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The Mg−Sn alloys, with basal or prismatic Mg2Sn laths, were employed to reveal the effect of precipitate orientation on twinning behavior quantitatively. The Mg−5wt.%Sn alloys with basal or prismatic Mg2Sn were compressed to study the twinning behaviors. Subsequently, an Orowan strengthening model was developed to quantitatively investigate the critical resolved shear stress (CRSS) increment of precipitates on twinning. The results revealed that the prismatic precipitates hindered the transfer and growth of tensile twins more effectively compared with the basal precipitates. The decreased proportion of tensile twins containing prismatic Mg2Sn might be attributed to a larger CRSS increment for tensile twins compared with that for basal precipitates. The obvious decreased twinning transfer in the alloy with prismatic Mg2Sn could be due to its higher geometrically necessary dislocation and enhanced CRSS of tensile twins. Notably, the prismatic precipitates have a better hindering effect on tensile twins during compression.

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 Al2Ca, and the formation of the Mg17Al12phase is suppressed in the presence of Ca. The decomposition of Al2Ca during the solution treatment is more difficult than that of Mg17Al12, and undissolved Ca was still present in the form of stable Al2Ca even after 120 h. With increasing treatment time, the coarse Al2Ca 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 Al2Ca particles were spheroidized and fined due to the diffusion of Ca and Al along the Al2Ca/Mg interface. As a result, the continuous network-like Al2Ca phase turned into a dispersion of spherical particles. The fine Al2Ca particles were homogeneously distributed in the matrix, which can be very beneficial to creep resistance and mechanical properties of Ca-containing alloys.