Observations of grain boundary sliding during superplastic deformation

Flat test specimens of a 10-micron grain size Al-33 pct Cu alloy were deformed in air over a range of strains at 450 °C (723 K) and at 10^-5 strain per second, and examined by SEM. Direct observations and measurements were also made in a SEM during superplastic deformation of a 5-micron Pb-Sn eutectic alloy at room temperature at a strain rate of about 10^-5 s^-1. Extensive grain boundary sliding was noted. Microcreep curves were obtained between initially adjacent grains and showed that deformation is cyclical in nature and varies by several orders of magnitude locally from point to point. Many surface grains are observed to change neighbors many times, to undergo some rotation and tilt, to show little change in size or shape, and yet to avoid intergranular cracking. Superplastic deformation is a highly heterogeneous process, not unlike that observed in the creep of conventional types of alloys at elevated temperatures, but differing in the extent of plastic deformation free of intergranular cracking, combined with extensive recovery in the form of boundary migration. Formerly a Graduate Student in the Department of Materials Science and Engineering at Massachusetts Institute of Engineering, then Research Manager of SAIL in Ranchi, India