Excess Carbon Enrichment in Austenite During Intercritical Annealing
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stenite reversion during intercritical annealing generates considerable interest due to its application to the production of dual-phase and transformation-induced plasticity steels.[1] Recently, two studies were reported on kinetics of isothermal intercritical annealing of asquenched martensite (i.e. a¢ to c transformations).[2,3] The growth of austenite was simulated assuming local equilibrium at the a/c interface. The simulated transformation kinetic curve approximately agrees with the measured one. However, it is highly possible for the interfacial concentrations to deviate from local equilibrium, since a finite intrinsic a/c interface mobility and/or solute drag effect may occur, as reported in c to a transformations.[4–7] As far as the current authors are aware, this possibility has been scarcely examined experimentally in a¢ to c transformations to date. Nevertheless, a simulation study was carried out recently by Santofimia et al.,[8] which pointed out the significance of intrinsic interface mobility on the evolution of austenite/martensite grain assemblies during annealing. A construction of free energy curves was
ZHEN-QING LIU, Ph.D. Student, is with the School of Materials Science and Engineering, Key Laboratory of Advanced Materials of Ministry of Education, Tsinghua University, Beijing 100084, People’s Republic of China, and also with the Department of Metallurgy, Tohoku University, Aoba-ku, Sendai 980-8577, Japan. Contact e-mail: [email protected] GORO MIYAMOTO, Associate Professor, and TADASHI FURUHARA, Professor, are with the Institute for Materials Research, Tohoku University, Sendai, Japan. ZHI-GANG YANG, Professor, is with the School of Materials Science and Engineering, Key Laboratory of Advanced Materials of Ministry of Education, Tsinghua University. Manuscript submitted July 3, 2013. Article published online September 7, 2013 4872—VOLUME 44A, NOVEMBER 2013
adopted by them to estimate the driving force for interface migration (i.e., one part of free energy dissipation). In this construction, the same chemical potential of carbon is assumed between austenite and ferrite at interface (see Figure 1). The interfacial carbon concentrations, XC, deviate from the equilibrium points, XCe, due to the free energy dissipation at interface.[9] The a/c interface velocity, v, is then expressed by[8] v ¼ M DG=Vm
½1
where M is the interface mobility, DG is the free energy dissipation at the interface for 1 mol of the material transferred across the interface, and Vm is the molar volume of the material transferred across the interface. In the case of austenite formation from ferrite, Eq. [1] can be specified as v ¼ M ½XcFe ðlaFe lcFe Þ þ XcC ðlaC lcC Þ ½2 ðXcFe VFe þ XcC VC Þ where laFe , lcFe , laC ; and lcC are the chemical potential of Fe and C in a and c, respectively; XcFe and XcC are the Fe and C concentrations of c at the corresponding interfaces, respectively; and VFe and VC are the molar volumes of Fe and C in c, respectively. It is supposed that XcC and VC are sufficiently small so that the term
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