Superplastic Behaviour and Microstructure Evolution in a Commercial Ultra-Fine Grained Al-Mg-Sc Alloy

摘要

An Al-6%Mg -0.3%Sc-0.3%Mn alloy was subjected to equal-channel angular extrusion (ECAE) at 325°C to a total strain of about 16 that resulted in an average grain size of about 1μm. Superplastic properties and micro structural evolution of the alloy were studied in tension at strain rates ranging from 1.4×10{sup}(-5) to 1.4 s{sup}(-1) in the temperature interval from 250 to 500°C. It was shown that this alloy exhibits superior superplastic properties in the wide temperature range 250-500°C at strain rates over 10{sup}(-2)s{sup}(-1). The highest elongation to failure of 2000% is attained at a temperature of 450°C and an initial strain rate of 5.6×10{sup}(-2) s{sup}(-1) with the corresponding strain rate sensitivity coefficient of about 0.4. Two different fracture mechanisms were revealed in high strain rate superplasticity. At temperatures higher than 300°C and/or strain rates less than 10{sup}(-1)s{sup}(-1), failure took place in brittle manner practically without necking and cavitation played a major role in the failure. In contrast, at low temperatures and/or high strain rates, fracture occurred in a ductile manner suggesting the fracture of samples by localized necking. At these conditions, evidence of very limited cavitation was found in the samples.