NEW PHASES IN MG-AL-CA SYSTEM

摘要

Mg-Al-Ca alloys exhibit appealing creep strength at temperatures over 100℃. It is believed that it is due to the substitution of the γ-Al_(12)Mg_(17) phase by laves phases of higher thermal stability. In the literature, the C15-Al_2Ca phase in the Al-Ca binary system and the C14-Mg_2Ca phase in the Ca-Mg binary system have been reported. Their relative stability in the ternary system may play an important role in the development of the Mg-Al-Ca alloys. Furthermore, recent experiments and first-principles calculations in the literature revealed the existence of a C36 laves phase in the Al_2Ca-Mg_2Ca pseudo-binary system in addition to the C14-Mg_2Ca and C15-Al_2Ca laves phases. In the present work, special quasirandom structures (SQS) for all three laves phases were constructed. The structures possess local pair and multisite correlation functions that mimic those of the corresponding random structures. First-principles calculations were carried out based on the SQS developed to predict the enthalpy of formation in the Al_2Ca-Mg_2Ca pseudo-binary system. It was observed that the enthalpy of formation of C36 is very close to that of C14 at the Mg_2Ca end and decreases with the addition of small amount of Al, while the enthalpy of formation of C14 increases with the addition of Al. It is thus energetically plausible that C36 is stable in the Al_2Ca-Mg_2Ca pseudo-binary system. Experimental investigations were done using diffusion couple of Mg-Ca alloy and pure Al. EPMA analysis of the diffusion zone found the existence of two ternary phases (Mg,Al)_2Ca and Al_2(Mg,Ca).