Evolution of the ωo phase in a β-stabilized multi-phase TiAl alloy and its effect on hardness

The intermetallic beta-stabilized Ti-43.5A1-4Nb-1Mo-0.1B alloy (in at.%), termed TNM alloy, is designed to be used at elevated temperatures, typically up to 750 degrees C. To understand the evolution of the microstructures during heat treatments and subsequent creep tests, an understanding of the phase transformations and decomposition reactions that occur is necessary. The present study deals with the development and growth mechanism of the omega(o) phase, which forms in the beta(o) phase during static annealing treatments and creep tests performed at 750, 780 and 800 degrees C using an applied stress of 150 MPa. In situ high-energy X-ray diffraction experiments were conducted to investigate the decomposition behaviour of the omega(o) phase during heating as well as to determine its dissolution temperature. High-resolution transmission electron microscopy was used to study the coarsening of coo grains during creep. The chemical composition of beta(o) and omega(o) was determined by means of energy dispersive X-ray microanalysis. In particular, the impact of the Mo content on the growth of the omega(o) grains within the beta(o) matrix was investigated. Additionally, nanohardness measurements in gamma, alpha(2), beta(o) and (beta(o) + omega(o)) grains were performed by cube corner indentation. The results show that 130 is the hardest phase in the TiAl-Nb-Mo alloy system when finely dispersed omega(o) precipitates are present. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.