A new relationship between white etching areas and butterfly cracks under rolling contact fatigue

A new relationship between white etching areas (WEAs) and butterfly cracks is found in M50 aeroengine bearing steel undergoing rolling contact fatigue. Contrary to the long-held belief that WEAs are caused by the rubbing and beating of crack faces, WEAs are found to precede crack formation in this steel. By adopting an improved plane-view focused ion beam lift-out technique, the circumferential section microstructure of WEAs in M50 is unveiled under transmission electron microscopy (TEM) for the first time. WEAs in M50 are found to consist of alternatively arranged fine-grained, coarse-grained and deformed domains aligned with the maximum shear stress direction, appearing as plastic flow bands under scanning electron microscopy. Densely distributed microcracks arise at the domain boundaries due to plastic localization and the mechanical property mismatch between the domains. This study provides the first TEM evidence of butterfly crack propagation after WEA formation. The difference in mechanical properties between WEAs and the matrix as detected by nanoindentation is proposed to facilitate butterfly crack growth. This mechanism addresses the paradox of asymmetric WEA formation at butterfly cracks. Carbon depletion is detected in both butterfly crack-containing and butterfly crack-free WEAs. Microcracks are identified as the sinks for depleted carbon. This new carbon depletion mechanism explains the correlation between carbon distribution and WEA plastic flow bands while addressing the long-distance carbon migration paradox in the previous hypothesis. The new insights from this study enhance the understanding of WEA formation in M50 with implications for predicting and modelling bearing damage, and developing long-life aeroengine bearing steels.