Tungsten fractures through a specific sequence, and the sequence matters more than any individual step.
Hussein et al. trace the cascade in molecular dynamics: first, mobile dislocations exhaust themselves (dislocation starvation). Then, twins nucleate to accommodate further deformation. Then, the twin boundaries propagate until they encounter surface asperities — roughness features at the crystal's exterior. The boundaries pin. Disconnections — step-like defects on the boundary — pile up at the pinned sites. The pile-up concentrates stress beyond the local fracture threshold. The crack nucleates.
Remove any step and the cascade breaks. Without starvation, dislocations handle the strain. Without twins, no boundaries form to pin. Without surface roughness, boundaries propagate freely. Without disconnection pile-up, stress stays distributed. The fracture mechanism is not a cause — it is a narrative, each chapter enabling the next.
The role of surface roughness is particularly counterintuitive. Surface asperities are typically treated as minor features — cosmetic imperfections that affect surface properties but not bulk failure. Here, they are the geometric trigger that converts an internal microstructural event into catastrophic fracture. The surface pins the interior. An external feature determines when an internal process becomes fatal.
This is fracture as plot, not as event. The brittleness of tungsten at low temperatures is not one thing going wrong but a sequence of things going right — each deformation mechanism activating in order, each one setting up the conditions for the next, until the final step (the pile-up at the surface pin) crosses the threshold. The material doesn't fail because it's weak. It fails because its responses are too well-organized.