High-temperature stability of nanocrystalline structure in a TiAl alloy prepared by mechanical alloying and hot isostatic pressing

A fully dense nanocrystalline compact of the Ti-47Al-3Cr (at. %) alloy was produced by mechanical alloying and hot isostatic pressing at 725 °C. Microstructure characteristics and grain growth behavior of this compact were studied after annealing for up to 800 h in the temperature range of 725 to 1200 °C, using analytical transmission electron microscopy techniques. The temperature and time dependencies of the grain sizes and the grain size distributions were determined. The grain growth occurred, with a time-and temperature-invariant single-peak grain size distribution (when normalized by the mean grain size), which was consistent with normal grain growth. The experimentally measured grain growth exponent decreased from 10 to 4.6 when the temperature was increased. The grain growth kinetics was described by a single thermally activated rate process limited by a permanent pinning force on the grain boundaries. The microhardness decreased on annealing and followed the Hall-Petch relationship with the parameters H_vo = 5.8 GPa and K_H = 1.6 MPa · m^0.5.