Linking electronic and dislocation parameters to fracture resistance in Nb–Ti–Cr alloys and Laves phases

The possible linkage of electronic and bonding variables to material parameters that describe dislocation nucleation, dislocation mobility, and the fracture resistance of metals and intermetallics is investigated in this paper. Using appropriate analytical methods, the effects of the number of d + s electrons per atom on the unstable stacking energy and the Peierls–Nabarro (P–N) barrier energy are computed for Nb–Ti–Cr solid solution alloys, Laves phase, and in-situ composites. Comparison of model computations with previously published experimental data reveals that the number of d + s electrons/atom exerts significant effects on the P–N barrier energy and the fracture toughness of the solid solution alloys, but has little effect on the unstable stacking energy, P–N barrier energy, and fracture toughness of the Laves phases. For in-situ composites, the electronic effects stem mainly from those exerted through the solid solution phase. A systematic investigation of the various components that contribute to P–N barrier energy indicates that the electronic effect is a manifestation of the influence of the charge density on the anisotropic shear modulus in the slip plane and direction, which in turn affects the lattice phase angle, the P–N barrier energy, dislocation mobility, and ultimately the fracture resistance. Ti addition in Nb–Ti–Cr alloys reduces the charge density, the anisotropic shear modulus, and the P–N barrier energy; the reductions enhance the dislocation mobility and improve the fracture toughness. In contrast, Cr addition increases the charge density, the anisotropic shear modulus, and the P–N barrier energy; these increases diminish the dislocation mobility and reduce the fracture toughness.