THE STRAIN-RATE SENSITIVITY OF THE FLOW-STRESS AND THE MECHANISM OF DEFORMATION OF SINGLE-CRYSTALS OF NI-3(ALHF)B

The strain rate sensitivity (SRS) beta = delta tau/delta(ln epsilon) of the flow stress tau, has been measured for crystals of Ni-3(Al, Hf)B over a range of temperatures T in the yield stress anomaly up to 500 K, and as a function of strain epsilon and of crystal orientation. beta obeys a Cottrell-Stokes low when plotted against tau - tau(y), where tau(y), is the yield stress at 0.01% strain. The slopes are about 1%, decrease with increasing T and are approximately independent of orientation. The results show that the yield stress and work-hardening rates (WHRs) are controlled by different mechanisms. The flow stress tau = tau(y), + tau(h), where tau(h) is due to work hardening. tau(y), is a reversible function of T, but independent of pre-strain epsilon and epsilon. tau(h) is a function of epsilon, T and epsilon; it controls the SRS of epsilon. At yield, both edge and screw dislocations propagate on (111), the latter via lateral glide of edge character superkinks which bypass screws locked by cross-slip on to (010), allowing source operation and slip lines to be formed rapidly. Work hardening is attributed to forest dislocations. The variation in WHR with temperature and crystal orientation is interpreted in terms of slip on the primary cube plane and of screws slipping on the (010) cross-slip plane.