Origin of surface oxidation induced the nucleation and propagation of microcracks in TNM alloy

This study investigated the impact of surface degradation caused by oxygen (O) and nitrogen (N) elements in the environmental atmosphere on the service failure of nearly lamellar TNM alloy. The results show that oxidation-induced phase transformations at the surface and subsurface regions strongly affect the alloy's tensile properties at high temperatures. The precipitation of Al₂O₃, which disrupts the integrity of the Ti₂AlN layer, has a weaker deformation resistance capacity compared to the Z-phase. Under stress conditions, numerous dislocations concentrate at the Al₂O₃ phase, causing Al₂O₃ particles to easily detach and initiating crack nucleation at the Z-phase/Al₂O₃ interface. This is the origin of oxidation failure. Cracks nucleate at the Al₂O₃ detachment region, propagate, and connect with surface microcracks induced by the rough and loose outer oxide layer structure. This leads to the rapid propagation of cracks into the subsurface. The consumption of β-stabilizing elements at the surface, along with the internal diffusion of O and N elements into the subsurface region, results in the formation of the hard brittle Z-phase at the subsurface lamellar structure and a transition from β₀ to α₂ at the lamellar colony boundaries. The precipitation of α₂ and Z-phase also accelerates crack propagation. In summary, the failure mode shifts from toughness fracture to brittle fracture.