MgCO3在AZ31镁合金中的细化效果及机理

为改善AZ31镁合金铸态组织,用MgCO3对其进行细化,采用扫描电子显微镜、X射线衍射仪和金相显微镜研究了细化工艺参数对AZ31镁合金显微组织及其物相组成的影响.结果表明:在AZ31中添加质量分数为0.6%的MgCO3,于760℃保温10 min细化效果最佳,α-Mg晶粒的尺寸由基体合金的570μm降至100μm,降幅约82.5%.少量多次添加MgCO3的细化效果明显优于单次添加MgCO3的细化效果.研究认为,细化机理是MgCO3反应后生成的部分Al4C3质点作为异质核心细化晶粒,多余的Al4C3质点钉扎晶界阻碍晶粒长大.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 MgCO3 in AZ31 magnesium alloy

processing parameters on the microstructure and phases were investigated.The results show that the best refining effect is obtained if adding 0.6wt% MgCO3 to the alloy at 760 ℃ and holding for 10 min,and the average grain size of α-Mg grain decreased from 570 μm to 100μm.The refining effect of repeated adding small amount of MgCO3 is obviously better than adding MgCO3 in one time.The grain refining mechanism is attributed to Al4C3 particles as heterogeneous cores generated after the reaction of MgCO3,and the rest of Al4C3 particles are pinned to the grain boundary to retard the grain growth.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,which immobilizes the grain boundaries.The distribution of alloying elements is also changed.