Abstract
A formal kinetic approach was applied to the spread of isothermal martensite over the neighboring austenite grains in a Fe-23.2 wt. (%) Ni-2.8 wt. (%) Mn alloy. The number of grains in a spread event changed with parent austenite grain size. However, isothermal martensite spread formed from fine-grained parent austenite and athermal martensite from a Fe-31 wt. (%) Ni-0.02 wt. (%) C alloy studied in a previous work followed the same microstructural path. The number of grains per spread- event found in the present study was shown to be consistent with the number of neighbors of the grain originating the spread event by means of a simple geometrical model of the parent austenite grain network. The study of the kinetics of isothermal martensite spread showed that the nucleation rate of the spread-event in isothermal martensite remained constant during the transformation. This result parallels the constant nucleation rate of the spread-event also found using the same methodology in athermal martensite formed in a Fe-31 wt. (%) Ni-0.02 wt. (%) C alloy studied in a previous work.
Highlights
Martensite is a displacive phase transformation with significant transformation strains which prompts microstructural heterogeneity, influences plate’s shape, and can lead to diversity in kinetics
Number of grains in a cluster of partially transformed austenite grains that constitute a spread event; a Coefficient obtained from microstructural path, a = 2g–1/3 ; Nvm
The transformation continues by subsequent “spread events” that spread the reaction to the untransformed grains
Summary
Martensite is a displacive phase transformation with significant transformation strains which prompts microstructural heterogeneity, influences plate’s shape, and can lead to diversity in kinetics. In isothermal martensite, nucleation of new martensite plates in untransformed grains and the consequent spread event takes place as a function of reaction time. V sp Volume of a single cluster of austenite grains partially transformed to martensite comprising a number γ of austenite grains, that is, volume of a single spread event; s. Surface area between a single cluster of austenite grains partially transformed to martensite and the untransformed sp matrix grains, that is, surface area of a single spread event; k. Number of grains in a cluster of partially transformed austenite grains that constitute a spread event; a Coefficient obtained from microstructural path, a = 2g–1/3 ; Nvm. Number of pre-existent nucleation sites at ∆t =Ms-T, where T is the transformation temperature; I v. The transformation continues by subsequent “spread events” that spread the reaction to the untransformed grains
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