N. de Diego

Current positron studies of structural modifications in age-hardenable metallic systems

The sensitivity of positron annihilation characteristics to the structural changes occurring in supersaturated metallic systems has been demonstrated and explained since the early 1980s. This knowledge is currently applied for positron annihilation spectroscopy (PAS) investigations regarding the main phenomena that govern the structural evolution of a supersaturated alloy when various thermal treatments are applied: (a) kinetics of vacancy-type defects retained after quenching; (b) solute migration; (c) formation of solute clusters and/or GP zones; (d) precipitation. The analysis of PAS data gives information on the chemical composition, the defect structure and the density of decomposition products of nanometric size. The present article, that includes an exhaustive repertory of works published since 1987, discusses the value of the contribution that PAS studies can give to physical metallurgy in the field of age-hardenable systems. Most examples are taken from the direct experience of the authors.

Mobility of interstitial clusters in alpha-zirconium

AbstractClusters of self-interstitial atoms (SIAs) formed in displacement cascades in metals irradiated with energetic particles play an important role in microstructure evolution under irradiation. They have been studied in the fcc and bcc metals by atomic-scale computer simulation, and in this article, we present the results of a similar study in a hexagonal close-packed (hcp) crystal. Static and dynamic properties of clusters of up to 30 SIAs were studied using a many-body Finnis-Sinclair type interatomic potential for Zr. The results show a qualitative similarity of some properties of clusters to those for cubic metals. In particular, all clusters larger than four SIAs exhibit fast thermally activated one-dimensional (1-D) glide, which is in a 〈11

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

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

0〉 direction in the hcp lattice. Due to the structure of the hcp lattice, this mechanism leads to two-dimensional mass transport in basal planes. Some clusters exhibit behavior peculiar to the hcp structure, for they can migrate two-dimensionally (2-D) in the basal plane. The jump frequency, activation energy, and correlation factors of clusters have been estimated, and comparisons drawn between the behavior of SIA clusters in different structures.

The stabilization of martensite in Cu−Al−Mn alloys

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.

On the structure and mobility of point defect clusters in alpha-zirconium: a comparison for two interatomic potential models

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.

On the interaction between a vacancy and self-interstitial atom clusters in metals

Abstract A many-body potential model based on the second moment of the tight-binding approximation is proposed for computer simulation of lattice defects in Zn. The constructed potential satisfactorily reproduces the experimental data: cohesive energy, equilibrium lattice constants, vacancy formation energy and elastic constants, except C 33. Mechanical and structural stability has been ensured and the stacking-fault energy has been determined to be close to the lower end of experimental estimates. As an application of the model, the monovacancy migration energies and the formation energy of single self-interstitials are calculated. A significant anisotropy of vacancy migration and a low value of interstitial formation energy is found, in full accordance with the results of previous experimental and theoretical studies. The shortcomings of spherically symmetric potentials applied to the hcp metals with c/a ratios strongly deviating from the ideal value are discussed.

Features of the interactions between a vacancy and interstitial loops in metals

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.