Crystallography Analysis of the β-Mg17Al12 Precipitates by the Secondary Constrained Coincident Site Lattice Model

For many engineering alloys, the multi-phase microstructures are usually obtained through different solid-state phase transformations to get better combined mechanical properties. The crystallographic features of the product phase, such as the morphology and its orientation with respect to the mother phase, are believed to play an important role on the properties of materials. For instance, precipitates with different morphologies and habit planes have been calculated to exhibit significantly different strengthening effects in precipitation-hardening Al alloys or Mg alloys (Nie & Muddle, 1998; Nie, 2003). Therefore, quantitatively description and interpretation of the precipitation crystallography are basic for control and design the precipitate microstructure to obtain certain desired properties. In the last several decades, a number of models have been developed to interpret or account for the experimentally observed crystallographic features of diffusional phase transformations. Some popular models such as the invariantline model (Dahmen, 1981; Dahmen et al., 1984), the O-lattice model (Bollmann, 1982; Zhang & Purdy, 1993), the constrained coincident site lattice (CCSL) model (Bonnet & Durand, 1975; Ye & Zhang, 2002), the g parallelism rules (Zhang & Weatherly, 2005) and the edge-to-edge matching model (Kelly & Zhang, 1999; Zhang & Kelly, 2005), have been successful in explaining some major crystallographic features of the transformation in specifi ed systems. Compared to the above-mentioned models, the recently proposed secondary CCSL (II-CCSL) model focus on the crystallographic features in the large-misfit phase transformation systems, where the lattice parameters between the mother phase and the product phases differs largely (Shi & Zhang, 2011). Guided by the distribution of the good matching sites (GMS) clusters, the constructed II-CCSL model emphasizes the preferred state in the periodically distributed good matching zones, which acts as a reference to evaluate the secondary misfi t. It can also provide the detailed interfacial defects structures (e.g., ledge)