CALCULATION OF MECHANICAL PROPERTIES OF α2-Ti-25Al-xNb ALLOYS BY FIRST-PRINCIPLES

Intermetallic alloys based on Ti3Al are potential high-temperature structural materials due to their low density, high specific strength, excellent creep behavior and good oxidation resistance, but their application has been hampered by the low room-temperature ductility and ambient brittleness. Numerous experiments have shown Nb is most effective additive to improve their ductility and toughness at low temperature, but the influence of Nb content on the mechanical properties of Ti3Al-based alloys has not been understood. In this work, using the first-principles pseudo-potential plane wave method, ultimate tensile strength sigma(b) of alpha(2)-Ti-25Al-xNb (x=0-12, atomic fraction, %) single crystal with D0(19) structure and bulk modulus B, Young's modulus E as well as shear modulus G of alpha(2)-Ti-25Al-xNb polycrystalline alloys have been calculated, and their ductile/brittle behavior is characterized and assessed by the Cauchy pressure (C-12-C-44) and the G/B ratio. The results reveal the ultimate tensile strength sigma(b) of alpha(2)-Ti-25Al-xNb crystals and the elastic moduli (B, E, G) of alpha(2)-Ti-25Al-xNb alloys monotonously increase with the addition of Nb in the whole range of x=0-12. Meanwhile a very sensitive ductile/brittle behavior of alpha(2)-Ti-25Al-xNb alloys to Nb content is also detected. The addition of Nb with low content is demonstrated to be profitable for weakening of the brittleness of alpha(2)-Ti3Al alloys, and the toughening tendency of alpha(2)-Ti-25Al-xNb alloys increases as increasing Nb addition in the range of x=0-6. Whereas in the range of x=7-9, relative to alpha(2)-Ti3Al alloys no toughening effect can be seen as Ti in Ti3Al being partially substituted by Nb. As x >= 10, the toughening effect of Nb addition is activated again, and an obvious improvement in the ductility and strength of alpha(2)-Ti-25Al-12Nb alloy is observed as comparing with alpha(2)-Ti-25Al-6Nb alloy. For this toughening and strengthening effect of Nb addition a reasonable explain was given by means of the analysis of the density of states (DOS) and the projective density of states (PDOS) Of alpha(2)-Ti-25Al-xNb (x=0, 6, 7, 12) crystals.