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 D_L controlled grain boundary sliding, D_L 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.