Effects of tantalum on the temporal evolution of a model Ni-Al-Cr superalloy during phase decomposition

The effects of a 2.0 at.% addition of Ta to a model Ni-10.0Al-8.5Cr (at.%) superalloy aged at 1073 K are assessed using scanning electron microscopy and atom-probe tomography. The gamma'(LI2)-precipitate morphology that develops as a result of gamma-(fcc)matrix phase decomposition is found to evolve from a bimodal distribution of spheroidal precipitates, to {001}-faceted cuboids and parallelepipeds aligned along the elastically soft < 001 >-type directions. The phase compositions and the widths of the gamma'-precipitate/gamma-matrix heterophase interfaces evolve temporally as the Ni-Al-Cr-Ta alloy undergoes quasi-stationary state coarsening after 1 h of aging. Tantalum is observed to partition preferentially to the gamma'-precipitate phase, and Suppresses the mobility of Ni in the gamma-matrix sufficiently to cause accumulation of Ni on the gamma-matrix side of the gamma'/gamma interface. Additionally, Computational modeling, employing Thermo-Calc, Dictra, and and PrecipiCalc, is employed to elucidate the kinetic pathways that lead to phase decomposition in this concentrated Ni-Al-Cr-Ta alloy. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.