Positron lifetime study on vacancy behavior in bcc-base intermetallics

体心立方结构的金属间化合物Co_2TiAl在高温下仍保持L2_1型有序结构，并具有优良的抗氧化性能，有望用作耐火材料的涂层材料。Co_2TiAl相的组成范围很宽，可以跨越化学计量比，因此在非化学计量比时会自然地引入结构缺陷。然而，缺陷结构尚未被揭示。本工作测量了1273K起水淬或水冷富Co和富Al的Co_2TiAl合金的正电子寿命。水冷Co_ Ti_ Al_3中的正电子平均寿命<50.8><24.0><25.2>约为160ps，与水冷Co_<50.8>Ti_<24.0>Al_3<25.2>中的正电子平均寿命相当，但远大于第一原理计算的理想晶格Co_ Ti_ Al_3中的正电子平均寿命120ps<50><25><25>。对于水淬火和真空冷却的样品，χ^2q^<-1>接近于1，它与实验获得的光谱和一个寿命分量的拟合衰减函数之间的差的平方成正比，这可以解释为由于空位对正电子的饱和捕获。这些结果清楚地表明，在<-4>富Co和富Al的Co_2TiAl中引入了许多原子分数大于10^的空位，以补偿偏离化学计量比。

The bcc-base intermetallic compound, Co_2TiAl, is expected to be useful as a coating material on refractory material, because it maintains L2_1-type ordered structure up to high temperature and has excellent oxidation resistance. The Co_2TiAl phase has a wide composition range, extending across the stoichiometric composition and hence constitutional defects have to be naturally introduced at non-stoichiometric compositions. However, the defect structure has not been revealed yet. In this work we have measured the positron lifetimes in Co-rich and Al-rich Co_2TiAl water-quenched or furnace-cooled from 1273K. The positron mean lifetime in the water-quenched Co_<50.8>Ti_<24.0>Al_<25.2> is about 160ps and nearly equal to that in the furnace-cooled Co_<50.8>Ti_<24.0>Al_<25.2>, but is much larger than about 120ps of the first-principles calculated positron lifetime in the perfect lattice, Co_<50>Ti_<25>Al_<25>. For both the water-quenched and furnace-cooled specimens, χ^2q^<-1>, which is proportional to the square of the difference between the experimentally obtained spectrum and the fitted decaying function of one lifetime component, is close to unity, which can be interpreted as due to the saturation trapping of positron by vacancy. These results clearly show that many vacancy of more than 10^<-4> atomic fraction is introduced in Co-rich and Al-rich Co_2TiAl, in order to compensate the deviation from the stoichiometry.