Simulation of precipitation and strengthening in MG-RE alloys

by Hong Liu

Institution: Monash University
Department: Department of Materials Engineering
Year: 2015
Keywords: Magnesium alloys; Heterogeneous nucleation; Elastic interaction; Variant selection; Phase field method; First-principles calculation; Strength Calculation
Record ID: 1036455
Full text PDF: http://arrow.monash.edu.au/hdl/1959.1/1134835


Magnesium - rare earth (Mg-RE) alloys have received considerable attention in the past decades for wider applications in the aerospace industry due to their relatively high strength and excellent creep resistance. Most rare-earth containing magnesium alloys, such as Mg-Y, Mg-Gd, and Mg-Y-Nd, are precipitation hardenable. A technical barrier to the wider applications of such alloys is the lack of a sufficiently large age hardening response. To further improve this response, an improved understanding is required of the nucleation and growth behaviours of the key strengthening phases, metastable β' and β1. These behaviours are examined in this research. Previous studies have found that the β' phase has a base centred orthorhombic structure (a = 0.321 nm, b = 22.240 nm and c = 0.521 nm) and a Mg₇RE composition in some Mg-RE alloys, such as Mg-Y, Mg-Gd, Mg-Dy and Mg-Gd-Y. However, the morphologies of β' particles in Mg-Y and in Mg-Gd alloys are different, as they have a faceted and nearly equiaxed shape in the Mg-Y alloys, but a truncated lenticular shape in the Mg-Gd alloys. The first stage of this research was focused on an investigation of the nucleation and growth behaviour of β' particles, and the factors that cause the difference in the equilibrium shapes of β'-Mg₇Y and β'-Mg₇Gd. It was found that during the nucleation and growth of β' particles, the habit plane of β' particles gradually changes from {11 ̅00}ₐ to {112 ̅0}ₐ. The equilibrium shape of β' precipitates in both Mg-Y and Mg-Gd alloys is determined by the competition between interfacial energy and elastic strain energy anisotropy. The research also indicates that the difference between the shapes of β'-Mg₇Y and β'-Mg₇Gd precipitates is due to the difference in lattice parameters between β'-Mg₇Y and β'-Mg₇Gd particles, i.e., compared with interfacial energy anisotropy, the anisotropy of elastic strain energy plays a dominant role in the shape difference. Two ways of increasing the aspect ratio are increasing the elastic strain energy anisotropy and decreasing the interfacial energy anisotropy. When the elastic strain energy anisotropy is increased, rather than when the interfacial energy anisotropy is decreased, the aspect ratio increases is larger. It was also found that one possible way to increase elastic strain energy anisotropy of β' precipitates is by selecting the appropriate alloying elements which have a larger atomic radius and occupy the Y atom lattice sites in the β'-Mg₇Y phase. The second stage of this research was to investigate the nucleation, growth and equilibrium morphology of β₁ particles. Previous research indicated that the β₁ phase has a cubic structure (a = 0.74 nm) and a Mg₃RE composition. In addition, the β₁ particles have a plate-like shape, and, in some Mg-RE alloys, such as WE54, individual β₁ plates always form with β' particles attached each end. However, the reason behind the latter phenomenon is not clear. The simulation results in this research suggest that the coherency elastic strain energy at the two edge facets of a β₁…