FCC/B2 phase boundary variant-sensitive fatigue cracking in a eutectic high entropy alloy at high temperature

High-entropy alloys (HEAs) show the potential for high-temperature structural applications, with their superior fatigue properties of particular significance. However, fatigue cracking can be initiated in these materials with phase boundaries (PBs) as a specific source of weakness. In this work, a model eutectic HEA is studied using both in situ and ex situ methods with emphasis on unravelling the roles of two variants of FCC/B2 PBs - (i) PBs between B2/Prior FCC (denoted here as Type I PB) and (ii) PBs between B2/eutectic FCC (denoted as Type II PB). Our work addresses two fundamental questions. First, do these two types of PB confer differences in behaviour on the microstructural scale? And second, under what conditions is fatigue cracking promoted or hindered? Our work demonstrates conclusively that the two variants of PB do indeed behave differently being influenced by a varying hardness mismatch on either side of the PBs - as confirmed by our nanoindentation results. Moreover, the PBs demonstrate different roles in fatigue cracking, being capable of both promotion and inhibition, depending on the angle between the crack direction and the directional morphology of the eutectic lamellar structure. In addition, certain microstructural orientations demonstrate the greatest resistance to fatigue cracking. These findings provide new insights for improving fatigue-resistant design by microstructural engineering, because the strengthening effect of PBs can be leveraged, and the eutectic lamellar direction can be optimised.