Correlative characterization of structural changes across a white etching layer boundary: Evidence for a single-slip mechanism

The formation of white etching layers (WELs) on railway track surfaces represents both a long-standing engineering challenge and a scientific puzzle. Because WELs develop under variable and complex local conditions, the mechanisms behind their formation remain under debate. This study reconstructs the sequence of events leading to WEL formation by examining structural transformations at the interface between WELs and the underlying pearlitic steel. WELs were reproducibly generated using a full-scale test rig under conditions of significant slip, but low speed and a few wheel passes. A correlative characterization approach, employing techniques ranging from synchrotron X-ray diffraction to atom probe tomography, revealed abrupt microstructural changes at the interface, including cementite dissolution, ferrite lattice distortion, and retained austenite formation. Since the experimentally produced WELs are indistinguishable from those found on in-service rails, we propose a model in which WELs form during a single, severe slip event. This model suggests that the accumulation of plastic strain over many cycles is not a necessary condition for WEL formation. Instead, a single event of rapid and severe plastic deformation leads to adiabatic heating, autocatalytic softening, and plastic flow of the material, which transforms into hard and brittle ultrafine-grained martensite upon cooling.