Mg_(96.17)Zn_(3.15)Y_(0.5)Zr_(0.18)合金高压下固溶处理时效硬化研究

在4GPa高压下对Mg96.17Zn3.15Y0.5Zr0.18合金进行600-800℃固溶处理，之后在200℃进行等温时效处理．利用TEM，HRTEM，SEM，XRD等分析方法研究了高压固溶及随后时效处理后合金的显微组织，并测试了4GPa高压下固溶处理后合金的时效硬化曲线．结果表明，在4GPa高压下固溶能大幅提高Zn在Mg基体中的溶解度，Zn的溶解度由常压下400℃固溶后的2．11％（质量分数，下同）提高到4GPa高压下700—800℃固溶后的约6．60％，获得了过饱和α-Mg固溶体．在随后的200℃温时效过程中，高压固溶合金在较短的时间内即可获得较高的近峰值硬度，4GPa下800℃固溶的合金近峰值时效硬度高达105HV比400℃固溶处理合金近峰时效硬度（81HV）提高了约30％．HRTEM观察表明，4GPa高压下固溶合金时效沉淀析出相具有很高的析出密度，且析出相中含有粒状准晶I-Mg3Zn6Y相.

Room temperature compressive properties and strengthening mechanism of Mg96.17Zn3.15Y0.50Zr0.18 alloy solidified under high pressure

The microstructures of Mg96.17Zn3.15Y0.50Zr0.18 alloys solidified under 2-6 GPa high pressure were investigated by employing SEM(EDS) and TEM.The strengthening mechanism of experimental alloy solidified under high pressure is also discussed by analyzing the compressive properties and compression fracture morphology.The results show that the microstructure of experimental alloy becomes significantly fine-grained with increasing GPa level high pressure during solidification process,and the secondary dendrite arm spacing reduces from 40 μm at atmospheric pressure to 10 μm at 6 GPa pressure.The morphology of the second phases changes from the net structure by the lamellar-type eutectic structure at atmospheric pressure to discontinuous thin rods or particles at 6 GPa pressure.Besides,the solid solubility of Zn in the Mg matrix is improved with the increase of the solidification pressure.Compared with atmospheric-pressure solidification,high-pressure solidification can improve the strength of the experimental alloy.The compressive stre ngth is improved from 263 to 437 MPa at 6 GPa.The fracture mechanism of the experimental alloy changes from cleavage fracture at atmospheric pressure to quasi-cleavage fracture at high pressure.The main mechanism of the strength improvement of the experimental alloy includes the grain refinement strengthening caused by the refinement of the solidification microstructure,the second phase strengthening caused by the improvement of the morphology and distribution of the second phases,and solid solution strengthening caused by the increase of the solid solubility of Zn in the Mg matrix.

Dynamic recrystallization kinetic of fine grained Mg-Zn-Y-Zr alloy solidified under high pressure

Fine grained Mg_（96.17）Zn_（3.15）Y_（0.79）Zr_（0.18） alloy with an average grain size of 20 μm was prepared by high pressure solidification. The dynamic recrystallization（DRX） behavior of the fine grained Mg alloy solidified under the pressure of 4 GPa was studied via isothermal compression experiments. The tests were performed under the strain rate of 0.001–1.0 s^（–1） and at a deformation temperature of 523–623 K on a Gleeble-3500 D thermal-mechanical simulation machine. The DRX kinetic of the fine grained Mg alloy solidified under high pressure was established, and the microstructures of the alloy under different hot compression conditions were analyzed by electron back-scattering diffraction（EBSD）. According to the experimental results, the DRX kinetic model of the fine grain Mg alloy solidified under high pressure was X_（DRX）=1-exp[-0.75445（（ε-ε_c）/ε~＊）^（1.066208）.The Avrami exponents of n and k were 1.066208 and 0.75445 respectively, higher than those in the conventional casting alloy. The DRX volume fraction of the fine grain Mg alloy solidified under the pressure had a tendency to increase obviously with the strain rate decreasing and the deformation temperature increasing, which is different from the one in the conventional casting alloy. When compressed at 523 K, the DRX volume fraction of the fine grained Mg alloy solidified under high pressure was 85% under the strain rate of 1.0 s^（–1） and could be up to 95% under the strain rate of 0.001 s^（–1）. The DRX volume fraction of the conventional casting alloy was only 67% although under the condition of 623–0.001 s^（–1）. It was shown that the fine grained Mg alloy solidified under high pressure had a strong DRX capacity.