Superplasticsty in Commercial Al 7475

摘要

Commercially produced sheets of superplastic grade Al 7475 with various thicknesses have been characterised in terms of their microstructure and mechanical properties. Gold grids deposited on polished surfaces of tensile specimens were used to measure the surface strains on mesoscopic and microscopic length scales, and fracture surfaces were examined using SEM. The results show that mechanically the alloy behaves like other superplastic materials, with a maximum strain rate sensitivity, m, of 0.6, and a grain size exponent, p, of 2.6. Thinner sheets had smaller grain sizes and therefore higher superplastic strain rates. The surface studies showed that the actual strain rate in the gauge length was considerably smaller than the ratio of the crosshead velocity to the nominal gauge length owing to extrusion of material from the gauge head. Evidence for grain boundary sliding and localised tensile separation of material were found. The latter effect was responsible for about 30% of the strain in the surface. The separated regions were bridged by filaments of similar dimensions to whiskers observed protruding from the fracture surfaces. These whiskers were rich in Mg and were oxidised. It is suggested that the solid oxide layer is responsible for stabilising the filaments against necking.