Dislocations and Brittle Fracture in Metals

Abstract

There now exists a considerable body of evidence that the type of brittle fracture observed in metals at low temperatures is always preceeded by some plastic deformation. Metals are not observed to fracture in the elastic range, and even under severely embrittling conditions, such that the macroscopic stress strain, curve appears to be entirely elastic, evidence can be usually be found for microscopic slip preceeding fracture. For example, the macroscopic ductility of polycrystalline iron at low temperatures may be varied by varying the grain size; the larger grain sizes in such a series will break brittlely, whereas the finer grain sizes will exhibit considerable ductility. If, in such a series, the fracture stresses for the brittle specimens are compared with the yield stresses of the ductile specimens, as in fig. 1 a, it is found that the fracture stresses of the brittle specimens vary in the same way with grain size as do the yield stresses of the ductile specimens. Further, a single curve serves to describe the functional relationship between grain size and fracture stress and grain size and yield stress, i. e., σ = σ0 + kd −1/2. Where σ is the yield or fracture stress depending upon whether or not the specimen is fine or coarse grained, d is the mean grain diameter, and σ 0 and k are constants depending on the material, the temperature and the strain rate. A similar relationship between grain size and yield stress has been repeatedly found at more elevated temperatures where ductile behaviour is observed [1–3]. A more direct test of the equivalence of the brittle fracture stress and the yield stress may be obtained by comparing the brittle fracture stress in tension with the yield stress in compression for a series of grain sizes as has been done in fig. 1 b. Within the limits of accuracy of the test methods used, it appears that the stress at which fracture occurs in tension is the same as the stress at which yielding, i e, slip, occurs in compression.