New understanding on the pearlite-induced brittle failure of bimetal composite interface

The pearlite-austenite interface is the primary cause of brittle failure in the Mn8/SS400 composite. Here, we present new microstructural understandings on this interfacial brittle failure. The transformation-induced stress from pearlite formation, coupled with thermal stresses during cooling, generates complex stacking fault (SF) configurations within adjacent austenite grains. Specifically, V-shape, Z-shape, W-shape, and mesh-shape SFs are observed near grain boundaries, while multidirectional stresses in the grain interior promote the formation of multi-layer nanotwins, X-shape, and U-shape SFs. These intricate SF configurations form locking structures that severely restrict plastic deformation and enhance local stress concentration. Consequently, widespread intergranular fracture dominates, accompanied by a few cleavage fractures. These findings reveal a critical microstructural mechanism underlying brittle failure and provide new insights into interface design strategies for improving composite toughness.