From Coherent to Semi-Coherent — Evolution of Precipitation Crystallography in an fcc/bcc System

A comprehensive knowledge about how crystallographic features are developed at the early stage of a phase transformation in an anisotropic misfit system (e.g., bcc/fcc) is limited, which impedes the deep understanding and quantitative modeling of microstructure evolution. In this work, we simulated coherency loss process during the early growth of a Cr precipitate (bcc) in a Cu matrix (fcc) by the combination of Monte Carlo and molecular dynamics. The simulation results reveal the fine details in the evolution of the crystallographic features, including the orientation relationship (OR) between two phases, the precipitate morphology, and interfacial dislocation structure at the early stage of the precipitate growth. Generation of a dominant set of dislocations is the governing event during the evolution process. The selection of the dominant dislocations was rationalized based on the minimization of interfacial energy in the major facet containing a single set of these dislocations. The initial OR developed between the coherent precipitate and the matrix is near the Nishiyama-Wassermann OR, and it changes discretely in response to the early generation of the dominant dislocations, towards the ideal OR corresponding to the O-line condition, near the Kurdjumov-Sachs OR. This tendency reflects the experimental observations in a Cu-Cr system, and provides helpful insight into the evolution of crystallographic features in physical reality.