| Structural materials with the ability to work under normal conditions at high temperatures are requiered for aerospace,automotive and other sections,and magnesium alloys that have good performance at room temperature will also face situations involved with high temperatures.Many kinds of magnesium alloys present poor mechanical properties in high temperature environments such as AZ series and AM series.Therefore,it is an effective way to solve this problem by replacing the conventional commercial magnesium alloys with rare earth heat-resistant magnesium alloys.At the same time,many structural materials inevitably suffer from friction and wear,and their wear resistance under high temperature is also worth studying.Based on the previous studies on the wear behavior of heat-resistant rare earth magnesium alloy at room temperature,it is indispensable to study the high temperature wear property of this kind of material.The main content of this thesis is the wear test of Mg97Zn1Y2 alloy under high temperature by the pin-disc wear device.The sliding speed range is 0.8～4m/s,the temperature range is 50～200℃ and the load range is 10～260N.Then draw the Coefficients of friction and wear rate curve,observe the worn surface morphology by SEM,measure the content of the wear surface elements by EDS,draw wear rate map and wear regime transition map of Mg97Zn1Y2 alloy at different temperatures and sliding speeds.Finally,observe the changes of the hardness of the worn surface and subsurface,draw the hardness curve and observe the microstructure evolution of the subsurface.Wear behavior of Mg97Zn1Y2 alloy can be divided into two types:a mild and a severe wear behavior.At the sliding speed of 0.8m/s,the main wear mechanisms of mild wear occurred in the range of 50-150℃,including abrasion + oxidation and delamination + surface oxidation.The main wear mechanisms of severe wear are severe plastic deformation + spallation of oxide layer and surface melting.At 200℃,the main wear mechanism of mild wear is delamination,and the main wear mechanisms of severe wear are severe plastic deformation and surface melting.The mechanisms of wear at 3m/s and 4m/s is similar to that at 0.8m/s,except that the abrasive + oxidative wear mechanism didn’t occur in the mild wear mechanism,and the temperature affected by the surface oxidation is advanced to 100℃ and 50℃ respectively the following.While,when the temperature is higher than 150℃,the test presented a similar result at both sliding speeds that is the happening of the phenomenon of adhesion wear in the mild wear and severe wear stages.Although the sliding speeds were different during the test,some phenomenona of wear behavior and reasons are similar.For the worn specimen surface,the hardness experienced a process of first increasing and then decreasing at different temperatures.According to the test results,we observed that the critical transition loads from the hardening stage to the softening stage of the surface material corresponded to the transition loads from the mild regime to severe regime one by one.From the microstructure evolution and hardness change of the subsurface of the samples after wear test,the subsurface material suffered a process of strain hardening due to plastic deformation in the mild regime,and only the plastic deformation phenomenon was observed in the zone affected by the friction;in the severe wear regime,the dynamic recrystallization occurd in this zone.Therefore,the FAZ contains two sub-zones:DRX and plastic deformation.The advent of DRX resulted in a reduction of subsurface hardness.The emergence of DRX is due to large friction heating accumulation,which makes the surface temperature reach dynamic recrystallization temperature,i.e.Ts≥TDRX. |
PDF Full Text Request