Molecular Dynamics Simulation Of Cu Precipitation At An Early Stage In Fe-Cu-Ni Alloy

Cu-rich precipitation plays an important role in the hardening of high-strengthlow-alloy (HSLA) steels. However, it is also confirmed to be the origin ofembrittlement of ferritic steel used as structural material for pressure vessels ofnuclear reactors (RPV). The Cu precipitation and related basic mechanisms are offundamental scientific interest. In order to improve the understanding of theformation of Cu-rich clusters and its effect on the mechanical properties of matreix,Fe-Cu-Ni ternary alloys with high solute concentrations are adopted under the agingand continuous cooling processes. In this paper, the early precipitation behaviors arestudied by molecular dynamics (MD) methods in Fe-3at%Cu andFe-3at%Cu-4at%Ni alloys with aging and continuous cooling process, respectively.Several analytical methods, including the mean square displacement (MSD), radialdistribution function (RDF), common neighbor analysis (CNA) and visualizationsare used to investigate the formation of the Cu-rich clusters, the structure of Cu-richclusters, influence of Ni on Cu precipitation, and Cu precipitation impact on thestructures and mechanical properties of the matrix. Partial results that obtained bythe MD simulations are supplemented and verified by the first-principlescalculations. The study has important significance in understanding the influence ofNi on the formation of Cu-rich clusters and effects of Cu-rich clusters on structuresand properties of the matrix. The main conclusions are stated as follows:(1) The formation of Cu-rich clusters can be confirmed as the following threestages in the aging and ccontinuous cooling simulation process: Cu and Ni atomscome together to become a local small solute clusters, and small clusters act as theprecipitation precursors; small and mobile clusters combine to be big ones but withloose structures; Ni is rejected to the Cu-rich cluster/Fe-matrix interface to formcore-shell structures during the growth. The Cu-rich clusters with large size exhibitthe ellipoidal shapes. (2), Researches show that Ni is able to connect small sizes of Cu-rich clusters togrow into big ones in the aging and continuous cooling processes. In the agingprocess, it is found that Ni increases the diffusion of Cu, increases the diameter sizeof Cu-rich clusters. In the continuous cooling process, it is shown that Ni increasesthe structural stability of the Cu-rich clusters with a certain "pinning" role in theformation of the Cu-rich clusters. In short, Ni promotes the precipitation of Cu-richclusters from the matrix.(3) The result obtained by MD simulations that Ni promotes small Cu-richclusters to be big ones is verified and supplemented by the first-principlescalculations. Results show that Ni atoms energetically favor at combination positionsbetween clusters, and the nature is that a large number of the first nearest neighborCu-Ni interactions leading to the decrease of the total system energy.(4) In the continuous cooling process, it is found that the coplanar Cu-richclusters/matrix interfaces are misfit centers of ferrite lattice and induce a partialtransformation from bcc towards the fcc lattice structure. The sequence of thetransformed structures in the matrix is as follows: bcc structures in the center; thenthe fcc structures; and a large number of other structures are distributed between bccand fcc lattice structures.(5) Stress-strain behavior is analyzed and suggesting that the brittleness offerrite is increased by the formation of Cu-rich clusters, and the matrix prefers tobreak in the stress concentration zones which caused by Cu-rich clusters and localstructural phase transition.