Many-body effects on calculated defect properties in h.c.p. metals

Defect calculations have been carried out with a simple short-range atomistic computer model which includes n-body forces from the embedded-atom method. The model was constrained to an ideal c/a ratio and parameters were chosen to approximate Ti for comparison with earlier short-range two-body calculations. The results contrast sharply with the earlier work and with other pairwise calculations in h.c.p. metals. Here vacancy diffusion is anisotropic with greater mobility in the basal plane and the migration energy is about half the formation energy. Divacancy binding is approximately isotropic and follows ‘vacancy bond counting’ (as does the binding energy of larger clusters), and the divacancy migration energy is only about 10% less than the vacancy migration energy. Both calculations give the configuration with the interstitial centred in a equilateral basal triangle under an octahedral site as most stable, but in the present work there is much less spread in energy between different configurations. Interstitial migration is complex, but not the same in the different models.