Deformation and microstructure evolution in three refractory metals at elevated temperatures

Refractory metals and their alloys are used in a range of significant technological applications requiring resistance to high temperatures.  Despite their widespread use, even unalloyed refractory metals still present some behaviors that remain mysterious.  Among these are the mechanisms that control plastic flow and microstructure evolution during deformation at elevated temperatures.  Results from studies of these behaviors are presented for three unalloyed refractory metals: Mo, Ta, and Nb.  The deformation in all these BCC refractory metals at elevated temperatures and slow strain rates is governed by the dislocation-climb-controlled creep mechanism.  This mechanism produces dislocation substructures that contain distinct subgrains.  The dynamic microstructural evolution that results in subgrains affects behaviors such as grain growth.  Dynamic grain growth that occurs during plastic deformation, in turn, leads to the phenomenon of dynamic abnormal grain growth (DAGG).  The links between deformation substructure and dynamic grain growth are explored through experiments using Mo and Ta.  DAGG is shown to produce large single crystals in the case of Mo.  A potential mechanism that drives DAGG is proposed and discussed.  Deformation substructure and subgrains also provide an explanation for the increase in creep resistance generally observed with increasing impurity content in refractory metals.  The effect of impurity content on creep resistance is explored for Nb, and a mechanism to explain it is proposed.