Microstructure, Texture Evolution And Superplasticity Of Mg-1.5Mn-0.3Ce Magnesium Alloy Processed By Severe Plastic Deformation

An improved T-shape channel pressing (TCP) die was applied to extrude deformation for meshed Mg-1.5Mn-0.3Ce magnesium alloy. The change of mesh curves indicated that meshed specimen was shear deformation from the middle to both ends during TCP deformation and the basic homogeneous deformation can be achieved after three passes pressed by route A. Texture of different passes of TCPed magnesium alloy was also observed. The experimental results showed that the texture of as-rolled magnesium alloy was reduced gradually, and started to change to TCPed texture when the intensity of texture rose with the increase of TCP passes. As the role of grain refinement was stronger than texture softening, the strength and elongation of TCPed magnesium alloy at room temperature were increased with the passes.Mg-1.5Mn-0.3Ce magnesium alloy was severely deformed by TCP for four passes, the grain size was greatly refined from 35μm to 2μm, and numbers of tiny Mg12 Ce dispersively distributed in intragranular and intergranular regions observed by TEM. The maximum elongation of 604% was obtained at 673 K and a strain rate of 3×10-3 s-1, and its strain rate sensitivity m was 0.36. The microstructure characteristics of tensile-to-failure sample showed that grain boundary sliding was the primary mechanism of the superplastic deformation, and intragranular slip would become more obvious at lower strain rate and higher temperature and played an accommodated role in promoting grain boundary sliding during the deformation.Equal-channel angular pressing (ECAP) was conducted on Mg-1.5Mn-0.3 Ce magnesium alloy to investigate microstructure, texture and mechanical properties. The microstructure of severe plastic deformation indicated that the grain size was greatly refined from 26.1μm to 1.2μm after six passes. At the same time, the texture of as-rolled magnesium alloy was reduced gradually and started to change to ECAPed texture when the texture intensity increased with ECAP passes. Mechanical properties showed that the tensile strength and yield strength were raised with ECAP passes before three passes which matched the relationship of Hall-Petch as the role of grain refinement was more significant than texture softening, and got the maximum values of 272 MPa and 263 MPa at the third pass deformation. While the passes continued to increase to four passes, the strength was declined due to the non-basic texture formed. The fractograph of ECAPed magnesium alloy revealed that fracture mode of the sixth pass was similar with as-rolled magnesium alloy which was fractured along the grain boundary, and there were more dimples for sixth pass with the biggest elongation of 16.8% at room temperature. The annealed microstructure evolution of ECAPed Mg-1.5Mn-0.3Ce magnesium alloy and the superplasticity at high temperature were investigated. The results demonstrated that grain growth was faster for one pass annealed at 673 K for 1 h, and lowed down with increase of passes because of the stabilization of grain boundary. The refined microstructure of the sixth pass was stable below 673 K as the grain size ECAPed magnesium alloy still retained less than 5μm. Measurement of activation energies (Q) for static grain growth indicated the presence of two different values depending on temperature ranges, that is, Q=38.2 kJ/mol in temperature range 523～673 K and Q=128.45 kJ/mol in temperature range 673～823 K. The low Q value in temperature range 523～673 K caused by decrease in dislocation density by enhanced recovery with increasing temperature. After six passes ECAPed, the maximum elongation of 687% was obtained at 573 K and a strain rate of 2×10-4 s-1.