On the diffusion of aluminium and titanium in the Ni-rich Ni–Al–Ti system between 900 and 1200°C

Abstract Interdiffusion coefficients in the Ni-rich portion of the Ni–Al–Ti system have been determined, in the temperature range 900–1200°C, concentrating particularly on the face-centered cubic (γ) phase. The values were derived from concentration profiles measured by electron probe microanalysis, using a modified form of the Boltzmann–Matano method. The diffusion couples were of three types: γ/γ, γ/γ′ and γ′/γ′, where γ′ denotes the L12 phase. The two major coefficients D Ni,γ AlAl and D Ni,γ TiTi are very nearly equal; moreover these vary only slightly with composition. The minor coefficients D Ni,γ AlTi and D Ni,γ TiAl are up to an order of magnitude smaller and show considerable composition dependence. When plotted on the ternary phase diagram, the D Ni,γ AlAl and D Ni,γ AlTi contours lie approximately parallel to the Al iso-concentration lines whereas the D Ni,γ TiTi and D Ni,γ TiAl contours follow the iso-concentration lines for Ti. The activation energies of all coefficients were found to be similar at the reported compositions. Observations are made concerning the accumulation of Kirkendall porosity in the interdiffusion zone, the locations of the γ/(γ+γ′) and γ′/(γ+γ′) phase boundaries and also their rate of migration. Data for the interdiffusion of Al and Ti on the Al/Ti site of the intermetallic phase γ′ are also reported, and the values are rationalised in the context of the diffusion mechanism which is known to operate in that phase. The kinetic data enable reasonable predictions to be made of the parabolic rate constant pertaining to the γ/γ′ interfaces, implying that local equilibrium is indeed adopted during migration under diffusional control.