A surface-physics-based hypothesis for ductile-to-brittle transitions in a variety of pure metals and crystal structures

Fractographic observations (from macroscopic to nanoscopic scales) for pure metals show that the characteristics of ‘brittle’ fractures produced at low temperatures in air are remarkably similar to those produced by adsorption-induced liquid-metal embrittlement (LME) at ‘high’ temperatures (where they would otherwise be ductile). These similarities include crystallographic fracture planes and directions for cleavage-like cracking, and nano-scale dimples on both cleavage-like and brittle intergranular fracture surfaces. Moreover, these similarities are observed for metals representing four crystal structures. The observations suggest that crack-growth processes are essentially the same for embrittlement at low temperature and for LME, with both involving localisation of plasticity resulting from easier dislocation-emission from crack tips. It is therefore proposed that, since adsorption can only affect bonding at surfaces (via electron-charge-redistribution between adsorbates and substrates), abrupt ductile-to-brittle transitions (DBTs) with decreasing temperature in air are caused by an intrinsic surface-charge-density perturbation at crack-tip surfaces below a critical temperature. Surface-science observations of abrupt surface-lattice reconstructions with decreasing temperature, and charge-density-wave considerations, provide support for this hypothesis. LME observations also suggest that the unusual behaviour of bcc tantalum in not exhibiting a DBT with decreasing temperature, and anomalous behaviour of some fcc and hcp metals, are probably associated with surface-bonding peculiarities.