How grain boundary characteristics influence plasticity close to and above the critical temperature of ultra-fine grained bcc Ta2.5W

Dislocation-grain boundary interaction is widely accepted as the rate-controlling process for ultra-fine grained bcc metals in their high temperature deformation regime above the critical temperature. However, the influence of different types of grain boundaries remains widely unexplored so far. To this end we present here an advanced high temperature nanoindentation study on Ta2.5W specimens consisting of two distinctively different grain boundary types, but with similar submicron average spacing. While one set of samples consisted of a predominant fraction of high-angle boundaries, the second set contained mainly low-angle boundaries. Fully recrystallized samples served as a coarse grained reference batch. Using advanced nanoindentation at elevated temperatures up to 823 K, we find a temperature invariant hardness in the case of the low- and a strongly pronounced temperature dependence for the high-angle grain boundary samples. This underlines the importance of grain boundary diffusivity for the predominant process of interfacial stress relaxation. Pronounced interaction of dislocations with oxygen impurity atoms was observed from 473 to 773 K for the coarse grained microstructure, yielding serrated flow as an indicator for a Portevin-Le Chatelier effect up to 573 K. Both grain boundary types showed a significant influence to the dislocation-impurity interaction, whereby the high-angle grain boundaries suppress discrete flow characteristics.