A First-principles Study On The Phase Stability And Elastic Properties Of B2-tial Based Alloy

Using the first-principles pseudo-potential plane wave method, the electronic structures, energetics and the elastic moduli of B2-Ti-Al-X (X=Cr, Mo, Nb, W, Zr and Hf) and B2-Ti₅₀(Al_50-xNb/Cr_x) (x=6.25,9.375,12.5, 18.75, 25 at.%) crystals are calculated. The site preference of TM elements X in B2-TiAl crystal and the mechanical stability of these B2 phases with preferential site of X are also investigated by a comparison of the heats of formation and the Born Criterion of stability. The Cauchy pressures (C₁₂-C₄₄) and the ratios of G/B have been adopted to characterize and assess the ductile/brittle behaviors of B2-TiAl-X crystals. The results show:(1) Cr, Mo, Nb, Zr and W prefer to occupy Al sites in B2-TiAl crystal, and Hf mainly substitutes for Ti atoms. The ductility of B2-TiAl alloys with the addition of Cr or Zr is evidently improved relative toγ-TiAl alloys, whereas no toughening effect can be seen for the addition of Nb, Mo or Hf in B2-TiAl alloys.(2) The B2-TiAl-Nb alloys stable existence with Nb content >8.0 at.%. The stability of B2-Ti₅₀(Al_50-xNb_x) alloys decrease as Nb addition increasing and the strength of B2-Ti-Al-Nb alloys enhanced obviously. The ductility of B2-Ti₅₀(Al_50-xNb_x) alloys is the best as the partial substitution of Nb for Al by 9.375 at.%, whereas decrease with the Nb content increase.(3) B2-TiAl-Cr alloys were stable existence with Cr content >9.0 at.%, and the stability of B2-Ti₅₀(Al_50-x-Cr_x) alloys increases as Cr addition increasing, with improving the ductility and enhancing the strengthen of B2-Ti₅₀(Al_50-x-Cr_x) alloys.For the effect of phase stability of B2-Ti₅₀(Al_50-xNb_x) and B2-Ti₅₀(Al_50-xCr_x) crystals. A reasonable explain has been given on the basis of the analysis the density of states (DOS).