CaCO3在AZ31镁合金中的细化效果及机理

用CaCO3作细化剂研究了对AZ31镁合金凝固组织的影响.结果表明:在AZ31中添加质量分数为0.5%的CaCO3,在760℃保温10 min后细化效果最佳,α-Mg晶粒的尺寸由基体合金的570μm降至209μm,降幅约63.3%.通过能谱分析、结合能和自由能的计算证实,细化机理是CaCO3反应后生成Al4C3,其中部分Al4C3质点作为异质核心,使晶粒细化,其余的Al4C3质点钉扎晶界也阻碍了晶粒长大.Al元素随固/液界面前沿被快速推至晶界,生成沿晶界生长的β-Mg17Al12相,起到进一步固定晶界的作用.合金元素的分布均有改变.

Refining Effect/ Mechanism of CaCO_3 in AZ31 Magnesium Alloy

Taking CaCO3 as a refining agent, the influence of CaCO3 on the microstructure of AZ31 magnesium alloy was investigated. The results showed that the best refining effect is obtained if adding 0.5wt% CaCO3 to the alloy at 760°C held for 10 min, i.e., the average grain size of α-Mg decreases to 209 μm from about 570 μm of the alloy or 63.3% is reduced. Based on the analysis of EDS, the calculations of binding energy and Gibbs free energy proved that, the grain refining mechanism is resulting from a part of Al4C3 particles as heterogeneous cores generated after the reaction of CaCO3, whereas the rest of Al4C3 particles are pinned to the grain boundary to retard the grain growth, and Al element is then quickly pushed to the grain boundary along the solid-liquid interface to generate the β-Mg17Al12 phase growing along the grain boundary, thus immobilizing the grain boundaries with the distributions of alloying elements changed.