利用扫描电子显微镜（SEM）和X射线衍射（XRD）等分析手段研究了Bi、Cr-Bi复合添加对Mg-8Al-4Zn合金铸态组织及时效沉淀相演变的影响。结果表明：Cr-Bi复合添加能有效改善合金的铸态组织,使连续网状相断开呈短杆或颗粒状。Bi、Cr-Bi复合添加的合金经350℃×12 h＋160℃时效,其沉淀硬化曲线呈现典型的温时效。Cr-Bi复合添加的合金时效初期硬化速率较高,20 h就达到峰值硬度的97%,时效48 h达到峰值硬度92.44 HV,过时效阶段硬度下降速度比较缓慢。Mg-8Zn-4Al-0.5Bi合金经350℃×12 h＋160℃×120 h时效后主要有MgZn2、Mg3Bi2和单Bi相,呈短杆或细小颗粒状弥散分布在基体上;Cr-Bi复合添加的合金经350℃×12 h＋160℃×144 h时效后,除短杆或细小颗粒状的MgZn2相外还有Cr、Al12Cr3等沉淀相,没有发现粗大的MgZn相,且沉淀相与单独添加Bi经120 h时效的合金相比更加细小、致密、弥散均匀分布。
Effect of Cr-Bi on as-cast microstructure and evolution of aging precipitations of Mg-8Zn-4Al alloys were investigated by X-ray diffraction（XRD）and scanning electron microscopy（SEM）.The results show that the as-cast microstructure of the alloy is improved obviously and the netwark phases become dispersive by adding Cr-Bi.After solution treatment at 350 ℃ for 12 h and then 160 ℃ aging treatment,the hardening curves of the experimental alloys show the typical characteristic of aging.The kinetics of ageing of the alloy is considerably accelerated and reaches 97% of its maximal hardness value only 20 h aging by adding Cr-Bi.The maximal hardness（92.44 HV） of the alloy containing Cr-Bi is obtained after ageing for 48 h and the rate of hardness decline after over-aging is relatively slow.The main precipitation phases of the alloy adding Bi aged at 160 ℃ for 120 h are MgZn2,Mg3Bi2 and Bi,which are rod-like and blocky and well distributed in the matrix.The main precipitation phases of the alloy adding Cr-Bi aged at 160 ℃ for 144 h are MgZn2,Al12Cr3 and Cr,the microstructure is more uniform and compact than that of the alloy adding Bi.
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