Principles of grain refinement and superplastic flow in magnesium alloys processed by ECAP

The superplastic properties of metallic materials are associated with the length scale and with the thermal stability of their grain structure. Whereas equal-channel angular pressing (ECAP) may be used to produce ultrafine-grained structures in f.c.c. metals through the homogeneous subdivision of the grains, research on two magnesium alloys reveals a different and heterogeneous process of grain refinement which is dependent upon the initial grain structure in the alloys. Experiments demonstrate that different structural features may be achieved using different processing routes and this leads to the development of a processing strategy for achieving an optimum microstructure. It is shown by mechanical testing that the optimum superplastic properties also depend on the processing route and, depending on the structural characteristics, the maximum elongations to failure may occur either in the early stages of processing by ECAP or after processing through large strains.