Irreversible quasi-surface metallurgical phase transformations are the specific response of some metallic materials—such as metals and alloys—subjected to high thermomechanical loads applied very near their surface during the manufacturing processes or after being put into operation. These solid/solid phase transformations can be observed, for example, on the tread of many rails in railroad networks frequented by freight trains. The severe thermal and mechanical loads imposed on the surface of the rails and in the immediate vicinity of the surface by the wheel/rail contact often result in highly localized irreversible metallurgical transformations. A new kinetic model based on a previous study is presented here, which accounts more realistically for the nucleation and growth of these irreversible solid/solid phase transformations resulting from high thermomechanical loads. This metallurgical behavioral model was developed in the framework of continuum thermodynamics with gradients of temperature and internal variables. 1. Introduction The irreversible quasi-surface solid/solid phase transformations observed in many cases in the real industrial settings such as metal forming processes and the subsequent operating phases are the first material responses to high thermomechanical loads. In metallurgical phase transformations of this kind, which often occur in the rails of straight railway sections [1, 2] and those frequented by heavy freight trains, the ferrite/pearlite phase is directly transformed into a martensite phase [3]. To account for this process, it does not suffice to take only the thermal history of the material into account like in the standard metallurgical phase transformations [4–6], but both the thermal and the mechanical histories have to be taken into consideration when modelling these irreversible metallurgical transformations because high combined thermomechanical loads are engendered in the wheel-rail contact area, in the presence of strong normal and tangential stresses possibility in addition to a significant increase in the temperature due to the friction process occurring in the contact area [7–10]. In a previous study [11], a kinetic model was presented and discussed for predicting the onset and development of these irreversible quasi-surface solid/solid transformations in the materials subjected to high localized thermomechanical loads applied near the surface. In order to account more accurately, the effects of these high localized thermomechanical loads in the material, a new metallurgical behavioral model, was developed here
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