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

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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.