Study On High Temperature Tensile Deformation And Mechanism Of Mg-7Gd-4Y-1.5Zn-0.5Zr Magnesium Alloy

An alloy of Mg-7Gd-4Y-1.5Zn-0.5Zr was obtained through melting and casting middle alloy. High temperature elongation-to-failure tests were carried out after double-stage homogenization treatment and solid solution treatment of 450 ℃ and 60 min, asome tests were designed to stop at fixed strains. High temperature elongation-to-failure tests were also conducted for specimens treated by additional single stage and two-stage aging treatments. The relationship among long period stacking order(LPSO), high temperature mechanical properties, and deformation mechanisms were investigated, as well as the effects of the dual-stage aging treatment on mechanical properties of this alloy.The alloy consists of ?-Mg matrix, eutectic structure Mg(GdYZn) phase,18R-type Mg12 YZn phase with LPSO, and pure Zr. after the double-stage homogenization treatment at 460 ℃×6 h+520 ℃×16 h, Dense striped structures also exist.The EF experiments were conducted at temperatures of 450, 475, and 500 ℃,and strain rates of 10-3 and 5×10-4 s-1, respectively. The results show that only the deformation at 500 ℃ provides a relatively long steady-state deformation zone. The peak stress is not affected strongly by a decreasing strain rate at the same temperature,which has a remarkable effect on elongation though. Dynamic recrystallization occurs during the EF tests based on observation under optical microstructure. The striped structure decomposes diffusively and disappear gradually during deformation at500 ℃, and the residual bulk phase near grain boundary is Mg12 YZn.Under high temperature deformation conditions introduced before, the dominate deformation mechanism is grain boundary sliding with a stress exponent of n≈2, and an activation energy between 180 and 240 kJ/mol. The relatively high activation energy is related to the inhibited dislocation movement by Mg12 YZn phase and striped structure which are stable during high temperature deformation.The tensile tests at fixed strains were conducted at 500 ℃ and 10-3 s-1 until true strain reached 0.2, 0.4 and 0.6, in which the eutectic phase dissolves and the residual bulk structure is the 18R- type LPSO phase, Mg12 YZn. The striped structure in the?-Mg matrix is 6H- type LPSO phase which dissolve gradually during deformation.The increase of deformation strength at high temperatures is attributed to the interaction between dislocation and the LPSO phases, and grain boundary pinning by the LPSO phases.EF tests were conducted on the specimens after single stage aging treatment of200 ℃× 24 h and dual-stage aging treatment of 90 ℃× 16 h+200 ℃× 24 h under150, 200, 250 ℃ and 10-3 and 10-4 s-1, respectively, then microstructures were observed under optical microscopy. The results show that all speciments show ahigher strength and a lower plasticity and some granular precipitates and fine grains can be observed on the cross section. The dual-stage aging treatment does not improve mechanical properties greatly.