Positron annihilation in deformed Co–Ni alloys

Positron lifetimes have been investigated in a series of deformed Co–Ni alloys in an attempt to identify the mechanism involved in the annihilation process at dislocations. Isochronal annealing measurements show that, although the mechanism is complex in the deformed alloys before annealing, a unique defect state may be attained which gives rise to a lifetime of 140 ps, This value is similar to the lifetime related to monovacancies suggesting that annihilation occurs at a vacancy trapped by a dislocation. The dislocation density was determined by transmission electron microscopy and the trapping rate per unit dislocation density was found to be 2 × 1015 s−1. The phase transition between the fcc and hcp structure for two of the alloys is shown to introduce defects.