Ductile-to-brittle transitions with decreasing temperature in BCC transition metals: fractographic observations and issues with conventional hypotheses

Transitions from ductile to brittle fracture, which can often occur abruptly with decreasing temperature, are described for bcc transition metals and alloys. High-resolution fractography showing that cleavage-like and brittle intergranular fracture surfaces were dimpled on a nano-scale indicate that ductile-to-brittle transitions (DBTs) involve localisation of plasticity, and not transitions from ductile behaviour to atomically brittle fracture, as is usually assumed. In other words, most brittle fractures are accomplished by dislocation activity rather than by ‘decohesion’ accompanied by some dislocation activity. For this and other reasons, the conventional hypothesis for DBTs based on steeply increasing yield-stress with decreasing temperature, but a relatively temperature-insensitive stress for decohesion, is not credible. Moreover, previous, often-cited, criteria for DBTs based on dislocation emission from atomically sharp crack tips leading to ductile behaviour are far too simplistic. Possible explanations for the fractographic observations are discussed by the authors separately in following papers in this issue.