Kinetic Study of the Interrelation among Three Types of γ→α Martensite Transformation in Iron Alloys

Abstract

The γ-α martensitic transformation in iron alloys can be classified into three types: type IA (packet), 1ype IB (lenticular), and type II (isothermal) transformations.
The interrelation among these three types of transformation were investigated using the equation of thermal nucleation rate, and the following conclusions were drawn from the obtained results.
(1) Using the idea of temperature dependence of the activation energy ΔW, it could be shown that the athermal transformation is an extreme case of isothermal transformation.
(2) Assuming the temperature dependence of ΔW is high enough in the calculation, the type I transformation can be obtained, whereas type II appears when the temperature dependence of ΔW becomes progressively smaller in the lower temperature range. The larger the value of autocatalytic factor c, the more burst-like transformation occurs. Physical metallurgical meaning of c can be discussed from the results of this study and also previous works by other researchers.
(3) It is also shown that the magnitudes of ΔW and v (mean size of martensite lath) markedly shift the transformation temperature range, while they do not appreciably influence the transformation rate.
(4) Taking into account the temperature dependence of the critical resolved shear stress and that of the shear modulus of alloys, behaviours of those three types of transformation can be explained qualitatively in terms of the temperature dependence of the activation energy.