Phase transformation behavior of a Mn containing beta-solidifying gamma-TiAl alloy during continuous cooling

摘要

The research was mainly focused on the phase transformation of Ti-42Al-5Mn (at%) alloy produced by vacuum induction melting (VIM) and vacuum arc remelting (VAR) followed by wrought process. The phase transformation kinetics of the alloy during continuous cooling under the cooling rates of 0.1 degrees C/s, 0.5 degrees C/s, 2 degrees C/s, 10 degrees C/ s, 50 degrees C/s, and 200 degrees C/s were identified by the Gleeble-3800 system, which is finally used to construct a schematic continuous cooling transformation (CCT) diagram of this alloy. The related phase composition, microstructure and property of the alloy were studied using X-ray diffraction (XRD), electron probe micro analyzer (EPMA), transmission electron microscope (TEM) and Vickers hardness (HV). It shows that six kinds of phase transformation are existed during continuous cooling from 1300 degrees C with the cooling rates of 0.1 degrees C/s, 0.5 degrees C/s, 2 degrees C/s, 10 degrees C/s, 50 degrees C/s, 200 degrees C/s and WQ, including beta -alpha(2)', beta -alpha, beta -gamma, beta -beta(o)/alpha -alpha(2), alpha(2)-alpha(2)/gamma and alpha(2)/gamma -beta(o,sec).It reveals that partial transformations would be inhibited when the cooling rate is relatively high. The martensitic structures (alpha(2)b') obtained by beta -alpha(2) were formed under the WQ condition after holding at 1300 degrees C for 5min, and it will be completely restrained below the cooling rate of WQ. For the beta -gamma, it is restrained if the cooling rates are higher than 50 degrees C/s. The alpha(2)-alpha(2)/gamma could be also suppressed when cooling with 200 degrees C/s, and the gamma lath within the supersaturated alpha(2)-grains would be formed at lower cooling rate below 50 degrees C/s. For 0.5 degrees C/s and 0.1 degrees C/s, a small amount of dotted beta(o) (beta(o,sec)) phases can be formed in the lamellar interfaces, which is proved the reaction products of alpha(2)/gamma -beta(o,sec). Furthermore, the effect of cooling rates on the microstructure, including interlamellar spacing (lambda) and gamma grains are also identified and discussed.