Study on the Partitioning of Alloying Elements Between bcc-Fe Matrix and bcc-Cu Precipitates and the Corresponding Effec
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THE precipitation of nanosized copper particles in iron has been studied extensively and has been proven to be an effective method to develop high-strength low-carbon (HSLC) steels due to their significant precipitation strengthening effect.[1–5] Given the low solubility of Cu in a-Fe, a high number density of nanosized Cu-rich precipitates forms in ferrite during aging in supersaturated Cu-bearing steels.[6] The precipitation kinetics of Cu-rich particles have been shown to involve several stages.[7,8] During the initial stage, coherent Cu-rich clusters form in the bcc Fe matrix, and these XUEYUN GAO and HUIPING REN are with the Key Laboratory of Integrated Exploitation of Bayan Obo Multi-Metal Resource, Inner Mongolia University of Science and Technology, Baotou 014010, P.R. China. HAIYAN WANG is with the Key Laboratory of Integrated Exploitation of Bayan Obo Multi-Metal Resource, Inner Mongolia University of Science and Technology and also with the School of Materials and Metallurgy, Inner Mongolia University of Science and Technology, Baotou 014010, P.R. China. Contact e-mail: windfl[email protected] CAINV MA and LEI XING are with the School of Materials and Metallurgy, Inner Mongolia University of Science and Technology. Manuscript submitted February 6, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
nanoprecipitates are confirmed to contribute to the peak-aged condition in steels that have excellent ductility and toughness.[9] As the particles grow, the strain energy and interfacial energy decrease, which leads to the martensitic transformation from the bcc structure into an intermediate 9R structure. With subsequent growth, the 9R Cu-rich precipitates transform into the thermodynamically stable fcc structure and the particle shape changes from a sphere to a rod with a loss in coherency. Previous studies[10–13] indicate that bcc Cu precipitates contribute the highest strengthening of approximately 300–400 MPa while nanosized 9R Cu particles provide a strength of approximately 200 MPa. During the late stage of evolution involving detwinned 9R and fcc Cu, the strengthening effect is weak because of the large size and incoherent relationship of the interface. Despite intensive studies of the evolution of the crystal structure and the strengthening mechanism of Cu-rich precipitates, the partitioning mechanism of the chemical composition and its impact on the stability of the nanosized precipitates and reduction in the coarsening remain subjects of debate.[14] With the presence of alloying elements such as Cr, Mo, Ni, Co, Mn and Al, the tensile strength of Fe-Cu-based ferritic steels can be further enhanced.[15] It has been suggested[16–18] that
some of these elements, i.e., Ni, Mn and Al, tend to segregate at the interface of the ferritic matrix/bcc Cu precipitates. The segregation layers serve as an effective barrier to the diffusion of copper, and the segregation of Ni and Mn leads to a decrease in the interfacial energy, thus reducing the coarsening rate of the Cu-rich particles.[11] Furthermore, the significant al
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