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

Interdiffusion coefficients in the Ni-rich portion of the Ni-Al-Ti system have been determined, in the temperature range 900-1200 degreesC, concentrating particularly on the face-centered cubic (gamma) 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: gamma/gamma, gamma/gamma' and gamma'/gamma', where gamma' denotes the L1(2) phase. The two major coefficients (D) over tilde (Ni,gamma)(AlAl) and (D) over tilde (Ni,gamma)(TiTi) are very nearly equal; moreover these vary only slightly with composition. The minor coefficients (D) over tilde (Ni,gamma)(AlTi) and (D) over tilde (Ni,gamma)(TiAl) are up to an order of magnitude smaller and show considerable composition dependence. When plotted on the ternary phase diagram, the (D) over tilde (Ni,gamma)(AlAl) and (D) over tilde AlTiNi,gamma contours lie approximately parallel to the Al iso-concentration lines whereas the (D) over tilde (Ni,gamma)(TiTi) and (D) over tilde TiAlNi,gamma 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 gamma/(gamma + gamma') and gamma'/(gamma + gamma') 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 gamma' 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 gamma/gamma' interfaces, implying that local equilibrium is indeed adopted during migration under diffusional control. (C) 2001 Acm Materialia Inc. Published by Elsevier Science Ltd. AII rights reserved.