Recently, Gaowu Qin's team from NEU's School of Materials Science and Engineering has made significant progress in the field of diffusive solid-state phase transformation, discovering a new mechanism of nucleation and growth of topologically close-packed (TCP) precipitates induced by non-interface dislocations, which is completely different from traditional cognition. The research paper titled "Structural pathway for nucleation and growth of topologically close-packed phase from parent hexagonal crystal" was published in the journal Acta Materialia in the field of metallurgy. NEU is the sole completing unit, with Junyuan Bai, a doctoral student from the School of Materials Science and Engineering, as the lead author, and Associate Professor Xueyong Pang and Professor Gaowu Qin as corresponding authors.
Solid-state phase transformation has always been an effective means to control structural and functional materials. For a given phase transformation, it is crucial to understand the formation pathway of the parent phase transforming into the product phase for the purpose of understanding and controlling the phase transformation. However, due to the transient nature of phase transformations, only a few non-diffusion (displacement) type phase transformation pathways between simple crystal structures have been extensively studied, such as the famous Bain pathway (bcc↔fcc) and Burgers pathway (bcc↔fcc). Solid-state diffusion-type nucleation and growth phase transformations include simple→simple (e.g. fcc→L12, hcp→D019, fcc→hcp, etc.) and simple→complex structure transformations. The former is generally achieved through atomic short-range shuffling or step mechanism of partial dislocation slip to complete the structural transformation, forming a widely accepted consensus. However, the simple→complex phase transformation in alloys has always lacked a profound understanding. This is because experimental techniques often cannot provide sufficient temporal-spatial resolution to capture atomic trajectories, while theoretical simulations are often limited by the insufficient accuracy of interatomic potential functions or the inability to capture small probability events such as solid nucleation on a traditional nanosecond time scale. Therefore, understanding the nucleation and growth mechanisms in solid diffusion-type phase transformation has always been a hot and challenging research topic in the field of solid-state phase transformation.
To elucidate the nucleation-growth pathways involved in the simple→complex diffusion-type phase transformation, this work, using magnesium alloy as a model material, systematically investigates the in-situ nucleation and growth pathways of a series of TCP phases (including Laves and Laves-like phases) within hcp structured Mg matrix through first-principles calculations. By introducing a new definition of the crystallographic structure of TCP, theoretical calculations have determined that the basic structural transformation unit (BSTU) in the hcp→TCP precipitation transition is a three-layer unstable hcp ordered structure. Through structural optimization based on first-principles calculations, these BSTU-ordered atomic clusters are not stable and will spontaneously collapse to form a TCP structure once they are generated in the matrix (as shown in Figure 1). The entire structural evolution process is guided by BSTU, leading to the generation of TCP disk-like precipitates in the hcp matrix starting from a 3-layer ordered structure, and thickening by a unit thickness of 2 atomic layers. The evolution law of the number of layers/thickness can be summarized as: N=3+2n (where N is the total number of layers of TCP precipitates, and n is the number of times it increases in thickness), as shown in Figure 2. The evolution pathway of this structure has been confirmed by multiple experiments, and this work further evaluates the atomic structure and size of critical nucleation of Mg2Ca and MgZn2 Laves phase during the precipitation process using this pathway.
This research found that the hcp→TCP transformation exhibits significant non-classical nucleation behavior, where the structural transformation depends only on the distribution of large-sized atoms. This leads to the occurrence of polytypism and deviation from the stoichiometric ratio in the actual TCP precipitation, thereby unifying the origins of differences in experimental results among different research groups at home and abroad. The hcp→TCP transformation is a conservative transformation that does not require additional atomic participation. The entire transformation process involves only shuffle-based displacement, in addition to atomic diffusion. Specifically, this study found that the extension of steps during the thickening process of TCP disks mainly depends on the formation of new BSTU on the TCP disk coherence surface, and the steps here do not have the interface dislocation properties considered by traditional solid-state phase transformation theory. This research not only improves the material science theory of the growth mechanism of nano-sized precipitate phase steps (i.e. the traditional interface dislocation-driven layer-by-layer step growth mechanism and the newly discovered non-interface dislocation tri-layer step generation mechanism), but also proposes new design strategies for enhancing the heat resistance and creep resistance of future TCP phase strengthened metal materials.
