Evaluation of the maximum transformation rate for determination of impingement mode upon near-equilibrium solid-state phase transformation

• The role of impingement mode in kinetic analysis is demonstrated.
• Thermodynamic term affects largely maximum transformation rate analysis in NES case.
• A new and simple method to determine impingement mode is proposed.
• Isothermal α/γ transformation in Fe–3Mn may be due to anisotropic growth.
• Non-isothermal α/β transformation in Zr may be due to non-randomly dispersed nuclei.

Considering the role of hard impingement mode in analyzing phase transformation kinetics, a maximum transformation rate analysis method was developed for pre-selection of impingement mode. Based on an extended analytical model, this method proposed for transformations with very large undercooling is reevaluated for transformations in the vicinity of equilibrium transformation temperature. Accordingly, a new peak maximum analysis method for both isothermal and non-isothermal transformations conducting near or far from the equilibrium state is proposed to determine the prevailing mode of impingement. The results obtained by application of the new method to numerically calculated transformations are in good agreement with the input values. Then, the method was applied to the isothermal ferrite/austenite transformation in Fe–3.28at.%Mn alloy as measured by dilatometry and non-isothermal hcp/bcc phase transformation in pure Zr as measured by differential scanning calorimetry.