Generation and Evolution Mechanism of Cracks in HMX Single Crystals around Phase Transition Temperature

Thermal-induced cracks in explosive crystals could hinder heat conduction, generate hot spots, and ultimately induce detonation, posing safety hazards for the storage and transportation of explosives and impacting their performance. The thermotropic phase transition occurring below decomposition and detonation temperature complicate the evolution and mechanism of the cracks during heating for some explosives. In this work, the generation and evolution of cracks induced by the heating at both 180 and 185 °C above the phase transition temperature were investigated for the classical energetic crystal HMX. Three types of cracks, fast cracks, slow cracks, and small cracks, were observed and the corresponding formation mechanisms and distribution characteristics were systematically studied by optical microscopy, scanning electron microscopy, 3D X-ray computed tomography, X-ray diffraction, and Raman spectroscopy. The results indicate that fast cracks form due to thermal expansion, tending to be perpendicular to the lattice b-direction. Slow cracks arise from the (101) (or (1̅10)) plane slipping and exist only in the subsurface. Small cracks originate from the β–δ phase transition of HMX, are denser and more disordered, and mainly occur close to the heated surface. These findings provide further insight into the relationship between thermal damage and the heat conduction characteristics of HMX crystals.