High temperature intermetallic alloy based on a new intermetallic compound γ₁ in the TiAl+Nb system

A new intermetallic phase γ₁ has been found in the Ti-Al-Nb ternary system. The chemical formula of the γ₁ phase was determined to be Ti₄Nb₃Al₉. The unit cell of the γ₁ phase contains 16 atoms and is 4 times larger than that of the γ-TiAl phase with L1 ₀structure. The relationship of lattice parameters between the γ and γ₁ phases is: a_γ1= √2 a_γ and c_γ1= 2c_γ The x-ray diffraction pattern of stoichiometric γ₁ phase, Ti₄Nb₃Al₉, is shown in the paper. The typical atom occupation in the unit cell as well as the reciprocal lattice of γ₁ phase is also shown in the paper. Nb atom in TiAl was confirmed to be substituted for Ti and preferentially occupies the Ti sublattice by the method of atom location by channeling enhanced microanalysis(ALCHEMI). For the case of low Nb content the site occupation of Nb atoms is random in the Ti sublattice. With increasing Nb content the distribution of Nb atoms in the sublattice gradually becomes ordered. The development of the Nb ordering on the Ti sublattice finally leads to the formation of a new ternary compound γ₁. The paper also showed the mechanical properties of several γ₁ based alloys that are located at the different phase region in the Ti-Al-Nb ternary phase diagram. It seems that the compressive yield strengths at the temperature from 20°C up to 1100°C are higher than conventional high Nb containing γ-TiAl alloy. The oxidation resistances of high Nb containing γ₁ based alloys could be improved by Y addition without loss of strength.