Microstructure and phase evolution in laser rapid forming of a functionally graded Ti-Rene88DT alloy

A graded Ti-Rene88DT superalloy material, which has potential uses in aero engines, was fabricated using laser rapid forming. A continuous compositional gradient from 100 wt.% Ti to 60 wt.% Rene88DT was achieved. The solidification behaviour and phase morphological evolution of the coin positionally graded material were investigated. The study showed that a series of phase evolutions along the compositional gradient occurred: alpha -> alpha + beta -> alpha + beta + Ti2Ni -> beta + Ti2Ni -> Ti2Ni + TiNi -> TiNi. The phase transformation and microstructural evolution along the compositional gradient were analysed by considering the thermodynamic conditions as well as using microstructure selection models based on the maximum interface temperature criterion. The results were summarized in a microstructural selection map for the Ti-Ni system. The hardness value of the graded material was measured along the compositional gradient and the results were explained in terms of the presence of the various phases. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.