A new intermetallic compound in TiAl+Nb composition area of the Ti–Al–Nb ternary system

Abstract

A new intermetallic phase γ 1 was determined in the TiAl+Nb composition area in the Ti–Al–Nb ternary system. The chemical formula of γ 1 phase was determined to be Ti 4Nb 3Al 9. The unit cell of γ 1 phase contains 16 atoms and consists of 4 γ-TiAl unit cells. The relationship of lattice parameters between γ and γ 1 phase is found as follows: a γ 1 = 2 a γ ; c γ 1 = 2 c γ The typical atom occupation in the unit cell as well as the reciprocal lattice of γ 1 phase has been given in the paper. The X-ray diffraction pattern of stoichimetric γ 1 phase, Ti 4Nb 3Al 9, 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 ∼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 γ 1. The structure transition from binary γ-TiAl to γ 1-Ti 4Nb 3Al 9 is schematically shown in the paper. High Nb content is favorable for the formation of γ 1 phase. Increasing Al content from 50 to 60 at.% is also favorable for the formation of γ 1 phase. The phase relationship in the TiAl+Nb area is determined. Various γ 1 base alloys with different microstructures can be designed.