J. del Río

Position annihilation in Kapton source-supporting foils

The fraction of positrons annihilated in the Kapton support of a positron source has been measured as a function of the atomic number, Z, of the sample studied. The data have been filtered by both exponential and logarithmic laws obtained from a backscattering model. Two formulas are given to correct the source contribution in a positron lifetime experiment. The results shown that the exponential fit is satisfactory for low atomic numbers while a logarithmic fit is better for Z>30.

Postirradiation recovery of a reactor pressure vessel steel investigated by positron annihilation and microhardness measurements

Positron lifetime and microhardness measurements have been performed on untreated, thermal-aged, neutron-irradiated, and postirradiation-annealed samples of reactor pressure vessel steels with the purpose of investigating the mechanisms of irradiation-induced hardening and recovery of the mechanical properties in these materials. The positron lifetime experiments have not revealed any evidence of the formation of a significant concentration of voids or vacancy clusters in samples irradiated at [approximately]290 C with fluences [le]2.71 [times] 10[sup 23] n/m[sup 2] (E[gt]1 MeV), but they suggest a dislocation annealing induced by the irradiation. Isochronal annealing experiments with neutron-irradiated samples show a simultaneous recovery in their positron lifetime and microhardness at [approximately]340 C. From the microhardness measurements, the yield strength of the irradiated material has been estimated. The results appear to be consistent with a model of hardening due to irradiation-induced dissolution of precipitates with formation of small metastable precipitates after postirradiation aging and recovery induced by the disappearance of these metastable precipitates.

A positron annihilation study of the formation and dissolution of L12 precipitates in Al—Li alloys

Abstract The formation and dissolution of δ′ precipitates has been followed by positron annihilation spectroscopy in two Al-Li alloys (Al–9·9 at.% Li and Al-8·6 at.% Li-0·04 at.% Zr). Lifetime measurements have been performed by annealing the samples from the as-quenched state to 750 K, which is well above the dissolution temperature for the δ′-phase Al3Li. The results suggest that small δ′-like regions are present in the as-quenched state, and that they dissolve at around 430 K, in agreement with previous results in the literature. The technique has proved to be very sensitive to the growth and dissolution of δ′ precipitates owing to the high positron affinity to Li in an Al matrix.

The stabilization of martensite in Cu−Al−Mn alloys

The stabilization of martensite in Cu-Al-Mn shape memory alloys has been studied by measurement of the reverse transformation temperatures after various heat treatments and by positron annihilation spectroscopy. The martensite in the Cu-Al-Mn system rapidly stabilizes at room temperature after quenching from a high temperature in the β field. Positron annihilation measurements indicate the presence of excess quenched-in vacancies in the as-quenched sample. This vacancy excess is eliminated by heating in the β field just above the Af, temperature. Stabilization in the Cu-Al-Mn system, as in the Cu-Zn-Al system, thus arises either by pinning of the martensite plates by the quenched-in vacancies or by changes in configurational order, made possible by the increase in diffusion rate caused by the excess vacancy concentration.

A positron study of sintering processes in ZnO-based ceramics

Positron annihilation spectroscopy has been used to study the sintering processes in pure and Bi-doped ZnO. The lifetime measurements in the as-pressed specimens reveal the presence of a long component, this being ascribed to pores. By following the evolution of the annihilation parameters as a function of the annealing temperatures between 673 and 1573 K it is suggested that a satisfactory sintering occurs above 1373 and 1173 K for pure and Bi-doped ZnO, respectively. From the lifetime measurements, the activation energy for the sintering processes in pure ZnO is found to be QR=(360+or-10) kJ mol-1, which is in agreement with previous values obtained from densification measurements.

Li clustering in Al-Li alloys studied using positrons

The behaviour of two Al-Li dilute alloys having two different Li contents (1.7 and 3.7 at.%Li) has been studied using positron annihilation. The lifetime value is about 20 ps higher than that measured for pure Al and is independent of the Li content. When the specimens are cold rolled the lifetime increases to a value that is lower than the one currently associated with dislocations in Al and does not depend on the Li concentration. After isochronal annealing the lifetime achieves the initial value tau =185 ps. The results are interpreted in terms of Li clustering.

The influence of Li on the nucleation of defects of quenched AlLi alloys

Abstract The nucleation and kinetics of defects formed by quenching in two AlLi alloys having Li content of 1.7 and 3.74 at.%Li have been studied by positron annihilation spectroscopy. It has been found that the defect formation is sensitive to the aging time at the temperature from which the samples are quenched. This fact has been related to the Li loss experienced by the alloys aged at high temperature. The quenched-in defects have been identified as vacancy-Li clusters and dislocation loops. The latter are formed by the collapse of the Li-rich vacancy complexes and are very sensitive to the Li content; as a consequence, the loops are decorated by Li-rich zones and are revealed as very effective positron traps in comparison to the vacancy-Li complexes, giving rise to an enhanced trapping.

A positron study on the microstructural evolution of Al-Li based alloys in the early stages of plastic deformation

The formation of voids by coalescence of microvoids initiated at precipitates has been proposed to explain the fracture mechanisms in alloys containing a large number of second phase particles whereas in binary Al-Li alloys with shearable particles the brittleness could be linked with the grain boundary fracture. Most of the microstructure studies of Al-Li alloys have been performed by deforming to fracture; however, little is known about the processes and mechanisms involved in the early stages of plastic deformation. Butler et al. have studied a quaternary Al-Li alloy and have found that there is a critical effective strain to cause voiding, which is about 0.06 and 0.1% for the aged and for the solution treated material respectively. It is very well established that positrons are very sensitive to vacancy-like defects. With the aim of clarifying the behavior of Al-Li based alloys in the very early stages of deformation, and detecting the eventual formation of microvoids, the authors have studied the response of the positron lifetime parameters to the degrees of deformation in age-hardenable Al-Li based alloys plastically deformed under tensile stress.

Temperature dependence of positron trapping at grain boundaries

Positron lifetime spectra were measured for fine-grained samples of the superplastic alloy Al - 5 wt% Ca - 5 wt% Zn at temperatures from 10 to 295 K. The lifetime attributed to annihilation from traps at the grain interfaces was found to increase with the temperature, while the corresponding intensity was observed to decrease. The quantitative analysis of the experimental results according to the diffusion-trapping model (Dupasquier et al 1993 Phys. Rev. B 48 9235) leads to the following conclusions: (a) the positron diffusion coefficient in the alloy matrix (a solid solution of Zn and Ca in Al) is limited by positron - phonon scattering as well as by positron - impurity interaction; (b) the phonon-associated term in the reciprocal of the diffusion coefficient is dominant at room temperature and scales at other temperatures with the same power law as holds for pure Al (Soininen et al 1990 Phys. Rev. B 41 6277); (c) the term associated with positron - impurity scattering is small except at very low temperatures, but the positron - impurity interaction seems to give a localization effect that is more important than the scattering; and (d) the specific trapping rate at the interface has a negative temperature dependence, as expected for trapping mediated by a precursor shallow state.

The nature of defects in electron irradiated and deformed indium

Abstract Positron annihilation spectroscopy is used to study the recovery behaviour in electron irradiated In. A long component of 270 ps in the as-irradiated samples has been attributed to monovacancies produced during the irradiation, which anneal out at 100 K to give rise to dislocation loops. The results obtained are compared with previous works performed in low temperature deformed In, concluding that the plastic deformation in In is mainly due to twinning.