不同冷速凝固的Mg-9Gd-0.8Al合金固溶行为及组织性能

为了探究Al元素在不同冷却速度下对Mg-9Gd合金组织细化效果及其对后续固溶处理的影响,利用铁模和铜模重力铸造制备了铸态Mg-9Gd-0.8Al合金,之后进行10~50 h的固溶处理。采用OM、SEM、TEM、EDS及XRD等方法研究了冷却速度对Mg-9Gd-0.8Al合金凝固和固溶行为及组织力学性能的影响。结果表明,铁模和铜模制备的铸态Mg-9Gd-0.8Al合金组织均由α-Mg基体、花瓣状(Mg, Al)₃Gd相、细条状Mg₅Gd相和方块状Al₂Gd相组成。铜模相比于铁模冷却速度加快,制备的合金基体晶粒和第二相显著细化,第二相体积分数总量增长幅度达56.1%。2种模具制备的合金固溶10 h后,Mg₅Gd相溶解、(Mg, Al)₃Gd相部分溶解、高熔点Al₂Gd相无变化,晶粒内析出层片状(Mg, Al)₂Gd新相,第二相总量趋于相等。固溶50 h后,(Mg, Al)₂Gd层片相回溶,残余(Mg, Al)₃Gd相发生熔断呈颗粒状,铜模制备的合金第二相颗粒比铁模的更细小。细晶强化和第二相强化使铜模制备的铸态合金性能较铁模制备的合金性能大幅提高,固溶10 h后合金屈服强度提升,伸长率基本不变。固溶处理50 h后,固溶强化、细晶强化和细小颗粒的第二相强化使铜模制备的固溶50 h态合金获得最优性能,屈服强度、抗拉强度和伸长率分别为141 MPa、234 MPa和22.4%。

Solid solution behavior,microstructure and properties of Mg-9Gd-0.8Al alloy solidified at different cooling rates

In order to explore the effect of Al element on the microstructure refinement of Mg-9Gd alloy under different cooling rates and its influence on the subsequent solid solution treatment, the as-cast Mg-9Gd-0.8Al was prepared by gravity casting with iron and copper mold, followed by 10-50 h solid solution treatment. The effect of cooling rate on the solidification, solid solution behavior, microstructure and mechanical properties of Mg-9Gd-0.8Al alloy was studied by OM, SEM, TEM, EDS and XRD. The results show that the microstructure of as-cast Mg-9Gd-0.8Al alloys prepared by iron mold and copper mold are all composed of α-Mg matrix, petal-like (Mg, Al)₃Gd phase, thin strip-like Mg₅Gd phase and square-like Al₂Gd phase. The cooling rate of copper mold is faster than that of iron mold, so the grains and the second phases of alloy are significantly refined, and the total volume fraction of the second phase increases by 56.1%. After the alloys prepared by the two molds are solid solutioned for 10 h, the Mg₅Gd phase is dissolved, the (Mg, Al)₃Gd phase is partially dissolved, high melting point Al₂Gd phase remaines unchanged and the intragranular lamellar (Mg, Al)₂Gd phase precipitates. The total volume fraction of the second phases tends to be the same. After 50 h of solid solution treatment, the lamellar (Mg, Al)₂Gd phase is dissolved and the residual (Mg, Al)₃Gd phase fuses to be particle-like, the particles of alloy prepared by copper mold are smaller than those of iron mold. Grain refinement strengthening and second phase strengthening greatly improve the properties of as-cast alloys prepared by copper mold compared with alloys prepared by iron mold. After 10 h of solid solution treatment, the yield strength of alloy is improved, while the elongation remain unchanged. After 50 h of solid solution treatment, solid solution strengthening, grain refinement strengthening and second phase strengthening of fine second phase particles make the alloy prepared by copper mold obtain best properties, yield strength, ultimate strength and elongation are 141 MPa, 234 MPa and 22.4%, respectively.