Creep behaviour of Mg-Al-Ca based alloys

Abstract

The role of intermetallic phases on the creep behaviour of two Mg-Al-Ca based alloys MRI153M and MRI230D was investigated by characterizing the microstructures of both the alloys in ascast as well as creep tested conditions. For comparison the AZ91 Mg alloy is also employed. The AZ91 alloy consists of Mg17Al12 phase, the MRI153M alloy consists of C36 ((Mg,Al)2Ca) and Mg17Al12 phases, and the MRI230D alloy consists of C36 ((Mg,Al)2Ca) phase. The creep tests were conducted at various temperatures using a stress level of 70MPa. The highest, medium and lowest creep resistance was exhibited by the MRI230D, MRI153M and AZ91 alloys, respectively. Microstructural characterization following creep tests revealed that the amount of Mg17Al12 phase was increased in case of AZ91 alloy, which deteriorates its creep resistance. In case of the MRI230D alloy the amount of C36 phase increased, which increased its creep resistance owing to the higher thermal stability of the C36 phase. In case of the MRI153M alloy the Mg17Al12 phase increased significantly. However, the volume fraction of C36 phase is less in it as compared to that of the MRI230D alloy and accordingly, the former alloy exhibited medium creep resistance.The two Mg alloys MRI153M and MRII230D contains Ca which suppresses the above mentioned â phase and gives rise to new intermetallic phase C36 ((Mg, Al)2Ca along the grain boundaries and other Mg dendritic regions. The intermetallic C36 having higher melting point is effective to pin dislocations at superior temperatures and increases the creep resistance of the alloy. The precipitation of the above mentioned C36 phase will result in increase of steady state creep resistance in MRI230D. Hence it is observed that MRI230D has superior creep and mechanical properties than the remaining at the elevated temperature.