Roles and Influences of Extra vacancies introduced during δ-γ massive-like Transformation of Carbon Steel

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  • 炭素鋼 δ-γmassive的変態時に導入される過剰空孔の役割とその影響

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<p>Ferrous materials are widely used for various industrial products, of which mechanical properties depend on the microstructures initially determined in solidification process. According to recent studies using in situ observation, a new class of phase transformation takes place following solidification from BCC-δ phase to FCC-γ phase. This is referred to as δ-γ massive-like transformation which is different from peritectic reaction expected from Fe-C binary phase diagram. The δ-γ massive-like transformation may leads to surface cracking in casting, because this is a solid-solid transformation without any liquid phase and thus the strain energy which originates from density difference between the two phases would not be relieved. Thus, elucidation of the reaction mechanism is needed for the control of microstructures. Our previous studies revealed that this transformation is governed by continuous nucleation of γ phase and it is also suggested that the formation of vacancies would play important roles in transformation. However, it is difficult to directly observe nuclei or vacancies by in-situ observation. Thus, in this study, using phase-field-modeling, we analyzed the roles of vacancies on transformation, and found that vacancies are introduced in the generated γ phase above thermal equilibrium concentration. It is also suggested that the excess vacancies in the γ phase are caused by both vacancy formation in γ phase and diffusion from δ phase.</p>

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