Atomic study on phase transformation of the strengthening phase in a die-casting Mg–Al–La alloy via an intermediate phase

Abstract

Phase transformation in structural alloys is of practical significance for their security services. Understanding phase transformation of strengthening intermetallic phases and its influence on mechanical properties is therefore crucial for the alloys’ safe applications and further development. In this work, investigations using an aberration-corrected scanning transmission electron microscopy combined with density functional theory calculations reveal that the strengthening intermetallic phase, η-Al3La, of a die-casting Mg − 3.5Al − 4.2La alloy is metastable, and it will transform to α-Al11La3 via an intermediate phase, ε-Al2.12La0.88, during annealing at 250 °C. For each transformation step, two orientation relationships are obeyed due to different nucleation sites or temperatures, or because of similar interfacial energy. Furthermore, a new planar fault, namely monolayer twin (MT), was observed, and it not only offer the nucleation site for ε-Al2.12La0.88 and the fast diffusion path for Al atoms, but also provide driving force for overcoming the energy barrier of η → ε → α transformation. Finally, both intermediate and product phases will obstruct the η-Al3La phase delivering 〈c + a〉 dislocations, and the level of solute Al atoms in Mg matrix will be clearly lowered during phase transformation, which result in deterioration in plasticity and creep resistance, respectively.