Positron lifetime study on vacancy behavior in bcc-base intermetallics

bcc基金属间化合物Co_2TiAl在高温下保持l2_1型有序结构，具有优异的抗氧化性能，有望作为耐火材料的涂层材料。Co_2TiAl相具有广泛的组成范围，延伸到化学计量组合物中，因此在非化学计量组合物中自然会引入结构缺陷。然而，缺陷的结构尚未被揭示。在这项工作中，我们测量了富co和富al的Co_2TiAl在水淬或炉冷1273K时的正电子寿命。水淬Co_<50.8>Ti_<24.0>Al_<25.2>中的正电子平均寿命约为160ps，与炉冷Co_<50.8>Ti_<24.0>Al_<25.2>中的正电子平均寿命约为120ps，但远大于第一原理计算的完美晶格中正电子平均寿命Co_<50>Ti_<25>Al_<25>中的正电子平均寿命。对于水淬和炉冷试样，χ^2q^<-1>与实验得到的光谱与拟合的单一寿命组分衰减函数之差的平方成正比，接近于一致，这可以解释为由于空位对正电子的饱和俘获。这些结果清楚地表明，在富co和富al的Co_2TiAl中引入了许多大于10^<-4>原子分数的空位，以补偿化学计量的偏差。

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.