Nb Alloying Effect On The Basic Mechanics Properties Of α₂, O And β₀ Phase In Ti₃al Based Alloy

As the first step of the Ti₃Al based mixed phase alloy designing, a first principles plane-wave psendopotentials method based on Density Functional Theory (DFT) has been used to investigate the Nb alloying concentration effect on the basic mechanics properties ofα₂, O andβ₀ single phase of Ti-Al-Nb alloy. The results show that:(1) The ultimate tensile strengthσb of D0₁₉-Ti-25Al-xNb crystals and the elastic moduli (B, E, G) ofα₂-Ti-25Al-xNb alloys monotonously increase with the addition of Nb in the whole range of x=0¹2 (atom fraction, at.%). Meanwhile a very sensitive ductile/brittle behavior ofα₂-Ti-25Al-xNb alloys to Nb content is also detected. The addition of Nb with low content is demonstrated to be profitable for the weakening of the brittleness ofα₂-Ti₃Al alloys, and the toughening tendency ofα₂-Ti-25Al-xNb alloys increases as increasing Nb addition in the range of x=0⁶. Whereas in the range of x =7⁹, relative toα₂-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 ofα₂-Ti-25Al-12Nb alloys is observed as comparing withα₂-Ti-25Al-6Nb alloy. For this toughening and strengthening effect of Nb addition a reasonable explain has been given by means of the comparison of the density of states (DOS) and the projective density of states (PDOS) of D0₁₉-Ti-25Al-xNb (x=0, 6, 7, 12) crystals.(2) When Nb alloying concentration in the range of x=13³0, theα₂-Ti₃Al alloy was almost transformed to be O-Ti₂AlNb alloy. The O-Ti-25Al-xNb (x=13³0) alloys have better ductibility and lower shear modulus (G) and Young's modulus (E) comparing withα₂-Ti₃Al-Nb alloys. The addition of Nb with low content (x=13¹9 ) was demonstrated to enhance the ductility of O phase alloy slightly, whereas in the range of x=19²4, relative to O-Ti-25Al-xNb alloys no toughening tendency can be seen. However, the elastic moduli (B, G, E) of O-Ti-25Al-xNb alloys monotonously increase with the addition of Nb in the range of 13²4 at.%. An obvious improvement in the ductility and tobogganing in the strength of O-Ti-25Al-xNb alloys is observed when x=24²6. As x> 26,the strengthening effect of Nb addition is activated again, while the toughening effect disappear.(3) It is only when the Nb content > 8.0 at.% thatβ₀-Ti-(50-x)Al-xNb alloys can exist stably, which consist with the experimental results well. The stability and elastic moduli (bulk modulus B, shear modulus G and Young's modulus E) ofβ₀-Ti-(50-x)Al-xNb alloys increase with Nb adding, and the ductility is obviously improved as x= 9.375 compared withγ-TiAl alloy. Whereas in the range of x =12.5²5, the ductility ofβ₀- Ti-(50-x)Al-xNb alloys is decreased with Nb addition.