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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。
The effect of Bi and Cr-Bi composite addition on the as-cast microstructure and aging precipitates of Mg-8Al-4Zn alloy was studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD) The results show that the Cr-Bi composite addition can effectively improve the as-cast microstructure of the alloy, and the continuous network phase is broken into short rod or granular. Bi, Cr-Bi composite added by the alloy at 350 ℃ × 12 h +160 ℃ aging, the precipitation hardening curve showed a typical temperature aging. Cr-Bi composite added early high-rate hardening of the alloy at a high rate of 20 h to reach 97% of the peak hardness, aging 48 h peak hardness of 92.44 HV, over-aging stage hardness slow down more slowly. After aging at 350 ℃ for 12 h and 160 ℃ for 120 h, Mg-8Zn-4Al-0.5Bi alloy mainly has MgZn2, Mg3Bi2 and single Bi phases dispersed in short matrix or fine particles dispersedly on the matrix. Cr- After aging at 350 ℃ for 12 h and 160 ℃ for 144 h, precipitated phases such as Cr, Al12Cr3 and the like were not found except for the short rod or the fine MgZn2 phase, and no coarse MgZn phase was found. Adding Bi by 120 h aging alloy is more compact, dense, dispersed evenly distributed.