4 - Bulk and Grain Boundary Diffusion in Intermetallic Compounds

Abstract

This chapter focuses on diffusion in ordered binary aluminides of Ni, Ti, and Fe. In these cases, a lot of new, reliable experimental data exist. The chapter also elaborates the diffusion mechanisms. Self-diffusion of the transition metal component in Ni3Al, Ti3Al, and TiAl mainly occurs by nearest neighbor jumps via vacancies in the transition metal sublattice. This mechanism does not dominate in NiAl. The triple-defect mechanism produces the main contribution to diffusion in the near-stoichiometric NiAl alloys. Under specific conditions, the antistructure bridge mechanism also becomes important in these intermetallics. The transition metal sublattice diffusion mechanism superimposed by the antistructure bridge (ASB) mechanism governs Al diffusion in Ni3Al, Ti3Al, and TiAl. Ga, which dissolves exclusively on the Al sublattice in Ni and Ti aluminides, is a suitable solute to simulate Al diffusion in these compounds because it diffuses by the same mechanisms as Al. Al diffusion in near-stoichiometric NiAl alloys mainly occurs by the triple-defect mechanism, and the ASB mechanism dominates Al diffusion in Al-rich compositions.