Essentials in diffusion behavior of nickel- and titanium-aluminides

Recently we have extensively investigated bulk and grain boundary diffusion in technologically important Ni- and Ti-aluminides. These compounds exhibit different lattice structures and different types and concentrations of lattice defects on their sublattices (vacancies, anti-structure atoms of constitutional and/or thermal origin). The study therefore provides fundamental insight into the interdependence of diffusion behavior and diffusion mechanisms on structure and ordering. An overview is presented, including the direct tracer diffusion measurements of the transition metal component and the determination of the Al diffusivity through interdiffusion data and by using Al-substituting solutes, like Ga. The interpretation of the experiments is strongly supported by EAM-potential calculations of defect properties and Monte-Carlo simulations of possible diffusion mechanisms. Among other features, the experimentally established curvature in the Arrhenius plot of Ti self-diffusion in TiAl, the surprising effect of the missing diffusion enhancement from structural Ni-vacancies in Al-rich NiAl, the paradoxically appearing deep minimum of the chemical diffusion coefficient in stoichiometric NiAl, and interphase diffusion along oriented Ti3Al/TiAl interfacesare successfully explained.