Reconsidering Hypotheses for Ductile-to-Brittle Transitions, based on Fractographic Observations, especially in Pure Tin

There is an enormous amount of literature on ductile-to-brittle transitions (DBTs) with decreasing temperature covering a wide variety of metals and alloys. Studies in bcc materials, like iron and steel, and other bcc transition metals have predominated, with fewer studies carried out for other crystal structures. DBTs (or their absence) in various crystal structures are briefly summarised in this paper, followed by detailed characterisations of the DBTs in β-tin, which has a unique, diamond-tetragonal structure. Tin been studied in less detail than other metals, and provides important insights into DBTs more generally.
A better understanding of DBTs has been obtained from high-resolution fractographic observations of brittle intergranular and cleavage-like fractures (in tin and other metals), which indicate that DBTs result from localisation of plasticity rather than a change to an atomically brittle decohesion process, as has been widely assumed in the past. It is proposed that DBTs are best explained by an abrupt change in crack-tip-surface bonding that facilitates the emission of dislocations from crack tips, thereby promoting the coalescence of cracks with nano-voids formed in the plastic zone ahead of cracks. Comparisons of low-temperature embrittlement with adsorption-induced liquid-metal embrittlement (LME) support this new hypothesis. The effects of variables such as strain-rate and constraint on DBTs in tin and other materials are then considered.