On the β to B2 ordering temperature in a Ti-22Al-26Nb orthorhombic titanium aluminide

Abstract

The past decade or so has seen considerable research and development activity on the titanium aluminides based on the Ti-Al-Nb system, especially those involving the Ti{sub 3}Al ({alpha}{sub 2}), Ti{sub 2}AlNb (O) and TiAl ({gamma}) compounds, for applications in high temperature aerospace propulsion systems. In the former two compounds, microstructure development often relies on the decomposition of the high temperature {beta} phase (b.c.c. structure). In Ti{sub 3}Al alloys containing Nb, a number of investigators have reported that rapid quenching from the {beta} phase field leads to an ordered b.c.c. or B2 structure. The {beta} to B2 ordering temperature is of interest and some of these studies have reported that this temperature varies with the Nb content. For instance, in alloys containing Nb in the range of 11 to 15 at. %, the ordering temperature is thought to be in the range of 1,100--1,130 C, but appears to rise sharply for Nb concentrations >20 at. %. In alloy compositions of the type Ti-(22--30)Al-(22--30)Nb, this temperature is believed to be around 1400 C, but is not known with certainty. The present work reports on the determination of the disordered {beta} to B2 ordering transition temperature in a Ti-22Al-26Nb orthorhombic alloy.