Atomic mechanisms of diffusional nucleation and growth and comparisons with their counterparts in shear transformations

Atomic structures and migration mechanisms of interphase boundaries during body- to face-centered cubic phase transformations

Atomic structures and migration mechanisms of interphase boundaries have been of scientific interest for many years owing to their significance in the field of phase transformations. Though the interphase boundary structures can be deduced from crystallographic investigations, the detailed atomic structures and migration mechanisms of interphase boundaries during phase transformations are still poorly understood. In this study, a systematic study on atomic structures and migration mechanisms of interphase boundaries in a body-centered cubic (b.c.c.) to face-centered cubic (f.c.c.) massive transformation was carried out using the phase-field crystal model. Simulation results show that the f.c.c./b.c.c. interphase boundaries can be classified into faceted interphase boundaries and side surfaces. The faceted interphase boundaries are semi-coherent with a group of dislocations, leading to a ledge migration mechanism, while the side surfaces are incoherent and thus migrate in a continuous way. After a careful analysis of the simulated migration process of interphase boundaries at atomic scales, a detailed description of the ledge mechanism based on the motion and nucleation of interphase boundary dislocations is presented. The ledge-forming process is accompanied by the nucleation of new heterogeneous dislocations and motions of original dislocations, and thus the barrier of ledge formation comes from the hindrance of these two dislocation behaviors. Once the ledge is formed, the original dislocations continue to advance until the ledge height reaches 1/|Δg|, where Δg represents the difference in reciprocal lattice vectors between two phases. The new heterogeneous dislocation moves along the radial direction of the interphase boundary, resulting in ledge extension. The interface dislocation behaviors greatly affect the migration of the interphase boundary, leading to different migration kinetics of faceted interphase boundaries under the Kurdjumov–Sachs and the Nishiyama–Wasserman orientation relationships. This study revealed the mechanisms and kinetics of complex structure transition during a b.c.c.–f.c.c. massive phase transformation and can shed some light on the process of solid phase transformations.

On the multiple orientation relationship of the Mg/γ-Mg17Al12precipitation system

The six orientation relationships (ORs) found in the Mg/γ-Mg17Al12precipitation system were summarized and systematically interpreted based on the atomic structure of the precipitate γ-Mg12Al17and the invariant deformation element (IDE) model for diffusional phase transformations. It was found that the pseudo-twinning relationship between the six ORs is a reflection of the pseudo-twinning relationship between the close- or near-close-packing planes ({\overline 8}\hskip.75{\overline 7}\hskip.75{\overline 7}), ({\overline 4}11), (033), (411) and (8{\overline 7}\hskip.75{\overline 7}) in the precipitate γ-Mg12Al17. As a result, the Pitsch–Schrader OR is the starting point for the other five ORs. Multiple morphologies, growth directions and habit planes could be rationally interpreted by the IDE model. This implies that a multiple orientation relationship between the variants of precipitates is favourable in order to minimize the gross energy of precipitation systems in which the matrix has a simple structure while the precipitate has a complicated structure, such as Mg/γ-Mg12Al17, Mg/δ-Zn2Zr3and Mg/η-MgZn2couples.

A simplified invariant line analysis for face-centred cubic/body-centred cubic precipitation systems

Several phenomenological crystallographic theories, including the invariant line model, O-line analysis and Δgparallelism rules, have been proposed and then successfully applied to the interpretation of crystallographic features for most face-centred cubic/body-centred cubic precipitation systems. However, the application of these methods requires the use of extra criteria and multiple rotations. A simplified invariant line analysis is proposed in this paper, to simplify the above theories from the well known confusions of additional criteria and multiple rotation around specific axes. One-step rotation dispenses with extra criteria or any input orientation relationship and so can deduce an invariant line when a Burgers vector is parallel to the habit plane. This simplified analysis makes the application of the theory more understandable, where it anticipates the invariant line, the habit plane, the orientation relationship between the matrix and the precipitate, and the distance between dislocations for which the Burgers vector is not inclined. The predictions are simplified, highly efficient and coincide well with experimental observations from lath-shaped precipitates in Cu–Cr and Ni–Cr alloys, as well as with the theoretical results obtained by O-line theory and Δgparallelism rules.