Solidification microstructure evolution in LA42 Mg alloy under various cooling rates

摘要

The effect of cooling rate on microstructure evolution of Mg-4La-2.5Al-0.3Mn (LA42) Mg alloy was investigated in this study. The results revealed that high-pressure die casting (HPDC) LA42 alloy exhibited small grain size and undeveloped dendritic structure as compared to permanent gravity casting (PGC) LA42 alloy. The dendrites in the HPDC LA42 alloy didn't show obvious secondary branches (in parallel), due to the narrow constitutional undercooling zone in the frontier of liquid/solid interface and relatively smaller constitutional undercooling degree at the side of the primary dendritic trunk. The PGC LA42 alloy displayed an anomalous eutectic morphology, owing to the faceted growth of the eutectic compound Al5La2 (strong hexagonal crystallographic feature). In contrast, the HPDC LA42 alloy showed a regular eutectic feature that was interrelated to the generation of fibrous eutectic compound (Al, Mg)(3)La, whose lamellar distribution suggested a non-faceted growth. The converting of the eutectic nature was primarily attributed to the transition of the eutectic intermetallic compounds related to enhancement of undercooling. The effect of cooling rate on eutectic lamellar spacing (lambda) for the HPDC LA42 alloy might depend on two factors: (1) the increasing of cooling rate during the eutectic nucleation stage decreased the critical nucleation radius and (2) the intensification of heat extraction at the eutectic growth stage raised the eutectic growth velocity (V-e).