqueness of metastable high entropy alloys (HEAs) that transform during plastic deformation. The linkages among alloy design induced metastablilty, processing induced “high enthalpy states” and evolving deformation micromechanisms are intriguing. One key aspect is the adaptive nature of deformation mechanisms because of activation of different micromechanisms during plasticity. This activation of deformation micromechanisms is attributed to transformation-induced plasticity in γ-f.c.c. phase and the evolving nature of c/a ratio in the ε-h.c.p. phase during deformation. As a result, metastable HEAs have good strength-ductility synergy and exceptional fatigue strength.

METALLURGICAL AND MATERIALS TRANSACTIONS A
VOLUME 52A, MARCH 2021—889-896

https://doi.org/10.1007/s11661-021-06138-3

Tianhao Wang, Shivakant Shukla, Bharat Gwalani, Subhasis Sinha, Saket Thapliyal, Michael Frank & Rajiv S. Mishra

Tuning deformation mechanisms is imperative to overcome the well-known strength-ductility paradigm. Twinning-induced plasticity (TWIP), transformation-induced plasticity (TRIP) and precipitate hardening have been investigated separately and have been altered to achieve exceptional strength or ductility in several alloy systems. In this study, we use a novel solid-state alloying method—friction stir alloying (FSA)—to tune the microstructure, and a composition of a TWIP high-entropy alloy by adding Ti, and thus activating site-specific deformation mechanisms that occur concomitantly in a single alloy. During the FSA process, grains of the as-cast face-centered cubic matrix were refined by high-temperature severe plastic deformation and, subsequently, a new alloy composition was obtained by dissolving Ti into the matrix. After annealing the FSA specimen at 900 °C, hard Ni–Ti rich precipitates formed to strengthen the alloy. An additional result was a Ni-depleted region in the vicinity of newly-formed precipitates. The reduction in Ni locally reduced the stacking fault energy, thus inducing TRIP-based deformation while the remaining matrix still deformed as a result of TWIP. Our current approach presents a novel microstructural architecture to design alloys, an approach that combines and optimizes local compositions such that multiple deformation mechanisms can be activated to enhance engineering properties.

Scientific Reports
Volume 11, Article number: 1579 (2021)

https://doi.org/10.1038/s41598-021-81350-0

Hang Z. Yu & Rajiv S. Mishra

As the forging counterpart of fusion-based additive processes, additive friction stir deposition offers a solid-state deformation processing route to metal additive manufacturing, in which every voxel of the feed material undergoes severe plastic deformation at elevated temperatures. In this perspective article, we outline its key advantages, e.g. rendering fully-dense material in the as-printed state with fine, equiaxed microstructures, identify its niche engineering uses, and point out future research needs in process physics and materials innovation. We argue that additive friction stir deposition will evolve into a major additive manufacturing solution for industries that require high load-bearing capacity with minimal post-processing. This paper delineates additive friction stir deposition, which offers a solid-state deformation processing route to metal additive manufacturing and renders fully-dense material with fine, equiaxed microstructures in the as-printed state.

Materials Research Letters
Volume 9, 2021 - Issue 2, Pages 71-83, 24 Nov 2020

https://doi.org/10.1080/21663831.2020.1847211

Priyanka Agrawal, Sanya Gupta, Saket Thapliyal, Shivakant Shukla, Ravi Sankar Haridas, Rajiv S. Mishra

An in-depth understanding of microstructure and resultant properties is paramount in the design of a novel alloy system, especially for additive manufacturing (AM). The present investigation aims to characterize a prototypical AM Al alloy with great potential for structural applications. An Al-1.5Cu-0.8Sc-0.4Zr alloy designed using integrated computational material engineering was printed using the laser powder bed fusion AM process. This novel alloy shows promising combination of strength and ductility in as-built and peak-aged conditions. This improvement in the tensile properties is attributed to the presence of both coherent L12 Al3Sc/Al3(Sc,Zr) precipitates and Cu-rich regions. The microstructures were studied via extensive microscopy at different length scales using X-ray microscopy, scanning electron microscopy, and transmission electron microscopy. Fractography revealed that the columnar grain boundaries in as-built condition allow easy slip transfer as compared to the equiaxed grains, with the apex of the melt pool acting as the crack nucleation site. However, the peak aged condition resulted in improved strength while marginally sacrificing ductility due to precipitates decorating dislocations, grain boundaries and melt pool boundaries thus acting as obstacles to slip transfer.

Keywords: Additive manufacturing; Aluminum alloys; X-ray microscopy; Texture; Fractography

Additive Manufacturing
Volume 37, 2021, 101623, ISSN 2214-8604,

https://doi.org/10.1016/j.addma.2020.101623

(https://www.sciencedirect.com/science/article/pii/S2214860420309957)

Subhasis Sinha, Mageshwari Komarasamy, Tianhao Wang, Ravi Sankar Haridas, Priyanka Agrawal, Shivakant Shukla, Saket Thapliyal, Michael Frank, Rajiv S. Mishra

Notch-tensile behavior of an Al0.1CrFeCoNi high entropy alloy (Al0.1-HEA) was studied using V-notch geometry and with local strain mapping using digital image correlation (DIC). A notch-strength ratio of 1.51 indicated notch strengthening. Further analysis of stress-strain response supplemented with microstructural analysis revealed that, while the presence of notch results in strengthening due to work hardening by twinning induced plasticity, the notch also contributes strongly to geometrical softening in the non-uniform ductility regime, and accounts for the onset of failure. The strain localization behavior of Al0.1-HEA due to the presence of V-notch was compared with three conventional alloys: Inconel 625 nickel-based superalloy, 304 stainless steel and Ti–6Al–4V titanium alloy. The study revealed that the nature of notch widening with increasing strain was dependent on material characteristics. The extent of notch widening impacted the local strain field and stress distribution, thereby influencing the propensity for crack initiation and growth. The experimental results were verified by finite element analysis.

Keywords: High entropy alloy; Notch; Tensile deformation; Digital image correlation; Finite element analysis

Materials Science and Engineering: A
Volume 774, 2020, 138918, ISSN 0921-5093

https://doi.org/10.1016/j.msea.2020.138918

(https://www.sciencedirect.com/science/article/pii/S0921509320300101)