NANOCRYSTALLIZATION OF Al-12.7Si ALLOY INDUCED BY SURFACE INTENSE PLASTIC DEFORMATION

Gradient nano-micro-structured surface layer was synthesized on Al-12.7Si alloy by means of surface mechanical attrition treatment. The microstructural evolution was examined by XRD and TEM. The surface intense deformation induced nanocrystallization mechanism was summarized as follows: upon the application of repeated loads, dislocation cells/sub-micro-grains form through slips, annihilations and recombinations of high density of dislocations in Al coarse grains. The misorientations between the dislocation cells/sub-micro-grains increased by continuously absorbing more dislocations. High density of dislocations in sub-micro-grains developing in above route led to the reduction of grain size and the increment of misorientations between the refined grains, and finally equiaxed nc grains with random orientations formed. The refinement of Si particles was found to be carried out through single- or multi-slips of internal dislocations and partial dissolution, then the refined Si particles distributed uniformly due to the plastic flows in Al matrix.