SOLIDIFICATION MICROSTRUCTURES AND CALCULATED PHASE-EQUILIBRIA IN THETI-AL-MN SYSTEM

The solidification behaviour of a number of Ti-Al-Mn alloys has been i nvestigated both experimentally and through CALPHAD methods. The alloy s were prepared as 1 kg buttons by plasma are melting and had a nomina lly constant Ti:Al ratio of 1.14 with Mn levels of 5, 10, 20 and 30 at .%. X-ray diffraction showed that the phases gamma-TiAl, alpha(2)-Ti3A l, beta-Ti (Al,Mn) and (Mn,Al)(2)Ti are present in the alloys in varyi ng quantities following solidification. The 5 and 10% Mn alloys both c omprise transformed lamellar dendrites of gamma/alpha(2) with interden dritic gamma, beta and (Mn,Al)(2)Ti. The quantity of interdendritic ma terial is significantly greater in the 10% Mn alloy. The 30% Mn alloy contains a large volume fraction of primary (Mn,Al)(2)Ti with only a s mall amount of gamma, beta and alpha(2). The 20% Mn alloy has acicular plates of (Mn,Al)(2)Ti which form a eutectic-like microstructure with the other phases. Melting endotherms, obtained by differential therma l analysis (DTA), for 5 and 10% Mn alloys compare well with CALPHAD ca lculations. Furthermore the CALPHAD prediction of a eutectic valley be tween the [beta + L] and [(Mn,Al)(2)Ti + L] phase fields is consistent with both DTA and microstructural observations on all the alloys. (C) 1997 Acta Metallurgica Inc.