Refinement and strengthening mechanism of Mg-Zn-Cu-Zr-Ca alloy solidified under extremely high pressure
(1. School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China;
2. School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, China;
3. State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China)
2. School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, China;
3. State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China)
Abstract: 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 Mg2Ca phase is formed under high pressure, but it is mainly segregated among dendrites under atmospheric pressure. The Mg2Ca particles are effective heterogeneous nuclei of α-Mg crystals, which significantly increases the number of crystal nuclei and refines the solidification structure of the alloy, with the grain size reduced to 22 μm 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.
Key words: high pressure solidification; Mg-Zn-Cu-Zr-Ca alloy; Mg2Ca particle; solution strengthening; grain refinement strengthening