Improvement of Mechanical Properties in Mg Alloys by Optimum Microstructure Control

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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.