Effect Of Heat Treatment On The Microstructure And Mechanical Properties Of Extruded Mg-Zn-Y Alloy

As a kind of light-weight metallic structural material, magnesium alloys have potential to be widely used in the fields of electronic communication, aerospace and transportation. In many studies on magnesium alloy, the Mg-Zn-Y alloys containing LPSO phase have been extensively paid attention for their high specific strength. The Mg-Zn-Y alloy of Mg97 Zn Y2 with specific atomic ratio(Zn/Y=1:2) was widely inverstigated during the past decade. However, the second phase of this alloy is in the shape of rough net under conventional cast condition. The strengthening effect of the intermetallic compounds wasn’t fulfilled for the as-cast Mg97 Zn Y2 alloy. Given the above-mentioned Zn/Y atomic ratio(1:2), the compositions of Mg-Zn-Y alloy were optimized in the present study to adapt to conventional cast condition. The as-cast Mg-Zn-Y alloy was extruded with the ratio 10:1 to refine the microstructure. The as-extruded Mg-Zn-Y alloy was heat-treated at 400℃, 490℃, and 550℃ for different time to disclose the stabilities of microstruture and mechanical properties under high temperature. Moreover, to simulate the environment of deep space, the effects cryogenic(liquid nitrogen) treatment and cryogenic-thermal(200℃) circle treatment on the microstructure and mechanical properties of Mg-Zn-Y alloy were experimentally studied.. Also, the research revealed the tensile behavior of the alloys treated by different processes based on the stress-strain curves and fracture surface. Some important conclusions were drawn as follows:On the condition that Zn/Y atomic ratio was fixed at 1:2, the optimal compositions of Mg-Zn-Y alloy was Mg97.75Zn0.75Y1.5, which was appropriate to be processed under conventional gravity cast. Compared to Mg97 Zn Y2, The ultimate tensile strength and elongation to failure of the alloy were improved by about 14% and 39% than ordinary casting condition. The extrusion process could significantly refine grains and microstructure of as-cast Mg97.75Zn0.75Y1.5. The net-shaped second phase was refined to tiny particles with uniformly distribution. The tensile strength was improved for one time after extrusion. The elongation to failure was increased from 6% to 30%.As-extruded Mg97.75Zn0.75Y1.5 alloy was heat treated at different temperature. The microstructure almost kept stable after being hold in 400℃ × 24 h and 490℃ × 4h. However, the yield strength was invreased to some degree. After being hold in 490℃ × 8h and 550℃ × 1h, the second phase particles were transformed to a rod-like shape. The mechanical property of as-extruded Mg97.75Zn0.75Y1.5 alloy drastically decreaseed and the tensile fracture was transformed from conical equiaxed dimples to cleavage planes and steps.. Analyses on tensile stress-strain curves showed that as-extruded alloy has obvious phenomenon of discontinuous yield. Heat treatment could effectively decreased the yield stage, and increased the absorbed energy during tensile.The dimension and distribution of second phase in as-extruded Mg97.75Zn0.75Y1.5 kept stabe after being treated for longtime cryogenic(liquid nitrogen) immersion and cryogenic-thermal(200℃) cycle treatment. The longer the cryogenic treatment, the higher the material’s yield strength. In addition, cryogenic-thermal cycle and continuously cryogenic treatment could contribute the amount of holes and tiny flaws on fracture surface. The elongation to failure and the strain hardening exponent decreased.