Activation energies for superplastic tensile and compressive flow in microduplex α/ß copper alloys

Abstract

Logarithmic stress against strain rate curves have been determined at various temperatures for a superplastic commercialα/β nickel-silver alloy strained in tension, and a laboratory prepared microduplex alloy of nominally similar composition strained in compression. The shapes of the curves were found to be affected by grain growth at high temperatures and strain softening at low temperatures. After taking these factors into account, it was apparent that with decreasing strain rate in both alloys a change in deformation mechanism occurred giving rise to a Region I of low strain-rate sensitivity. By confining activation energy (Q) measurements to temperatures at which steady-state deformation occurred, it was found thatQ for Region II was very similar to that measured for grain boundary diffusion in theα phase of a nickel-silver alloy of similar composition, whileQ for Region I was substantially higher than that for lattice diffusion. Values of strain-rate sensitivity andQ were found to be similar for each direction of applied stress.