AbstractThe 7475 Al–0·7Zr (wt-%) cold rolled alloy sheet fabricated by powder metallurgy exhibited remarkable superplasticity for strain rates between 2×10−3and 3×10−1s−1at 793 K. During the initial deformation stage, the strain rate sensitivitymincreased with increasing strain and attained a value greater than 0·5 after about 50% elongation. Thus, the microstructural evolution which occurred during this initial deformation stage was examined, especially at high strain rates. On heating to the tensile testing temperature of 793 K, the alloy developed a very fine subgrain structure, stabilised with finely dispersed Al3Zr precipitate particles. The average subgrain size was less than 1μm, and the misorientation angle of most subboundaries was less than 5°. In the initial deformation stage showing an increase inmvalue, the subboundary misorientations and, thus, the contribution of boundary sliding tended to increase with strain. Even the subboundaries with misorientation of 5–15°could play an important role through boundary sliding in the limited strain rate range showing superplasticity. Subgrains rotated as a group owing to the different sliding rates at different boundary misorientations, including those of original grain boundaries. The accommodation dislocations for the boundary sliding were absorbed into the subboundaries. Both subgrain rotation and dislocation absorption were considered to contribute greatly to the increase in subboundary misorientation.MST/1367