Deformation And Wear Behaviors Of The Mg-Y-Zn Alloys

Early in 1970's, the Mg-Zn-Y-Zr system of casting alloys had been studied in Chinaand it was found that a close relation between the microstructure and the property of thealloy and the ratio of Y/Zn existed in the alloy system. Padezhnova E M et al. of the USSRfound three ternary equilibrium phases in the 1980's, i.e. W-Mg3Zn3Y2, Z-Mg3Zn6Y and X-Mg12ZnY phases. In recent years, Z.P. Luo et al. identified the Z- Mg3Zn6Y as a stableicosahedral quasicrystalline phase, and determined the X-Mg12ZnY phase as an 18R LPSstructure by an electron diffraction technique.Inoue A. et al. fabricated an Mg97Zn1Y2 (at. %, i.e. WZ73) alloy with excellent tensileyield strength above 600 MPa by rapid solidification (RS) techniques of powder metallurgyin 2001. They also report the Mg-Y-Zn alloy, whose composition proportion is in the rangeof Mg-1³at%Zn-2⁶at%Y, has the excellent property. From that time, people pay attentionto the long period stacking order phase (LPS) during the research of Mg-Zn(Cu, Ni)-Re(-Zr)due to the better property of resistance to high temperature. The experiment includes thecontent below: by the analysis the microstructure and the property of as cast/ rolled/extrudedalloys, the mechanisms of deformation, fracture and wear in Mg-Y-Zn alloys at differenttemperature were determinated. The microstructure and the property were characterized bythe serial experiments. In order to gain the results of microstructure,phase and the texturemorphology of deformation alloys, the X-ray diffraction (XRD), general optical microscopy(OM), and scanning electron microscopy (SEM) were used; the hardness and tensileproperty were tested, and after that the SEM and transmission electron microscopy (TEM)were used to analyze the mechanism of deformation. The friction and wear properties of theexperimental alloys were conducted at MG-2000 type machine, meanwhile the wearmechanisms were discussed for explain the wear phenomenon.The paper describes the mechanism of deformation and fracture of as cast, rolled andextruded alloys, and characterized the friction and wear property. The datum and theconclusions were expressed as follow:(1) The cellular discontinuous precipitation can be completely suppressed in the AZ91Dalloy with addition of 2% Sn, and after aging treatment, the AZ91D-2%Sn alloy has the finer α-Mg grains and Mg17Al12 particles nearby the grain boundary compared to the AZ91Daging alloy. The AZ91D ageing alloy has higher ultimate fracture strength than the 2% Snalloy due to large numbers of grains including the cellular discontinuous precipitationforming nearby the grain boundary in the AZ91D, in which theβphase can effectively formthe obstacle for the dislocation movement, meanwhile the soft behavior induced by the highfraction of twins, correspondingly the better compressive failure strain, is the main reasonthat the strength of 2%Sn alloy is lower than the AZ91D.(2) Pin-on-disc dry sliding tests of Mg97Zn1Y2 and AZ91 against a steel conterface werecarried out in load ranges of 20–280 and 20–380 N, respectively. Five different wearmechanisms were found to operate under given conditions. They are abrasion, oxidation,delamination and thermal softening and melting for AZ91 alloy. Under the given conditions,for the Mg97Zn1Y2 alloy, the dominant wear mechanisms in the load range of 20–200N areabrasion and delamination. In the load range of 240–280 N, thermal softening is animportant wear mechanism. Surface melting is the wear mechanism as the load is over 280 N.The good tribological property in Mg97Zn1Y2 alloy at high load was due to the superiorthermal stability of the intermetallic and high elevated temperature mechanical properties.(3) For the rolled Mg₉₆Zn₁Y₃ alloy, the high density of dislocation, twin boundary andthe sub-grain boundary cause the high strength compared to the as cast alloy. For the R2alloy, the twin can be observed in the interior of grain intersecting with the LPS phase whendeformation is conducted by seventh pass at relative low working temperature. Themorphology of the dislocation and the influence of twin boundary in R2 alloy can be themain reason for the higher strength of R2 alloy. The fracture along the phase boundary andthe cleavage fracture can be considered to be the main fracture mechanism for the as-castalloy, while the fracture caused by the stress congregation at the interface between the matrixand Mg12YZn phase in grains is the main fracture mechanism for the rolled alloy at roomtemperature. The result can testified that the cleavage fracture can be suppressed by the highdislocation density, the size and the distribution of the second phase in magnesium alloy.(4) The hot-extruded Mg₉₆Zn₁Y₃ alloys with the extrusion ratio of 12:1 exhibits abimodal distribution of fine recrystallized and large elongated grains. The mechanicalanisotropy of extruded Mg₉₆Zn₁Y₃ plate shows the tensile samples machined along theextruded direction have a better yield strength and a better elongation-to-failure than thetensile samples machined along the transverse direction during the deformation. The higherductility for extruded direction alloy is attributed to the fragmentation of LPS phase into the particle in the interior and the boundary of the elongated original grains, and the lowerstrength in traversed direction alloy is induced by the stress concentration at the phaseinterface between the bulk Mg12YZn compound andα-Mg matrix. However, the micron andsub-micron particles distributed on the grain boundary as well as in the interior of originalgrains in the extruded alloy controls dominantly the fracture mode of tensile samplesmachined along the extruded direction.(5) The Mg96.6Zn0.4Y3 alloy has a worse elongation of less than 8% at room temperatureand 11% at 250℃, though it contains a fine grain size of less than 1μm. The worse elongationwas induced by the strain concentration at the phase interface between the Mg24Y5 and thematrix. The deformation mechanism of the Mg96.6Zn0.4Y3 alloy between 300℃and 350℃was the dislocation creep controlled by the grain boundary diffusion.(6) After laser surface melting of the Mg₉₆Zn₁Y₃ alloy, two distinct zones were formedsequentially near the surface, improving considerably the alloy's microstructure andhardness. The microstructure of the laser surface melted zone consists of fine dendrites andcoarse dendrites growing epitaxially from the liquid-solid interface. The microhardness ofthe fine dendrites in laser surface melted zone is improved to 77-83 HV as compared to 69-70 HV of the substrate. The wear in sliding can be decreased by laser surface melting, thefine dendritic microstructure exhibits good wear resistance as compared with the as-castmaterial and can increases the transition load from 160 N to 200 N. The wear mechanismvaries from abrasion and delamination at low and mediate load to thermal softening andmelting at high load.