Improvement of Mechanical Properties in Mg Alloys by Optimum Microstructure Control

研究了镁合金在室温和高温下的变形机制。在RT的变形不仅发生在基底位错滑移，但也通过非基底位错滑移。非基面滑移的激活是由于不相容应力的出现，以满足晶界处的应变连续性条件。由于这些位错是a型的，因此需要额外的机制来诱导c方向上的应变。对于直径小于10 μm的细晶合金，发现另外的机制是晶粒：边界滑动，其贡献高达总应变的8%。对于直径大于50 μm的粗晶合金，额外的机制是孪生。观察到的孪晶均为{ 1012 }型，而不考虑张力轴与c轴之间的几何关系。这是由于各向异性位错活动引起的应力集中导致孪晶的形成。在高温变形机制方面，对Mg-Y合金进行了蠕变变形和显微组织观察。速率控制机制为a型位错攀移。通过稀释添加Zn观察到抗蠕变性的显著改善。这是由于形成了与Zn和Y分离的稳定堆垛层错。Zn的添加似乎降低了堆垛层错能并稳定了扩展位错构型，这反过来又阻碍了位错攀移并提高了抗蠕变性。

Deformation mechanisms of Mg alloys at room temperature and elevated temperature were investigated. Deformation at RT was found to occur not only by basal dislocation slip but also by nonbasal dislocation slip. The activation of the nonbasal slip was attributed to the emergence of incompatibility stress to satisfy a strain continuity condition at grain boundaries. Since these dislocation are of the a type, additional mechanisms were necessary to induce strain in the c direction. For fine-grain alloys of less than 10 μm in diameter, the additional mechanism was found to be grain : boundary sliding that contributed to as much as 8% of total strain. For course-grain alloys of more than 50 μm in diameter, the additional mechanism was twinning. The observed twins were all of { 1012 } type regardless of the geometrical relationship between the tension axis and the c axis. This was attributed to the twin formation by stress concentration as a result of anisotropic dislocation activity. With regard to deformation mechanisms at elevated temperatures, creep deformation and microstructure ob servation were performed in Mg-Y alloys. A rate-controlling mechanism was dislocation climb of the a type. A significant improvement in creep resistance was observed by dilute addition of Zn. This was attributed to the formation of stable stacking faults segregated with Zn and Y. The Zn addition appeared to reduce the stacking fault energy and stabilize extended dislocation configuration, which in turn hampered dislocation climb and improved creep resistance.