Phenomenological Model for Deformation-Induced Ferrite Transformation

Abstract

A phenomenological model is proposed to describe the conditions under which an ultrafine ferrite (UFF) microstructure forms in low-alloyed steels as a result of deformation-induced ferrite transformation (DIFT). Assuming that corner points in the dislocation cell substructure of microshear bands provide suitable nucleation sites for ferrite, the model predicts that UFF forms in low-carbon steels independent of the steel chemistry and is promoted by increasing the strain rate. Analyzing ferrite growth rates suggests that the temperature range for UFF formation depends primarily on a minimum interfacial velocity criterion that must be fulfilled. In addition to these general trends, a more quantitative comparison of the model with experimental results for a Mo-containing steel is provided. This comparison suggests ferrite growth rates during DIFT that are somewhat larger than during conventional cooling transformation. These enhanced growth rates can be attributed to the presence of fast diffusion paths.

Ferrite Grain Size Distributions in Ultra-Fine-Grained High-Strength Low-Alloy Steel After Controlled Thermomechanical Deformation

“…It is evident that immediate WQ after heavy deformation at 1073 K (800°C) (just above A r3 ) is able to retain the largest grain size below~6 lm in low-carbon steels, as shown in Figures 7(d), (l), and 8(c). This observation can be supported by the work of Beladi et al [21] and Huang et al [37] The average grain size of the 1073-0.9-1s-WQ sample (1.6 lm), which agrees well with the predicted DSIT-ferrite grain size, followed the work of Militzer and Brechet [38] for the true strain (0.9) and strain rate (10/s) used in present study. The average grain size of the 1073-0.9-1s-WQ sample can be used then as D a0 in the following equation, for the prediction of average a-grain size (D a ) in the 1073-0.9-100s-WQ sample, as a function of time (100 seconds) and temperature (1073 K [800°C]) of isothermal holding: [39] …”

“…Overall, the average a-grain sizes of the specimens, where DSIT is expected to happen in the present study (Table II), are similar to the range of values (2.1 to 3.0 lm) reported earlier for the plain C-Mn or microalloyed steel specimens, air-cooled or slowly cooled after heavy deformation, around A r3 . [21,30] Zhang et al [24] and Militzer and Brechet [38] carried out initial modeling studies for the prediction of fraction and size of DSIT ferrite grains, as the function of (average) initial c-grain size, e, _ e, and T def . However, it should be remembered that DSIT is not synonymous with the development of UFF grain structure.…”

Multi-Phase-Field Simulation of Flow Stress and Microstructural Evolution during Deformation-Induced Ferrite Transformation in a Fe–C Alloy

“…The value of K1 is assumed to be the same as that reported by Umemoto et al 27) The area of the austenite grain boundary, Sgb, is calculated from the length of the austenite grain boundary, lg, which, in turn, is calculated through the MPF simulation as follows: On the other hand, in order to describe the evolution of the density of the activated ferrite nucleation sites on the microshear band, we determine using the following equation: 28) .............. (21) where ε0 is the critical strain for the initiation of microshear band formation and is a parameter representing the strain-rate sensitivity of the microshear band formation process. Here, and εref are the applied strain rate and the referential strain rate, respectively.…”

“…In order to determine the input parameters for the MPF model, we performed a preliminary simulation of the DIFT in a Fe-0.15wt.%C alloy at a temperature of 750°C and strain rate of 0.1 s -1 ; these conditions are same as those employed by Choi et al 32) Then, the parameters to be used for the subsequent MPF simulation were identified by comparing the simulated flow stress curve with the experimentally determined one. The physical values and input parameters obtained using the above-mentioned procedure are shown as follows: [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] For Eqs. (3) and (4) .…”

A New Constitutive Strain-Dependent Garofalo Equation to Describe the High-Temperature Processing of Materials—Application to the AZ31 Magnesium Alloy

“…[20,21] The main laws used correspond to Avrami and Voce, and the Garofalo equation is only used to supply some of the necessary parameters. [22,23] Another approximation for the construction of the constitutive equation extended for a large range of strains is based on dimensional analysis of various variables that are of physical significance such as the stacking fault energy, grain size, dislocation density, etc. [24] This model needs a large amount of microstructural data and works with a large amount of variables.…”