Microstructure, Mechanical Properties And Creep Behavior Of Mg-Gd-Y-Zr Alloys

In this paper, the Mg-9Gd-lY-0.5Zr (wt pct)(GW91) alloy was produced into piston trial by squeeze casting. Specimens were sampled from piston top and piston skirt respectively. The microstructure, mechanical properties, tensile-creep and compression-creep behavior of the specimens in the peak-aged condition (T6) were investigated. Meanwhile, the Mg-10Gd-3Y-0.5Zr (wt pct)(GW103) ingot was prepared. The microstructure, compression-creep and indentation-creep behavior of GW103alloy in the peak-aged condition (T6) were investigated.The microstructure of GW91piston in as-cast condition (F) consists of α-Mg phase and the network-shaped Mg24(GdY)5eutectic phase. After solution heat treatment, almost all the Mg24(GdY)5eutectic phase is dissolved in the α-Mg matrix, meanwhile a cuboid-shaped Mg5(GdY) appears both at the grain boundaries and inside the grains. After peak-aged heat treatment, metastable β’(Mg3.5(GdY)) or β1(Mg3.5(GdY)) phases are evenly deposited inside the grains. The average grain sizes of piston top and piston skirt are140um and80um respectively.The research on the mechanical behavior of piston indicates that with increasing temperature the ultimate tensile strength and yield strength of piston decrease and the elongation increases. When the temperature is under300℃, the strength decreases slowly. At room temperature the ultimate strength of piston top and skirt are238MPa and255MPa with a elongation of16%and14%, respectively. At300℃, the ultimate strength of piston top and skirt are also up to210MPa and223MPa with a elongation of22%and27%. The piston has the asymmetry character of tensile and compression. For the same part of piston, the compression yield strength is higher than thetensile yield strength. After tensile test the grains have great deformationand few twin appears, meanwhile, after compression test the grains deformslightly and a lot of twins appear. It is easier for hcp. structure magnesiuminducing twins under compression stress than tensile stress and the softcoefficient for compression is higher than the one for tensile, which may leadsthe asymmetry character of tensile and compression.The creep behavior of piston reveals that temperature and stress affectthe tensile and compression creep behavior of GW91piston. The steadystage creep rate increases with increasing temperature and stress.Temperature has great effect on the crept microstructure. With increasingtemperature, the grains grow bigger and phase appears inside grains.Though stress have little influence on crept microstructure, stress plays animportant role on creep deformation and the origin of cracks.For the piston tensile creep tested at200℃/50-120MPa range the creepmechanism is grain boundary slide. At250℃/50-120MPa and300℃/50-80MPa ranges the creep mechanism is corresponding to dislocationclimb. At300℃/120MPa range the creep mechanism is related to crossslide. The steady-state creep rate and fracture time is better followed by themodified Monkman-Grant relationship. Brittle rupture with few plastictearing ridges is the dominant character of the creep fracture. For the pistoncompression creep tested at200℃/50-120MPa and250℃/50-80MPa thecreep mechanism is grain boundary slide. At250℃/120MPa and300℃/50-120MPa the creep mechanism is dislocation climb. Creep behaviordisplays the asymmetry character of tensile and compression, which may berelated to the difference influence of tensile and compression stress ondeposition of phase.The research on the creep behavior of GW103alloy indates thattemperature and stress affect the compression and indentation creep behaviorof GW103alloy. The steady-state creep rate increases with increasingtemperature and stress. Temperature has great effect on the microstructure after crept. With increasing temperature, the grains grow bigger and thephase deposites inside the grains. Though the stress give litte effect on thecrept microstructure, creep deformation and origin of cracks can be affectedby stress.When GW103alloy compression creep tested at300℃/50-120MParange the creep mechanism is dislocation climb. At250-300℃/120-150MParange the creep mechanism is related to cross slip. When GW103alloyindentation creep tested at250℃/120-150MPa the creep mechanism iscorresponding to dislocation climb. At300℃/80-227.5MPa range the creepmechanism is cross slip. At300℃/227.5-505MPa range cross slipcombined with twins may be the rate-control mechanism.Indentation creep can be converted into compression creep by relatedconversion factors, that indentation creep test can be done instead ofcompression creep test to identify alloys’ creep factors.