A metallographic study of superplasticity in a micrograin aluminium bronze

Abstract

The microstructures of micrograin Cu-9 to 10% Al-0 to 4% Fe alloys, which are superplastic at 800° C, have been determined. Metallographic studies after deformation at 800° C over a range of strain-rates encompassing the three stage strain-rate hardening behaviour common to superplastic materials show that in the low strain-rate range, below that for high values of the strain-rate sensitivity exponent (m), clumps of grains slide together as units with considerable flow in the matrix close to sliding interfaces. After deformation in this low strain-rate range there is no evidence for dislocation motion within the grains. With increasing strain-rate, through and beyond the strain-rate range where peak values ofm are recorded, evidence for dislocation motion steadily increases; the tendency for clumps of grains to slide together diminishes; and there is decreased flow in the matrix about the sliding interfaces. The strain-rate for maximumm shows a strong dependence on the proportion ofβ phase in the microstructure and the presence of iron which acts to refine the grain size. These observations are explained in terms of a flow mechanism whereby the high strain-rate sensitivity range occurs intermediate between a low strain-rate range, where sliding is accommodated by diffusion, and a high strain-rate range, where accommodation is by dislocation movement through the matrix.