Structural evolution and validity of Hall-Petch relationship in an Al-3% Mg alloy with submicron grain size

A submicron grain size was produced in an Al-3% Mg solid solution alloy by introducing intense plastic deformation through either equal-channel angular (EGA) pressing or torsion straining. The microstructure after ECA pressing was very heterogeneous with some areas with equiaxed grains and other areas with elongated grains but the microstructure after torsion straining was reasonably homogeneous. The average grain sizes of the alloys produced by ECA pressing and torsion straining were similar to 0.2 and similar to 0.09 mu m, respectively. After static annealing for one hour at temperatures in the range from 323 K (50 degrees C) to 803 K (530 degrees C), both materials exhibited extensive grain growth up to grain sizes of > 100 mu m. The microhardness of the Al-3% Mg alloy was measured as a function of grain size. The results show that there is an increase in microhardness with decreasing grain size which persists to the smallest grain size examined experimentally (90 nm).