C. Hidalgo

Positron-annihilation studies of a superplastic Al-Ca-Zn alloy

Abstract The Al‒(5 wt%)Ca‒(5 wt%)Zn alloy has been studied by positron annihilation and transmission electron microscopy. Positron trapping has been observed in specimens of Al‒(5 wt%)Ca‒(5 wt%)Zn from lifetime and Doppler-broadening measurements. The trapping has been attributed to incoherent matrix-precipitate interfaces and to grain boundaries. The results can only be interpreted if the grain boundary contains more than one kind of trap for positrons. This hypothesis is in agreement with the present knowledge of the grain boundary structure.

Positron trapping at grain boundaries

A simple model is developed for the trapping of positrons at grain boundaries. It is shown that there is a linear relationship between any linear annihilation parameter and the inverse grain size. An effective grain boundary width is defined, which depends on the positron diffusion length and on the strength of the grain boundary for positrons. The effect of detrapping on this effective width is also considered. The model is tested by using the experimental results available in the literature.

The recovery of electron irradiated zinc and cadmium by positron annihilation spectroscopy

Positron lifetime and Doppler-broadening measurements have been performed in Zn and Cd specimens irradiated with 3 MeV electrons at 20 K. Isochronal annealing of the irradiation induced defects between 20 and 280 K has been studied. No evidence of three-dimensional vacancy agglomerates was found.

Correlation between positron lifetime results and magnetic behaviour upon relaxation and crystallization of an iron-based amorphous alloy

Abstract Positron lifetime, magnetization work and coercive force have been measured in an as-quenched, relaxed and crystallized Fe 81.5 B 14.5 Si 4 alloys glass. The magnetic softening and the simultaneous decrease of the positron lifetime is discussed in terms of a relaxation of amorphous structure. The positron lifetime is found to increase slightly upon crystallization, supposedly due to trapping at defects at the boundary of small crystallites formed in the beginning of crystallization. The onset of crystallization is accompanied by a rapid increase of coercive force and a less drastic change in magnetization work. The dependence of the positron lifetime on measuring temperature is discussed in terms of the microstructure.

A positron study of a fine-grained Zn-Al-Mg alloy

Abstract Positron lifetime spectra have been studied in the alloy Zn-0.123 wt.% Al-0.063 wt.% Mg for samples having different grain sizes. The results suggest that grain boundaries in this alloy can act as traps for positrons; the results indicate also the presence in the alloy of very stable defects.