Investigation On Wear Behavior Transition Of Mg-Gd-Y-Zr Alloy At Ambient And Elevated Temperatures

Magnesium alloys have excellent mechanical and process properties such as high specific strength and stiffness,good damping capacity and machinability.Especially in the industrial field of pursuing lightweight,the research and development of magnesium alloys have important engineering value.However,the relatively weak creep resistance at high temperature limits the wide application of magnesium alloys in automotive,aerospace and other industries.In recent years,the addition of rare earth elements has been proved to be an effective means to improve the heat resistance of magnesium alloys.Among them,the Mg-Gd-Y-Zr alloys with the advantages of outstanding performances have the potential to be applied for the tribological field at ambient temperature or even high temperature,such as being a substitute for aluminum alloy wear parts,or used for worm drive,light load gears and other components working under dry sliding conditions,and even as candidate materials for sliding bearings and pistons working under lubrication conditions.At present,more research has been carried out on the wear behavior of traditional commercial magnesium alloys such as AZ,AM,AS and ZE system.The mild to severe wear transition(MSWT)of magnesium alloy has become a widely accepted important understanding and attracted more attention.From the perspective of engineering application,it is of great significance to accurately predict the transition load of MSWT under different wear conditions.However,for Mg-REs alloys with excellent heat resistance and mechanical properties,there is still a lack of systematic research on their wear behavior and MSWT mechanism.The addition of rare earth elements also changes the microstructure and surface state of magnesium alloy during the sliding wear process.Therefore,it is necessary to comprehensively study the wear transition process in a large range of sliding parameters.In this thesis,the wear behavior of as cast Mg-10Gd-1.4Y-0.4Zr alloy at ambient temperature and elevated temperature was studied.The wear tests for ambient temperature,elevated temperature(50?200?)and different sliding speeds(0.2 ? 4 m/s)were carried out on a pin-on-disc wear tester.The surface morphology characteristics of wear specimens were observed under SEM,the distribution and content of main alloy elements on the surface were analyzed by EDS,and the microstructure evolution process of subsurface was observed under laser confocal microscope,The hardness level of worn surface and hardness distribution of subsurface were measured by microhardness tester,and the wear mechanism transition diagrams under different sliding conditions were established.Based on the dynamic process of dynamic recrystallization(DRX)transformation,the MSWTtransition load prediction model of the test alloy was given.The results showed that the mild wear mechanism of the test alloy at ambient temperature includes abrasive wear,oxidation wear and delamination wear,and the severe wear mechanism included severe oxidation,severe plastic deformation,adhesive wear and surface melting;The MSWT at low sliding speeds(0.2 and 0.5 m/s)and high sliding speeds(above 0.5 m/s)were controlled by surface oxidation and severe plastic deformation,respectively.At the temperatures of 50?200? and sliding speed of 0.8m/s,the mild wear mechanism mainly included abrasive wear,oxidation wear,delamination+surface oxidation and slight plastic deformation.The severe wear mechanism included severe plastic deformation,severe plastic deformation+oxide layer spalling,severe plastic deformation+surface oxidation,severe plastic deformation+adhesive wear and surface melting.Under the sliding conditions of20?200? and 3 m/s,the main mechanisms of mild wear were slight plastic deformation,scratch and surface oxidation.In severe wear,the main mechanisms were severe plastic deformation and surface melting.In the process of elevated temperature wear,the relationship between test temperature and transition load is approximately linear.In the sliding wear process of ambient temperature,the friction affected zone under mild wear was composed of deformed grains containing twins,and DRX structure appeared in severe wear.At 50?200? and 0.8 m/s,the friction affected zone was composed of mechanically mixed layer and plastic deformation zone in case of mild wear,and consisted of(solidification zone),DRX zone and plastic deformation zone in case of severe wear.The intermetallic phase precipitation and static recrystallization(SRX)microstructure transformation occurred in subsurface at 150??200? did not have a decisive impact on MSWT mechanism,but would change the wear rate and subsurface hardness distribution under certain conditions.At the temperatures of20?200? and sliding speed of 3 m/s,the subsurface microstructure also experienced plastic deformation and strain hardening during mild wear,while DRX microstructure transformation and softening also occurred during severe wear.Under different sliding conditions at ambient temperature and elevated temperature,the range of friction affected zone in subsurface was basically consistent with the hardness distribution.It could be considered that the DRX microstructure transformation caused by friction heat promoted the softening of the surface material,and then leads to the occurrence of MSWT.The prediction model of MSWTtransition load based on DRX dynamics is effective for the ambient temperature and elevated temperature wear process of Mg-10Gd-1.4Y-0.4Zr alloy,and the MSWT process still follows the Contact Surface Temperature Criterion.