Superplasticity in PM 6061 Al alloy and elimination of strengthening effect by reinforcement in superplastic PM aluminum composites

Plastic-flow behavior of a powder-metallurgy (PM) processed 6061 matrix alloy has been investigated in a wide range of elevated temperature between 430 and 620°C. It was found that the 6061 Al alloy exhibits superplasticity in a relatively wide range of temperature from 520 to 620°C at a high strain rate of 10−2 s−1. Deformation behavior of the present alloy could be divided into three regions when the presence of threshold stress for plastic flow was assumed. They are DL controlled grain boundary sliding, DL controlled dislocation climb creep and powder-law breakdown, respectively. When temperature is as high as 590°C, however, the activation energy increases significantly higher than that for self-diffusion in aluminum and flow stress decreases further than normally expected. This phenomenon is likely attributed to the presence of liquid phase above 590°C. Comparison of the data in Region I below 610°C with those for a number of superplastic aluminum composites indicates that strengthening effect by reinforcement does not exist. Several speculations including diffusional relaxation in vicinity of reinforcements were made to explain this phenomenon.