A deformation twin mediated sliding-opening zig-zag fracture mechanism in multi-principal element alloys

Multi-principal element alloys (MPEA) have emerged as a class of high-performance materials. Of particular interest is the excellent fracture toughness in face-centered cubic MPEAs with low stacking fault energy, where abundant deformation twins are commonly observed. To understand the fracture mechanism in such MPEAs, here we perform in situ nanomechanical testing inside transmission electron microscope, atomistic simulation, and theoretical analysis to investigate the mechanism of interaction between a propagating crack and a coherent twin boundary (TB) in CoCrFeNi MPEA. We unveil a hitherto unknown microscopic mechanism of crack-TB interaction in MPEAs, where sliding along secondary nanotwins (NTs) nucleated from the primary TB followed by sub-crack nucleation some distance ahead of the main crack constitute a consecutive sliding-opening crack propagation mechanism, leading to zig-zag fracture around the primary TB. The origin of this mechanism lies in the frequent nucleation of NTs from steps on the primary TB, which can effectively blunt and deflect a propagating crack. Quantitative analysis indicates nearly doubled fracture resistance associated with this unique mechanism of crack-TB interaction in MPEAs. These findings bring about a missing aspect in TB-modulated fracture in MPEAs and enrich our understanding of the enhanced damage tolerance of alloys with low stacking fault energy.