中文摘要：

采用第一原理赝势平面波方法计算了D019结构的α2-Ti25Al-xNb（x=012，原子分数，％）晶体的弹性模量（B，G和E）和抗拉强度（σb），并利用Cauchy压力（C12-C44）与C／B比值表征和评判了不同浓度Nb合金化时σ2-Ti-25Al—xNb台金的韧脆化倾向．结果表明：在x=2-12时，σ2-Ti-25A1xNb晶体的抗拉强度（σb）与α2相合金的弹性模量（B，E和G）随Z增加而增大；在x=0-6时，α2-Ti-25A1—ZNb合金脆性有一定改善，且x值越大韧化效果越好；但在x=7—9时，相对于α2-Ti3Al，合金脆性不但没有得到弱化，反而随x增加而加剧；随后，当x进一步增大时，合金脆性又随x增加再次得到改善，至x=12时，α2-Ti25Al-xNb合金的韧化效果最好．通过电子态密度（DOS）和投影电子态密度（PDOS）等电子结构的分析，初步解释了Nb的这种强化与韧化作用．

英文摘要：

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 σb of α2-Ti-25Al-xNb （x=0-12, atomic fraction, %） single crystal with D019 structure and bulk modulus B, Young＇s modulus E as well as shear modulus G of α2-Ti-25Al-xNb polycrystalline alloys have been calculated, and their ductile/brittle behavior is characterized and assessed by the Cauchy pressure （C12-C44） and the G/B ratio. The results reveal the ultimate tensile strength σb of α2-Ti-25Al-xNb crystals and the elastic moduli （B, E, G） of α2-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 α2 Ti-25A1 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 α2 Ti3A1 alloys, and the toughening tendency of α2-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 α2 Ti3A1 alloys no toughening effect can be seen as Ti in Ti3A1 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 α2-Ti-25Al-12Nb alloy is observed as comparing with α2-Ti 25A1-6Nb 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 pr