Investigation On Friction And Wear Behavior And Mild To Severe Wear Transition Of Mg97Zn1Y2 Alloy At Room And Elevated Temperatures

Magnesium alloys are the lightest structural materials that have been widely used in the electronics,aerospace,and automotive industries due to their attractive properties such as low density,high specific strength,good casting and machining properties.However,poor resistance to wear has been a serious impediment against the wider application of commercial Magnesium alloys,preventing them from being used as good as Al alloys in automotive engine components.It is recognized that the microstructure and mechanical properties of magnesium alloys have strong dependence on environment temperature.Therefore,development of magnesium alloys with good mechanical properties at elevated temperatures has always been one of the most important areas in magnesium alloys research.The rare earth magnesium alloys have excellent room and elevated-temperature mechanical properties,and the rare earth elements have a high solid solubility in magnesium,and can obtain excellent solid solution strengthening and aging strengthening effects.At present,there are few reports on the friction and wear behavior of magnesium alloys,and the existed reports are mostly studied from the perspectives of traditional tribology,that is,the characterization has been made by wear parameters such as sliding speed and load.There is rare research on the essence of wear behavior,especially at elevated temperature.Liang Ce et al.studied the friction and wear behavior of AZ magnesium alloys at room temperature,and established the surface DRX critical temperature criterion for mild to severe wear transition of magnesium alloy by applying the softening kinetics theory of thermal activation,established and verified the applicability of the mathematical prediction model.However,it is whether suitable for elevated temperature is still unknown,which needs further experimental verification.Based on such background,we have studied the friction and wear behavior of Mg97Zn1Y2 alloy at room and elevated temperatures,and verified the mathematical prediction model.Dry sliding tests were performed on Mg97Zn1Y2 alloy using a pin-on-disc configuration at room temperature(20?)and elevated temperatures(50-200?).At room temperature,the sliding speed range was 0.8~4.0m/s,and the load range was10~380N.Friction and wear characteristics of Mg97Zn1Y2 alloy were investigated as a function of the load and sliding speed.Morphologies,compositions and hardness of worn surfaces were characterized by scanning electron microscope with EDS(SEM,energy dispersive X-ray spectrometer)and microhardness tester.Five wear mechanisms,namely abrasion,oxidation,delamination,thermal softening,and surface melting,have been observed,Abrasion oxidation and delamination operated in mild wear regime,while thermal softening and surface melting dominated in severe wear regime.The wear rate map and the wear transition map of Mg97Zn1Y2 alloy are obtained according to the load ranges of wear mechanism operating at different sliding speeds.At elevated temperatures,the sliding speeds were 0.8 m/s,3.0 m/s and4.0 m/s,respectively.Under each sliding speed,wear tests were conducted by applying load from a low load 10 N or 20 N until 260 N or occurrence of surface melting at test temperatures of 50?,100?,150?and 200?,respectively.The elevated-temperature wear transition maps of Mg97Zn1Y2 alloy tested at three sliding speeds were established on applied load versus test temperature.After the analyses of the plastic strain distributions beneath the worn surfaces and microstructural evolution in subsurface of Mg97Zn1Y2 alloy,we can conclude that the most important microstructure evolution is the transformation from a plastic deformed to a DRX microstructure before and after mild-severe wear transition.Combining the researches of plastic deformation,microstructural evolutions,mechanical properties and oxidation characteristics,correlation between friction-induced microstructural evolution,strain hardening in subsurface and wear properties of Mg97Zn1Y2 alloy can be established.In mild wear regime,the high hardness values of worn surfaces could be attributed to strain hardening of surface materials and/or formation of MML,and thus leading to a low slope of wear rate.In severe wear regime,deformation zone partly has been replaced by DRX zone.Meanwhile,the presence of Mg12ZnY phase plays a role of fiber reinforcement before and after mild-severe wear transition.Surface softening originating from DRX was dominant factor for the rapid growth of wear rate.Based on the analyses of mild-severe wear transition and microstructural evolution of Mg97Zn1Y2 alloy,it can be concluded that the mild-severe wear transition of Mg97Zn1Y2 alloy at room and elevated temperatures follows the contact surface DRX temperature criterion.