The Effect of Prestrain on Deformation of Ni₃Al Single Crystals

Abstract

Prestrain of bcc metals at temperatures T>0.2T_m decreases the flow stress at lower temperatures (T<O.15T_m) where the yield strength has a large negative temperature dependence. This investigation has examined the influence of prestrain on the flow stress of Ni₃A1, for which the yield strength has a large positive temperature dependence above 25°C. Nickel-rich Ni₃Al single crystals with axial orientations near <001> or <123> were prestrained in compression up to 20% shear strain at -196°C and subsequently compression tested at 550°C. Specimens near the <123> axial orientation were also prestrained at 550°C and then tested at -196°C. The initial flow stress of samples prestrained at - 196°C and tested 550°C was reduced up to 50% compared to samples solely compression tested at 550°C. The magnitude of the reduced flow stress and its extent as a function of plastic strain were dependent upon the amount of prestrain and orientation. Prestraining at 550°C and subsequent testing at -196°C increased the flow stress by as much as 60% over samples solely tested at -196°C. Dislocation substructures obtained from selected samples coupled with arguments based on dislocation dynamics and obstacle strengthening are used to explain the results.