The strain rate sensitivity of the flow stress and the mechanism of deformation of single crystals of Ni3(Al Hf)B

Abstract

The strain rate sensitivity (SRS) β = δτ/δ (In σ) of the flow stress τ, has been measured for crystals of Ni₃(Al, Hf)B over a range of temperatures T in the yield stress anomaly up to 500 K, and as a function of strain E and of crystal orientation. β obeys a Cottrell-Stokes low when plotted against τ - τ_y, where τ_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 τ=τ_y + τ_h, where τ_h is due to work hardening. τ_y is a reversible function of T, but independent of pre-strain ∊ and σ τ_h is a function of ∊, T and σ; it controls the SRS of τ. 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.