Study of the Mg-Cd system by positron annihilation methods

Abstract

The problem of preferential positron annihilation in binary alloys has not been satisfactorily solved in the past. We examine this effect experimentally in Mg-Cd alloys using the new technique of Doppler broadening measurements with background reduction which allows us to observe positron annihilation with core electrons. Conventional positron lifetime spectroscopy is applied as well. We measure the Doppler spectra and positron lifetimes for selected alloys in the whole concentration range of the Mg-Cd system. The analysis of experimental results is supported by theoretical calculations. The conclusion is given that a small positron preferential annihilation occurs at Mg sites. Besides, the temperature dependencies of the peak counting rate of the angular correlation curve are measured for several samples and the vacancy formation energies are established. In the case of Mg-20 at.% Cd alloy, the increase of the peak counting rate characteristic for generation of thermal vacancies shows a discontinuity close to the order-disorder transition temperature.

Reverse transformation of deformation-induced martensite in austenitic stainless steel studied by positron annihilation

“…PAS was applied to determine vacancy migration energy in SS [37,38]. Sensitivity of PAS is sufficient for investigation of the phase transformations in different materials [39,40], and this technique has been profitably applied in microstructural characterization of steels which are candidates for structural materials in nuclear facilities [41,42].…”

Preferential Positron Annihilation in Binary Alloys

“…In our previous studies [2,3] we emphasised the effect of the volume dependence of the annihilation rate when evaluating the PPA. Here we use the well known Brandt and Reinheimer formula [10] (λ = 2 + 134n_ (λ is in ns -1 and n-in atomic units, a.u.))…”

“…Here we use the well known Brandt and Reinheimer formula [10] (λ = 2 + 134n_ (λ is in ns -1 and n-in atomic units, a.u.)) to estimate the volume dependence of the annihilation rate in Sn-In alloys (see [2,3] for details). The result of this procedure is also shown in Fig.…”

“…The influence of the change of the atomic volume with the alloy composition was already mentioned above. Even if such an effect may be neglected (as in the Sn-In case) and the concentration dependence of the annihilation rate does deviate from the straight line (see [2,3]), it is still not clear at which sites the PPA occurs and a theoretical analysis of the contributions of alloy constituents to the total annihilation rate has to be done. Possible positron traps represent a further important point as their influence should be excluded from resulting experimental annihilation rates, which may not be always possible [8].…”

“…In our previous papers we studied preferential positron annihilation in Fe-Al [2] and Mg-Cd [3] systems. Here we present preliminary theoretical and experimental results for Sn-In alloys and discuss them together with results previously found in Fe-Al and Mg-Cd.…”