Microstructure, Tensile Properties And Dry Sliding Wear Behavior Of Mg-Sn-Yb Alloys

In this dissertation, the effect of different process and Yb content on the microstructure,tensile property and dry sliding wear behavior of Mg-2Sn (at.%) alloy is investigated.Compare with casting process, hot extrusion effectively refines the grain size andsignificantly crushes secondary phase particles. Tensile test and dry sliding wear testdemonstrated that extruded Mg-2Sn-0.5Yb (at.%) alloy exhibited the best comprehensiveperformance. The difference of tensile properties for extruded Mg-2Sn-xYb (x=0,0.1,0.5at.%) alloy is attributed to the different strengthening roles of grain refinement, solidsolution strengthening and precipitation strengthening of Mg2Sn and Mg2(Sn,Yb) particles.The good dry sliding wear behavior in extruded Mg-2Sn-0.5Yb (at.%) alloy at high load isdue to the superior thermal stability of the intermetallic and high elevated temperaturemechanical properties. Furthermore, the deformation behavior of extruded Mg-2Sn-0.5Yb(at.%) alloy in tensile test at different temperatures and strain rates is also discussed.The investigated results in this dissertation are summarized as following:1. The microstructures of as-cast Mg-2Sn and Mg-2Sn-0.1Yb (at.%) alloy are composed ofα-Mg, eutectic phase (α-Mg+Mg2Sn) and Mg2Sn phase. In addition to α-Mg and Mg2Snphase, an amount of fine Mg2(Sn,Yb) particles are observed at grain boundary in as-castMg-2Sn-0.5Yb (at.%) alloy. With increasing Yb concentration, the grain size issignificantly reduced. Compare with as-cast Mg-2Sn-xYb (x=0,0.1,0.5at.%), the grainsize of extruded Mg-2Sn-xYb (x=0,0.1,0.5at.%) alloy is significantly refined due todynamic recrystallization during extrusion and secondary phase particles are crushed.Tensile testing results indicated that extruded Mg-2Sn-0.5Yb (at.%) alloy exhibits thehighest tensile strengths and available elongation to failure at RT. Namely, the yieldstrength, ultimate tensile strength and elongation to failure are185MPa,259MPa and12%, respectively. However, at both100°C and200°C, the extruded Mg-2Sn-0.1Yballoy has the highest tensile strengths. These differences in tensile properties is attributed to the different strengthening roles of grain refinement, solid solution strengthening andprecipitation strengthening of Mg2Sn and Mg2(Sn,Yb) particles.2. Strain-hardening and warm deformation behaviors of extruded Mg-2Sn-0.5Yb alloy(at.%) alloy were investigated in uniaxial tensile test at temperatures of25°C~250°Cand strain rates of1×103s1~1×101s1. The data fit with the Kocks-Mecking typeplots were used to show different stages of strain hardening. Besides III-stage andIV-stage, the absence of the II-stage strain hardening at room temperature should berelated to the sufficient dynamic recrystallization during extrusion. The decrease of strainhardening ability of the alloy after yielding was attributed to the reduction of dislocationdensity with increasing testing temperature. Strain rate sensitivity (SRS) wassignificantly enhanced with increasing temperature, and the corresponding m-value wascalculated as0.07~0.12, which indicated that the deformation mechanism wasdominated by the climb-controlled dislocation creep at200°C. Furthermore, the grainboundary sliding (GBS) was activated at250°C, which contributed to the higher SRS.The activation energy was calculated as213.67kJmol-1. In addition, the alloy exhibited aquasi superplasticity at250°C with a strain rate of1×103s1, which was mainly relatedto the fine microstructure and the presence of the Mg2Sn and Mg2(Sn,Yb) particles.3. Pin-on-disc dry sliding tests of extruded Mg-2Sn, Mg-2Sn-0.1Yb and Mg-2Sn-0.5Yballoy against a steel conterface are carried out in load ranges of20N~240N,20N~320N,20N~380N, respectively. The results indicated that wear rate, coefficient offriction and wear resistance changed with increasing applied load due to different wearmechanisms. Six wear mechanisms, namely adhesion, abrasion, oxidation, delamination,thermal softening and melting, were observed for three extruded alloys. The wearbehaviors of three extruded alloys are controlled by change in microstructure andmechanical properties with surface temperature rise during dry sliding. The extrudedMg-2Sn-0.5Yb alloy exhibited good wear resistance compared with the other twoextruded alloys, which was mainly attributed to a large number of volume fraction ofMg2Sn particles, the formation of thermal stable Mg2(Sn,Yb) particles and good elevated temperature mechanical properties.