Microstructure development and hardness of a powder metallurgical multi phase Gamma-TiAl based alloy

Abstract

A β-solidifying TiAl alloy with a nominal composition of Ti–43.5Al–4Nb–1Mo–0.1B (in at.%), termed TNM™ alloy, was produced by a powder metallurgical approach. After hot-isostatic pressing the microstructure is comprised of fine equiaxed γ-TiAl, α2-Ti3Al and βo-TiAl grains. By means of two-step heat-treatments different fine-grained nearly lamellar microstructures were adjusted. The evolution of the microstructure after each individual heat-treatment step was examined by light-optical, scanning and transmission electron microscopy as well as by conventional X-ray and in-situ high-energy X-ray diffraction. The experimentally evaluated phase fractions as a function of temperature were compared with the results of a thermodynamical calculation using a commercial TiAl database. Nano-hardness measurements have been conducted on the three constituting phases α2, γ and βo after hot-isostatic pressing, whereas the hardness modification during heat-treatment was studied by macro-hardness measurements. A nano-hardness for the βo-phase is reported for the first time.