Refinement and strengthening mechanism of Mg−Zn−Cu−Zr−Ca alloy solidified under extremely high pressure

超高压下凝固Mg−Zn−Cu−Zr−Ca合金的组织细化与强化机制

Mg−Zn−Cu−Zr−Ca samples were solidified under high pressures of 2-6 GPa.Scanning electron microscopy and electron backscatter diffraction were used to study the distribution of Ca in the microstructure and its effect on the solidification structure.The mechanical properties of the samples were investigated through compression tests.The results show that Ca is mostly dissolved in the matrix and the Mg_(2)Ca phase is formed under high pressure,but it is mainly segregated among dendrites under atmospheric pressure.The Mg_(2)Ca particles are effective heterogeneous nuclei ofα-Mg crystals,which significantly increases the number of crystal nuclei and refines the solidification structure of the alloy,with the grain size reduced to 22μm at 6 GPa.As no Ca segregating among the dendrites exists,more Zn is dissolved in the matrix.Consequently,the intergranular second phase changes from MgZn with a higher Zn/Mg ratio to Mg7Zn3 with a lower Zn/Mg ratio.The volume fraction of the intergranular second phase also increases to 22%.Owing to the combined strengthening of grain refinement,solid solution,and dispersion,the compression strength of the Mg-Zn-Cu-Zr-Ca alloy solidified under 6 GPa is up to 520 MPa.

在2~6 GPa下对Mg−Zn−Cu−Zr−Ca合金进行凝固。利用SEM和EBSD等分析手段研究高压凝固过程中Ca的分布及对合金凝固组织的影响,并采用压缩试验研究合金的力学性能。结果表明,与常规铸造合金中Ca多偏聚在枝晶间不同,高压凝固合金中Ca多固溶于基体中以及形成Mg_(2)Ca质点相。Mg_(2)Ca质点相为α-Mg晶体强有效的异质晶核,极大增加高压凝固过程总晶核数目并细化凝固组织,6 GPa下合金晶粒尺寸细化至22μm。由于枝晶间无Ca偏聚,更多的Zn固溶到基体中,导致晶间第二相由Zn/Mg比较高的MgZn相(常压)逐渐转变为Zn/Mg比较低的Mg7Zn3相,并且晶间第二相体积分数增加至22%。细晶强化、固溶强化以及弥散强化使6 GPa凝固的Mg−Zn−Cu−Zr−Ca合金强度高达520 MPa。