Investigation On Microstructure, Mechanical Property And Deformation Mechanism Of Several Mg-Zn-Y Alloys

Magnesium alloys are promising light weight environment friendly structural materials for using in aerospace, modern automobile and electric communication fields due to their high specific strength and modulus, good heat dissipation,vibration absorption and rich in reserves.It is prerequisite for promoting magnesium alloys’ industrialization development as a structural material to improve their service property including strength and formability at room temperature. And optimizing compositions, improving the processing and heat treatment techniques are important methods to improve their comprehensive performance.This article regards the ternary Mg-Zn-Y series alloy as research object. This alloy containing quasicrystalline I phase shows outstanding performance. To further improve the property of Mg-Zn-Y series alloy, the microstructure was optimized by regulating the content of alloying elements and the combination of hot formation based on the effect of alloying elements on the formation of quasicrystalline I phase.The effect of Zn, rare element Y content and the mass ratio of Zn to Y on the morphologies and the formation of the secondary phases in the as-cast alloys were investigated. The Magnesium alloy with excellent cast properties was hot-extruded and rolled and the influence of the processing on the microstructure, texture and mechanical properties was discussed. The deformation mechanism of the deformed alloy suffered static and dynamic load was analyzed. Main conclusions are as follows.Adding Y element to Mg-Zn alloy can promote the formation of icosahedra quasicrystalline phase(I_Mg3Zn6Y) and W_Mg3Zn3Y2. The microstructure mainly contains the two precipitates in the casting condition. The quantity and morphology of secondary phases varied with changing the content of Zn and Y element. The shape of the second phase is affected by the mass ratio of Zn/Y. The second phases are granulous when the value of Zn/Y is about 5, while they are exist as trident eutectic and strip phase when the value of Zn/Y is other than 5. The properties of the as-cast alloy increase with increasing the content of second phases, especially, the ultimate tensile strength(UTS) and the elongation can reach 191.8MPa and 9.8% respectively when the second phases are granulous. The improvement of the properties derives mainly from the high hardness, low surface energy and coherent with matrix of quasicrystalline Mg3Zn6 Y phase.Most α-Mg grains became extremely fine through recrystallization after hot extrusion at 350℃ with the extrusion ratio of 12.75. The average grain size of the recrystallized α-Mg is only about 2μm and the second phases distributed in the matrix as fine particles especially after hot extrusion. The extruded alloys show excellent property; especially the UTS, YTS(yield tensile strength) and the elongation of the extruded Mg-5Zn-1Y are 353.6MPa, 325.8MPa and 13.4%, respectively. The excellent properties of the alloy derived mainly from the extremely fine recrystallization grains and the granulous I phase, which can enhance the strength of the alloy. Moreover, strong(0002) fiber texture forms during extrusion inspire texture enhancement.The fully recrystallized equiaxed grains with the average size of 10μm are obtained in Mg-5Zn-1Y and Mg-6Zn-1Y alloy with the extrusion ratio of 12.75 after annealed at 425℃ for 15 min and there is almost no effect on grain size of the second phases after the anneal treatment. The strength decrease from 353.6MPa to 279.2MPa, while the elongation increase to 29.8% for the Mg-5Zn-1Y alloy after annealing treatment. EBSD results show that the excellent ductility mainly caused by the co-ordinate deformation of secondary twin {1012}-{1011} due to texture weakness when annealing. The optional aging process at 225℃/8h for Mg-Zn-Y alloy is determined through test the heat analysis during the heating process and the hardness at different aging conditions.The second phase became extremely fine and LPOS(long period stacking ordered) phase precipitate during aging, which can enhance the alloy slightly. But the strength of the alloy decreases after aging because the coarse grains sharply decrease the strength due to its sensitive to the size of the grains.Strain fatigue test show that the strength increases with cycle increasing and with the increase of the amplitude, cycle hardening is obvious when the strain amplitude is below 0.7% within the amplitude 0.3%-1.1% at the tensile stage. The cycle soften appears to some extent in the compression stage and the hysteresis loop is almost asymmetry. The hysteresis loop is obviously asymmetry and the area that the curve covered is increasing when the amplitude is above 0.7%. The main deformation mechanism at low amplitude is dislocation gliding and anelasticity. And the high density dislocation formed when dislocation gliding is hindered by the second phases which inspire the cycle hardening. Butthe main deformation mechanism at high amplitude is dislocation gliding and twinning-detwinning. And the detwinning stage leads to the cycle softening.The ultimate compressed strength enhanced obviously while the yield strength changed a little with increasing of the loading rate in the SHPB test for Mg-5Zn-1Y alloy. The property along extruded direction(ED) and along the transverse direction is different obviously at the same loading rate. It depends on the deformation mechanism caused by the extremely high loading rate and the(0002) texture. The {1012}-{1011} secondary twin will form because of the compress stress when loaded along the ED. The secondary phase combined well with the matrix is benefit to improve the properties at high strain rate.