K. P. Gopinathan

Effect of titanium impurities on helium bubble growth in nickel

Abstract Positron lifetime measurements in He-implanted Ni and Ni-Ti alloys containing dilute concentrations of Ti, during isochronal annealing, are reported. In the initial annealing stage of Ni-Ti alloys, only a single lifetime ranging from 160 to 180ps is observed, in contrast with the two lifetimes seen in pure Ni. This indicates saturation positron trapping at helium-bound Ti-vacancy complexes, formed in high concentrations. Lattice statics calculations of the He binding energy at various defect complexes in Ni-containing Ti give credence to the above interpretation. Above 800 K, two lifetimes are resolved in Ni-Ti alloys, where the longer lifetime τ2 increases with a sharp reduction in its intensity. This is indicative of He bubble growth. The bubble radius r B and bubble concentration C B are obtained from an analysis of positron lifetime parameters. These results indicate that, for a given annealing temperature, r B is smaller by a factor of two and C B higher by nearly an order of magnitude in Ni...

Helium bubbles in neutron-irradiated copper-boron studied by positron annihilation

Abstract The influence of helium, introduced by the 10B(n, α)7Li reaction, on the evolution of defect structure in copper containing a few hundred ppm boron has been studied by detailed positron lifetime and two-photon angular correlation measurements, supplemented by TEM studies. In the as-irradiated state of Cu-B, two lifetime components have been resolved. The shorter lifetime, τ1, = 167 ps of 97% intensity, has been understood as due to positron trapping at small helium-vacancy complexes, while the longer lifetime τ2 = 450 ps of 3% intensity is explained as due to helium-free voids. Marked changes in the annihilation characteristics observed at 670 K are interpreted in terms of the nucleation of microbubbles, controlled by thermally activated helium migration to vacancy traps. Corroborative evidence for the onset of helium clustering is obtained from the change in the average size of positron traps as deduced from the smearing of the measured angular correlation spectra. Helium bubbles and helium-free...

Positron annihilation studies of rare-earth mixed valence compounds

Positron lifetime has been measured in SmS under external pressures from 0 to ∼ 16 kbar, and in (Sm0·8Gd0·2)S at temperatures from 10 K to 300 K. An abrupt drop in the lifetime was observed at the valence transition at a pressure of 6·5 kbar in SmS and at a temperature of ∼ 120 K in (Sm0·8Gd0·2)S. Measurement of the Doppler broadening of the annihilation photons was made in (Sm0·8 Gd0·2)S as a function of temperature. The line shape parameter of the Doppler broadened spectrum also showed an abrupt change at the transition. Angular correlation of annihilation photons was measured in the semiconducting phase of SmS and in the metallic phase of (Sm0·8Tb0·2)S. The correlation profile in the mixed valence (metallic) phase was broader than that in the semi-conducting phase. These results have been explained on the basis of the electronic structure of the semiconducting and the metallic phases of these materials.

Effect of internal pressure on helium bubble growth in palladium

Abstract A study of helium bubble growth by positron lifetime measurements made on helium-implanted palladium (100at.p.p.m. He) is reported. The temperature of formation of stable bubbles which retain helium in post-implantation annealing is identified to be 750±20K. The average bubble radius, bubble concentration and helium pressure in the bubble are obtained from an analysis of experimental data. The variation in the deduced bubble parameters in the annealing temperature range 973–1473 K is compared with that calculated on the basis of surface diffusion controlled migration and coalescence model, allowing for pressure effects as suggested by Mikhlin in 1979. The comparison is found to be satisfactory. The observed results are explained as due to suppression of bubble mobility at high internal pressures and subsequent enhancement of the mobility on pressure relaxation.

Positron annihilation study of helium clustering in alpha irradiated copper

Abstract Helium clustering in alpha irradiated copper has been investigated by positron annihilation spectroscopy. Pure copper samples have been homogeneously helium implanted using a cyclotron, yielding helium concentrations of 100 appm and 400 appm. Post-implantation positron lifetime and Doppler broadened annihilation lineshape measurements have been carried out on these Cu samples as a function of isochronal annealing temperature. An annealing stage observed in the isochronal annealing curve viz., a marked reduction in the resolved lifetime τ2 and an increase of its intensity I 2, is explained as due to the formation of helium bubble embryos. At higher annealing temperatures, τ2 corresponding to helium bubbles increases and saturates while its intensity I 2 decreases, indicating an increase in the size of the bubble with a concomitant decrease in the bubble concentration. This stage is interpreted to be the bubble growth stage. From an analysis of positron lifetime parameters in the growth stage, heli...

Positron annihilation studies of SmS and (Sm0.8Gd0.2)S across valence transitions

Abstract Positron life-time measurements have been made in SmS under pressure upto 15 kbar and in (Sm 0.8 Gd 0.2 )S at temperatures from 10 K to 300 K. A discontinuous change in life-time was observed at the valence transition. This is explained on the basis of the changes in the lattice parameter and the electronic structure of these compounds.

A positron lifetime study of cellular precipitation in Al-22 at.% Zn alloy

Abstract The cellular precipitation of β-Zn which occurs on ageing quenched Al-22 at.% Zn alloy at temperatures within the two-phase region has been studied by positron lifetime experiments in conjunction with transmission electron microscopy and X-ray diffraction (XRD). The as-quenched alloy shows a single lifetime component of τ1 = 155 ps. On ageing the as-quenched alloy within the two-phase region, the positron lifetime spectrum is seen to develop a second component, τ2 = 230 ps, whose intensity I 2 increases with increasing ageing time and saturates in about 100 min. The occurrence of the second component in the aged samples is understood in terms of positron trapping at the misfit dislocations produced at the α-Al/β-Zn interface. The increase in I 2 with increasing ageing time correlates with the increase in the volume fraction of β-Zn precipitates as obtained from XRD measurements. The variation in positron annihilation parameters with ageing time, which reflects the kinetics of cellular precipitati...

Critical helium concentration and its effect on bubble growth in palladium

Abstract Helium bubble growth in palladium, implanted with different helium concentrations from 25 to 250 appm, has been investigated by positron lifetime spectroscopy during isochronal annealing. It is found that the bubble growth behaviour for helium above a critical concentration of 100 appm is different from that for doses below it. In palladium with

Helium implantedAlHf as studied by181Ta TDPAC

Time differential perturbed angular correlation (TDPAC) measurement onAlHf reference sample has shown that a fraction 0.88 of probe nuclei are defect free and are occupying the substitutional sites in fcc Al matrix, and the remaining are associated with Hf solute clusters. Measurements on helium implanted sample indicate the binding of helium associated defects by Hf solute clusters. Isochronal annealing measurements indicate the dissociation of the helium implantation induced defects from Hf solute clusters for annealing treatments beyond 650 K. On comparison of the present results with that reported inCuHf subjected to identical helium implantation, it is inferred that the Hf solute clusters inAlHf bind less strongly the helium associated defects than inCuHf.

Cold-worked and hydrogen chargedNbHf as studied by TDPAC and positron lifetime measurements

Time differential perturbed angular correlation (TDPAC) and positron life-time measurements on cold-worked and hydrogen chargedNbHf indicate a strong binding of hydrogen decorated vacancy clusters with Hf impurities. Both TDPAC and positron lifetime results are mutually consistent on this aspect. Transformation of H-vacancy clusters into H-bubbles is indicated by the positron lifetime behaviour at annealing temperatures beyond 900 K. Impurity effects like oxygen pickup and coating of voids cannot be ruled out at elevated temperatures.