On mechanical properties and superplasticity of Mg–15Al–1Zn alloys processed by reciprocating extrusion

Abstract

A superior combination of specific strength and low-temperature high-strain-rate superplasticity of two-phase Mg–15Al–1Zn alloys could be achieved with reciprocating extrusion directly from as-cast billets. In the as-extruded state, the yield strength and ultimate tensile strength were 306 and 376 MPa, respectively. The optimum elongation of at least 1610% in company with a high m value of 0.7 was obtained at the strain rate of 1 × 10 −2 s −1 when tested at 325 °C. The pronounced strengthening mechanism was attributed to the high volume fraction of fine-grained hard Mg 17Al 12 phase in the refined α-Mg matrix. The excellent superplasticity at high strain rates was also mainly contributed by the high volume fraction of Mg 17Al 12 phase which displays easier grain boundary sliding than α-Mg phase. The apparent activation energy for superplastic flow is thus smaller than that of the boundary diffusion in Mg. In addition, the mechanisms for the filament formation and cooperative grain boundary sliding are discussed.