Study On Microstructure And Mechanical Properties Of High Strength And Heat Resistant Mg-Gd-Y Alloy

Magnesium alloy is the lightest metal structure materials, and is regarded as “the green engineering structure materials of 21 st century”; as a result, it has the great applications in the aerospace, national defense war industry, automobile and electronic products. Although with light structure magnesium alloy have low mechanical properties at room and elevated temperature as compare to Al alloy which restrains its scope of applications. A new developed the series Mg-Gd-Y alloy receives widespread attention, because of its high strength and good creep resistance at elevated temperature. However, the existing research predominantly concentrated in the metal mold casting, few literatures study on sand mold castings. Moreover, the existing methods to manufacture Mg-Gd-Y series alloy sheets have high mechanical properties, but costs relatively high and difficult to easily obtain consecutive production, which limits its application values. In this dissertation, Microstructure and the mechanical properties of tensile test bar of sand mold casting Mg-9Gd-4Y-0.5Zr(wt.%) alloy(abbreviated GW94 as follow) as cast, solutionized and peak-aged states had been studied deeply, furthermore, analyzed it’s strengthening and toughing mechanism as well as fracture mechanism. Moreover, the microstructure and mechanical properties evolution during hot rolling process of ingot metal mold casting Mg-9Gd-4Y-0.5Zr(wt. %) alloy as solutionized and as-rolled had also been characterized and analyzed, the results showed as follow:(1) The microstructure of as-cast GW94 Mg alloy was mainly composed of equiaxed dendrite of α-Mg solid solution, coarse eutectic phases of Mg24(Gd,Y)5 at the grain boundaries and a few of square particles of Mg5(Gd,Y). The solution treatment was 525℃×6h after optimization. The microstructure of solutionized GW94 Mg alloy was mainly composed of supersaturated α-Mg solid solution, residual phases of Mg5(Gd,Y) as well as cuboid-shaped phases formed during solid treatment.(2) The elongation-to-failure of peak-aged alloy increased with increase in temperature, which showed an abnormal behavior that the ultimate tensile strength increased with the increase in temperature. This could be attributed to the thermal stability of β′ phases and the movement of more slip system at elevated temperature.(3) The fracture modes observed of as cast GW94 alloy was intergranular fracture-based cleavage fracture. However, the observed fracture modes of supersaturated GW94 alloy was transgranular cleavage fracture and transgranular quasi-cleavage fracture and cleavage fracture of peak-aged GW94 alloy, which was seem to be a mixture, but quasi-cleavage fracture at room temperature peak-aged GW94 alloy seemed to be dominant. The fracture modes of peak-aged GW94 alloy transformed to microvoid accumulation fracture in intergranular structure at elevated temperature and caused fracture.(4) The as-rolled GW94 alloys were mainly composed of large deformed grains, fine dynamically recrystallized(DRXed) grains and twins were also found. With the increase in rolling passes, the grains gradually homogenized. Furthermore, the volume fraction of deformed grains and fine DRXed grains gradually decreased but the volume fraction of twins were observed gradually increased. With the increase of rolling reduction in the last pass, the grains did not homogenize and the volume fraction of fine DRXed grains gradually increased but the volume fraction of twins observed gradually decreased, in addition, the grain boundary obviously bent. So static annealing between two passes led to homogenizing the grains and making the DRXed grains formed during rolling process grown.(5) With the increase in rolling passes, the segregation components on the grain boundary arrange in a straight line along rolled direction(RD) and the number of streamlines gradually increased. The phases of Mg5(Gd,Y) 、 Mg3(Gd,Y) formed in segregation components, however the big phases formed in solid solution dissolved gradually during rolling passes. The volume fraction of all the second phases gradually increased with the increase in rolling passes.(6) The observed texture in rolled sheets was annular multi-peak texture. The rolled process resulted in tending to the basal texture and strengthening it, however, static annealing between two passes led to weakening the texture.(7) With the increase in rolling passes, ultimate tensile strength and tensile yield strength of as-rolled alloy gradually increased, but elongation-to-failure gradually decreased, and fracture mode was cleavage fracture but noteworthy, the tongue patterns gradually increased.