Fracture of ultrafine grained magnesium alloy AZ31 under superplastic deformation conditions

Equal Channel Angular Pressing (ECAP) with an imposed hydrostatic pressure conducted at a relatively low temperature of 150 °C was used to obtain exceptional superplastic properties of magnesium alloy AZ31. The mechanism of the significantly improved ductility was studied by examining the fracture surfaces and internal cavities in superplastically deformed specimens by scanning electron microscopy. The micrographs of the fracture surfaces exhibit unusually deep elongated dimples. The micrographs provide an indication that the dimples were formed at an early stage of the tensile test, possibly due to pull-out of relatively large precipitates, as the size of the dimples is comparable with the precipitate size. However, the load-bearing capacity of the ligaments of intact material between the dimples was still high enough to maintain overall integrity of the specimen during the continued tensile test, while the pores formed kept getting elongated along with the extension of the specimen as a whole. The high ductility of the material within the ligaments suggests that further increase of superplastic ductility can be achieved by appropriate heat treatment eliminating large precipitates. We believe that the benefits of the combined effect of increased strength and ductility found for ECAP-induced bi-modal grain structure could then be utilised to an even greater extent.