Study of Point Defects in B2-type Ordered Alloys by Means of Positron Annihilation

近年来，金属间化合物或有序合金作为新型功能材料在形状记忆合金、含氢金属、半导体材料等领域得到了广泛的应用。此外，它们还用于高强度、轻质、耐热的材料。在这类材料中，晶格缺陷通常会降低它们的效率，因此需要全面了解和控制缺陷的浓度和分布状态。本文利用正电子寿命测量方法研究了有序合金中空位和反结构原子等点缺陷的成分/温度依赖性。结果表明：CuZN(B_2结构)：热平衡空位浓度较高，平均寿命饱和出现在有序-无序转变温度T_c(740)K以下。在T_c附近，由于有序度的降低，平均寿命增加了约5ps。Au-Cd(B2)：Au-47.5%Cd合金在300K附近发生马氏体相变，平均寿命与温度的关系可以用我们估计的缺陷浓度来描述。马氏体相的空位生成能和空位迁移能均高于母相。因此，在马氏体相向母相的反向相变过程中，空位浓度急剧增加。在马氏体相变过程中，母相中大量空位被保留在马氏体相中。这些多余的空位是马氏体相出现时效现象的原因。Cu3AU(L12)：由于晶格参数的增加和原子排列的改变，正电子平均寿命增加了约2ps。高温无序态空位的形成能为1.42&lt；正负0.09 eV。

Recently, intermetallic compounds or ordered alloys, are being used in verious fields as new functional materials such as shape memory alloys, hydrogen-containing metals, semiconducting materials and so on. Also, they are used in high-strength, light-weight, heat-resistant materials. In such materials, the lattice defect usually reduces their efficacy, it is desired to fully understand and control the concentration and the state of distribution of the defects.In this present study, the composition/temperature-dependence of point defects such as vacancies and antistructure atoms (ASA) in ordered alloys were investigated by means of the positron lifetime measurement. The results are summarized as follows:CuZn (B2-structure):The concentration of vacancies in thermal equilibrium is fairly high, and the saturation of the mean lifetime occures below the order-disorder transition temperature,Tc (740)K). At temperatures near Tc, the mean lifetime increases by about 5 ps, owing to the decrease in the degree of order. The temperature dependence of the mean lifetime can be delineated by using the defect concentrations estimated from our method which gives the equilibrium concentration of defects.AuCd (B2):Au-47.5%Cd alloy shows martensitic transformation at around 300K. Both the formation and the migration energy of vacancies are higher in martensite phase than in parent-phase. Consequently, the vacancy concentration increases aburptly on the reverse transformation from martensite to parent phase. On martensitic transformation, the high concentration of vacancies in parent-phase are retained in martensite-phase. These excess vacancies are responsible for the aging phenomena appearing in martensite phase.Cu_3Au (L1_2):The positron mean lifetime increases about 2 ps owing to the increase in the lattice parameter and also the change of atom arrangement. The formation energy of a vacancy in high-temperature disorderedstate is found to be 1.42<plus-minus>0.09 eV.