Monte carlo-method simulation of the deformation-induced ferrite transformation in the Fe-C system
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25/3/04
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Monte Carlo–Method Simulation of the Deformation-Induced Ferrite Transformation in the Fe-C System MINGMING TONG, JUN NI, YUTUO ZHANG, DIANZHONG LI, and YIYI LI A Monte Carlo (MC) technique has been used to model deformation-induced ferrite transformation (DIFT) in an Fe-C binary system on a mesoscale. The effects of strain rate, strain, and recrystallization of the matrix on DIFT are investigated. Increasing the strain rate slightly retards the onset of DIFT. The volume fraction of ferrite increases gradually as the strain increases before the volume fraction of ferrite reaches its saturation value. After the volume fraction of ferrite becomes saturated, it oscillates around its saturation value. The recrystallization of austenite slightly retards the onset of the DIFT. Although the recrystallization of austenite reduces the equilibrium volume fraction of ferrite significantly, it cannot completely suppress DIFT. The stress concentration has been shown to induce the nucleation of ferrite near the grain boundaries and phase boundaries. The significance of the reverse transformation has been investigated. We found that there is a temporal oscillation of the volume fraction of ferrite and the stored energy after they arrive at their saturation values. We conclude that this oscillation and the effect of the strain rate on DIFT are both brought about by the reverse transformation from induced ferrite to undeformed austenite. The diffusion behavior of carbon atoms in the systems is different for different strain rates. The simulation shows that the dynamic recovery of austenite cannot occur in the system during deformation under the present conditions. The results of the simulation show that, other than the oscillation of the equilibrium volume fraction of ferrite and the unusual diffusion behavior of carbon atoms, the simulation agrees well with the corresponding experimental results. The temporal oscillation of the volume fraction of ferrite and stored energy and the unusual diffusion behavior are two new phenomena that have not been reported by other researchers.
I. INTRODUCTION
SINCE refining the grain size is a low-cost way to increase both the strength and the toughness of metals, methods of refining the grain size have been investigated intensively. Among the various refining methods, deformation-induced ferrite transformation (DIFT) is of great value, because it can refine the grains significantly.[1,2] Moreover, compared to other refining methods, the cost of DIF T is very low.[3,4] The DIF T is a dynamic process, with deformation-induced transformation occurring during deformation.[5] In the course of deformation, some of the energy expended during loading is stored in the matrix, mainly in the form of dislocations. This stored energy increases the free energy of the austenite phase so that it is higher than that of the ferrite phase, causing the austenite to transform to ferrite at a certain temperature. At this temperature, in the absence of the stored energy introduced by deform
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