Temperature and orientation dependence of the yield stress in Ni{in3}Ga single crystals

Abstract

Single crystals of the intermetallic compound Ni 3Ga with Ll 2 structure were deformed in compression at temperatures between 77 °K and 1000 °K to investigate the anomalous temperature dependence of the strength in this type of compound. At an elevated temperature, the activated slip plane changed from the {111} to the {100}, or exactly the noncrystallographic plane near the {100}, the transition temperature being dependent on orientation. CRSS for {111}〈110〉 slip increased abnormally from 77 °K up to 900 °K. and Schmid's law did not hold for this system, i.e., the larger the stress component on the {100} cross slip plane the higher the CRSS. Whereas, CRSS for {100} type slip decreased steeply with increasing temperature and the flow stress by this slip was very sensitive to strain rate, suggesting that {100}〈110〉 slip is controlled by the Peierls mechanism. From the characteristic orientation dependence of CRSS together with the results of electron microscopy, anomalous positive temperature dependence of CRSS for {111} slip was interpreted in terms of a model in which the macroscopic yield stress is determined by the dynamical motion of screw dislocations creating and breaking away sessile portions. The sessile portions are formed by cross slip of a unit dislocation constituting a super-dislocation from the {111} slip plane onto the {100} plane to reduce the total energy of the dislocation.