High strain rate superplasticity in a fine-grained PM 7475 Al alloy

Superplastic behavior of powder-metallurgy (PM) processed 7475 +0.7Zr Al alloy having a fine grain size of 2 mu m was investigated. Strain-rate-change tests were conducted at elevated temperatures to examine the strain rate-stress relations. The deformation behavior was analyzed by assuming the presence of threshold stress for grain boundary sliding. After the threshold-stress compensation, the plastic flow was found to correlated well with lattice self-diffusion in pure aluminum. A maximum tensile elongation of up to 1000% was observed at a very high strain rate near 10(-1)s(-1) and 515 degrees C. A deformation map was constructed to identify the dominant deformation mechanism for the PM alloy. It was the grain boundary sliding controlled by lattice diffusion in aluminum. Evidence of short fiber or whisker-like formation were observed on the fractured samples over a wide range of temperature.