Austenite to Ferrite Transformation in Fe - 9 %Cr Alloys. II. Determination of Growth Rates

The paper continues theoretical analysis of austenite to ferrite transformation in an Fe-9 %Cr alloy demonstrating a typical site-saturation behaviour under the condition of diffusionless growth of a phase. In the previous paper the K coefficient in the Avrami equations being the asymptotes of Calm's solution of the site-saturation transformation kinetics was used that contained a constant activation energy indicating the small value of the work of critical nucleus formation. This approach permitted to determine activation energy of transformation that was found to be equal to about 300 kJ/mole. In the present paper a new method of finding Calm's equation coefficients from bending point at isothermal curves plotted on double logarithmic scale is proposed. Actually this bending point is the intercept of the two above-mentioned asymptotes of the Cahn's curve, and equating the formulae describing theses asymptotes allows to find the coefficients in question. Given the average austenite grain size, it permits to find numerical values of growth rates. The activation energy thus acquires the exact physical meaning of the energy barrier at the interphase interface; its value was found to be 286 kJ/mole, which is close to previous results. The growth rates were thus determined as a function of temperature and the TTT diagram for the alloy is calculated that shows close correspondence with experiment.