Application of, and Precautions for the Use of, the Rule of Additivity in Phase Transformation

The rule of additivity was first proposed for predicting the incubation time for nucleation of solid phases during continuous-cooling phase transformations, and has since been widely used for both the nucleation incubation and the entire process of phase transformation. While having been successfully used to calculate the transformed volume fraction during continuous cooling in many steel alloy systems, there is experimental evidence which shows that the rule of additivity is not rigorously valid for describing the incubation time for nucleation. We have recently used the classical nucleation theory to demonstrate the limits of validity of the rule of additivity. The general condition for the rule of additivity to be valid for phase transformation is that the kinetics of phase transformation be a state function independent of temperature history. In practice, the relative error caused by using the rule of additivity may be negligibly small in many cases. For better accuracy, the incubation time for nucleation should be excluded when applying the rule of additivity to phase transformation. Applications and precautions, when using the rule of additivity in nucleation incubation, phase transformation, and determination of equilibrium phasetransformation temperature, will be discussed.