热处理工艺对Mg-6Zn-3Al镁合金显微组织和力学性能的影响

摘要

通过金属型铸造制备新型Mg-6Zn-3Al(ZA63,质量分数,%)镁合金,并利用光镜(OM)、扫描电镜(SEM)、透射电镜(TEM)、X射线衍射仪(XRD)和力学性能测试研究ZA63合金在铸态、固溶和时效处理后显微组织和力学性能的变化规律.结果表明:合金铸态组织主要由基体 α-Mg 、晶界处呈连续或半连续分布的(α-Mg+Mg2Zn3+Mg7Zn3+Mg32(Al,Zn)49)共晶和晶内孤立的颗粒相组成.合金在350 ℃固溶12~36 h时,随着时间延长,固溶效果逐渐增强,且在28 h时获得了较好的组织和241 MPa的抗拉强度及11.12%的伸长率.随后在180 ℃时效 6~72 h后,合金的抗拉强度进一步提高,力学性能随保温时间延长呈先增加后减小的趋势,其中时效24 h时同时出现抗拉强度、伸长率和硬度的峰值298 MPa、9.78%和96.3 HV,比铸态的214 MPa、8.54%、62.2 HV分别提高39.25%、14.52%和54.82%.180 ℃时效24 h后,析出相的主要形态有板条状和短棒状.%A new type Mg-6Zn-3Al (noted ZA63, mass fraction, %) magnesium alloy was prepared by permanent mould casting, and the microstructure and mechanical properties of the alloy under the conditions of as-cast, solution and aging treatment were systematically investigated by OM, SEM, TEM, XRD and mechanical tests, respectively. The results indicate that the as-cast alloy is mainly comprised of matrix α-Mg, eutectic phases (α-Mg+Mg2Zn3+Mg7Zn3+Mg32(Al,Zn)49) with continuous or semi-continuous precipitated along grain boundaries and a few divorced granules within grains. When the alloy is treated at 350 ℃ for 12-36 h, the solution effects are gradually enhanced with increasing solution time. At 350 ℃ for 28 h, the alloy reveals relatively homogeneous microstructure, and the ultimate tensile strength (UTS) and elongation (EL) of 241 MPa and 11.12% are achieved, respectively. Subsequently, when the alloy is aged at 180 ℃ for 6-72 h, the mechanical test demonstrates that, the UTS further increases, the mechanical properties increase firstly and then decrease, the peak values of UTS, EL and micro-hardness of the alloy aged at 180 ℃for 24 h are simultaneously obtained, which are 298 MPa, 9.78% and 96.3 HV and increase by 39.25%, 14.52% and 54.82% compared with those of the as-cast alloy (214 MPa, 8.54% and 62.2 HV), respectively. There are mainly two types of precipitates of the alloy after aging at 180 ℃ for 24 h, namely, the first precipitate is lath-like, the secondary one is a short-rod.