Investigating the thermal-chemical-mechanical coupling effects on the cracking behavior of machine gun barrel: Microstructural insights

The presence of severe cracks at the inner bore of the gun barrel accelerates the erosion failure, whereas the evolution and failure mechanism of crack tips under the thermal-chemical-mechanical coupling effects needs further investigation. Herein, the elemental distribution, phase structure, and strain surrounding the perpendicular and circumferential cracks in a failed gun barrel were investigated in detail by utilizing scanning electron microscopy (SEM), transmission Kikuchi diffraction (TKD), and transmission electron microscopy (TEM). Results indicated that the perpendicular crack was covered by double continuous layers composed of inner FeO oxides and outer Fe_0.96S sulfides. Notably, a high-density precipitation of FeO oxides together with severe lattice distortion and localized amorphization was observed at the crack tip, accelerating the growth of the cracks. For the circumferential crack, the presence of fine recrystallized grains alongside coarsened M₂₃C₆ carbides was observed at the crack tip. There was a high level of strain concentration along high-angle grain boundaries at the forefront of the circumferential crack tip, resulting in the cracking along grain boundaries. Furthermore, the models for propagation of perpendicular and circumferential cracks under the thermal-chemical-mechanical coupling effects were proposed respectively.