Development of microstructure in solution-heat-treated Mg-5Al-xCa alloys

Abstract

The development of the microstructure in Mg-Al-Ca alloys was investigated both in the as-cast condition and after solution heat treatment at 415 °C for 0.25 -120 h, using secondary electron imaging, electron probe microanalysis and X-ray diffraction analysis in combination with light microscopy and image analysis. The main intermetallic compound in the Ca-containing alloys is Al 2 Ca, and the formation of the Mg 17 Al 12 phase is suppressed in the presence of Ca. The decomposition of Al 2 Ca during the solution treatment is more difficult than that of Mg 17 Al 12 , and undissolved Ca was still present in the form of stable Al 2 Ca even after 120 h. With increasing treatment time, the coarse Al 2 Ca phase, which was continuously distributed like a network around the α-Mg grains in the as-cast state, underwent thinning and necking in the initial stage. Then it became disconnected and fragmentized, and finally the fragmentized Al 2 Ca particles were spheroidized and fined due to the diffusion of Ca and Al along the Al 2 Ca/Mg interface. As a result, the continuous network-like Al 2 Ca phase turned into a dispersion of spherical particles. The fine Al 2 Ca particles were homogeneously distributed in the matrix, which can be very beneficial to creep resistance and mechanical properties of Ca-containing alloys.