How pressure transforms combustion: first direct observation of phase and internal crack dynamics in combusting HMX crystals

Combustion front structure and phase transition induced fracture are central to combustion dynamics in energetic materials, yet their direct observation under high pressures has remained elusive. Here, we report the first real-time visualization of HMX single crystal combustion using fourth-generation synchrotron X-ray phase-contrast imaging, capturing the transient interplay between melting, bubbling, and cracking at micron-scale resolution. Our observations reveal a sharp pressure-driven topological transition in the picture of the combustion front: at 0.1 MPa, the classical three-phase (solid-liquid-gas) structure prevails with a thick molten layer exhibiting intense bubble dynamics; at 7.0 MPa, the melt layer thins below 1 μm and bubbling is completely suppressed. Concurrently, the β→δ phase transition induced cracks shift from catastrophic fragmentation at ambient pressure to markedly slower propagation at elevated pressure, where they serve as conduits for hot gas infiltration. These findings provide decisive experimental evidence for theoretical models and establish critical constraints for multiscale modelling of HMX combustion.