用正电子湮没技术研究Zr和Nb在TiAl合金中的行为

Behavior of Zr and Nb in TiAl alloy investigated by positron annihilation technique

测量了TiAl,Ti50 Al4 8Zr2 和Ti50 Al4 8Nb2 的正电子寿命谱 ,并利用正电子寿命参数分别计算了合金基体和缺陷态的自由电子密度。TiAl合金基体的自由电子密度比金属Ti和金属Al基体的低 ,当Ti和Al组成TiAl合金时 ,Ti原子和Al原子的部分价电子被局域化 ,TiAl合金中金属键和共价键共存。TiAl合金晶界缺陷的开空间较大 ,晶界缺陷处的自由电子密度较低 ,金属键结合力较弱 ,材料易发生沿晶脆断。在TiAl合金中分别加入Nb和Zr元素 ,合金基体和晶界的自由电子密度升高 。

The positron lifetime spectra of TiAl, Ti 50 Al 48 Zr 2 and Ti 50 Al 48 Nb 2 alloys were measured. The densities of free electrons of the bulk and microdefects in these alloys were calculated by using the positron lifetime parameters. The density of free electrons is lower in the bulk of TiAl alloy than that of Ti or Al metal which implicates that some valence electrons of Ti and Al atoms are localized when they aggregate to form TiAl alloy. The bonding characteristic of TiAl is a mixture of metallic and covalent. The large open defects occur on the grain boundary in TiAl alloy and the bonding strength of grain boundary is weak due to the low density of free electrons there, and the brittle fracture along grain boundary is prone to occur. As TiAl alloy is alloyed with ternary element of Zr or Nb, the densities of free electrons in the bulk and the grain boundary will simultaneously increase. The brittle fracture along grain bourdary and cleavage fracture of alloys are restrained. [