A new intermetallic compound in TiAl plus Nb composition area of the Ti-Al-Nb ternary system

A new intermetallic phase gamma(1) was determined in the TiAl + Nb composition area in the Ti-Al-Nb ternary system. The chemical formula of gamma(1) phase was determined to be Ti4Nb3Al9. The unit cell of gamma(1) phase contains 16 atoms and consists of 4 gamma-TiAl unit cells. The relationship of lattice parameters between gamma and gamma(1) phase is found as follows: a(gamma 1) = root 2a(gamma); C-gamma 1 = 2c(gamma) The typical atom occupation in the unit cell as well as the reciprocal lattice of gamma(1) phase has been given in the paper. The X-ray diffraction pattern of stoichimetric gamma(1) phase, Ti4Nb3Al9, is consistent with the calculated diffraction pattern based on the proposed crystal structure and atom occupation. The space group of the lattice is P4/mmm. Nb atom in TiAl was proved to substitute for Ti atom and preferentially occupies the Ti sublattice using ALCHEMI technique. For the case of low Nb content the site occupation of Nb atoms is random on the Ti sublattice. The electron diffraction investigation indicated that when the Nb content is increased to similar to 12 at.% the ordered distribution of Nb atoms on the sublattice appears. This is a phenomenon of continuous ordering occurring in the TiAl + Nb system with increasing Nb and Al contents. The development of the continuous ordering finally leads to the formation of a new ternary compound gamma(1). The structure transition from binary gamma-TiAl to gamma(1)-Ti4Nb3Al9 is schematically shown in the paper. High Nb content is favorable for the formation of gamma(1) phase. Increasing Al content from 50 to 60 at.% is also favorable for the formation of gamma(1) phase. The phase relationship in the TiAl + Nb area is determined. Various gamma(1) base alloys with different microstructures can be designed. (c) 2004 Elsevier Ltd. All rights reserved.