L.M. Caspers

The application of TEM to the study of helium cluster nucleation and growth in molybdenum at 300 K

Abstract The addition of transmission electron microscopy (TEM) to monitor helium cluster nucleation and growth in molybdenum at ambient temperatures, has provided a useful extension to the information obtained in detailed helium desorption spectroscopy (HDS) studies. In particular, advantage has been taken of HDS helium filling techniques which allow the production of well characterized specimens containing a known number of helium atoms at a controlled low density of helium trapping centres. The problems of very dense damage structures found after conventional helium implantations have thus been avoided. This paper summarizes the filling techniques relevant to the production of TEM specimens together with the main results to date, e.g. the platelet morphology of helium cavities, their growth by loop punching, their transformation to groups of small bubbles, electron beam induced loop punching, and helium trapping at dislocations. The application of these results to the defect structures found in convent...

On the low-temperature nucleation and growth of bubbles by helium bombardment of metals

Abstract A model for the nucleation and growth of helium bubbles at low temperatures is described. The model characteristics are based on recently published results of helium desorption experiments and on atomistic calculations, which indicate that mass transport is achieved by punching out and migration of self interstitials. Simulations are performed up to a helium content of 30 at % resulting in the prediction of the bubble density, bubble volume, fraction of helium in bubbles and point defect concentration. The results show good agreement with experiment and a possible mechanism of blistering is discussed.

Precipitation in low energy helium irradiated molybdenum

Abstract The precipitation of low energy helium injected into molybdenum has been monitored by thermal helium desorption spectrometry (THDS). Precipitates nucleating at vacancies have been studied in the range from n = 1 to n = 2500 He per precipitate. The behaviour of the growing precipitates with respect to helium trapping, self-interstitial capture and emission, and helium release is discussed against the background of results of TEM work done for precipitates with n > 500 He. Evidence is found for (i) “trap mutation” at n = 10 He, i.e. formation of bound self-interstitials around the helium filled vacancy, and (ii) emission of at least one of these “mutation produced self-interstitials” (MPI) at n ∼ 12 He. For larger precipitates (n > 20 He) the emission of single MPI is not apparent. At n = 1000 signs of helium binding to MPI-loops punched out by the precipitates are observed in the spectra.

Clustering of krypton in tungsten observed by helium desorption spectrometry

A helium desorption spectrometer with small desorption volume incorporated in a large multipurpose uhv target chamber is described. With the spectrometer results have been obtained on substitutional Kr in W (100) that are consistent with previous work. Further experimental results obtained with higher Kr dose indicate the occurrence of Kr vacancy clustering. Reduction of these clusters to pairs of substitutional Kr atoms by annealing to temperatures above 1500 K can be observed by the occurrence of new characteristic helium desorption peaks.

Room temperature precipitation of helium in molybdenum observed by tem and THDS; Helium platelet formation

Abstract A combination of two well established techniques, thermal helium desorption spectrometry (THDS) and transmission electron microscopy (TEM), has led to a method of obtaining useful information on the nucleation of helium clusters in molybdenum at room temperature, and on the cluster growth processes. Although previous knowledge of helium behaviour in metals indicated that helium gas aggregates would be three-dimensional, in fact platelets or discs of helium were discovered. Other surprising observations included the increase of platelet thickness by the mechanism of loop punching, and the transformation of individual platelets into tight clusters of small bubbles rather than a single bubble. This paper describes the results in detail and includes details of the THDS results and helium filling techniques, and some theoretical aspects of the platelet nucleation process.

The interaction of He with a 12 〈1 1 1〉 {1 1 0} edge dislocation in W and Mo

Abstract The positions of the metal atoms around a 1 2 〈1 1 1〉 {1 1 0} edge dislocation in Mo and W are calculated using the Wilson-Johnson potentials. The boundary conditions are given by anisotropic elasticity theory. The He-metal potential, also developed by Wilson and Johnson, is used to calculate the position with maximum energy gain for a He-atom.

In-situ TEM observations of loop punching from helium platelet cavities in molybdenum

Abstract In an extension of earlier HDS/TEM work on the nucleation and growth of helium clusters in molybdenum, we have demonstrated as expected that TEM specimens with (011) surfaces can successfully limit the loss of 〈111〉 loops to the free surface. In particular the property that two of the 〈111〉 glide directions lie parallel to the foil surfaces has allowed us to see clearly that the electron beam can induce the in-situ punching of 〈111〉 interstitial loops from some of the helium platelets. Direct electron beam heating of the specimen was eliminated as a mechanism but two other possibilities were suggested, one based on the excitation of helium atoms to increase the internal cavity pressure, the second based on direct electron-molybdenum atom energy transfers to overcome an activation barrier for loop nucleation. Both fit the observations in the sense that the TEM beam density (electrons/unit area) seemed to be the controlling parameter. The TEM electrons also were able to initiate the glide of interstitial loops evidently pinned by helium atoms at the loop periphery. Once glide was possible it appeared to be remarkably easy, a point demonstrated by the frequent observation of loop oscillation along their glide cylinders.

The energy of helium filled platelets and bubbles in molybdenum

Abstract Theoretical models are developed to describe the observed behaviour of helium platelets in molybdenum, which grow and then collapse into clusters of bubbles. Firstly a fully dynamic simulation of helium in a two-dimensional structure is performed. Then simplified models of platelets and bubbles filled with helium are discussed, the calculated energies of which indicate that platelets should be higher in energy for 3 He/vacancy. A cluster of several bubbles is shown, under conditions of high internal pressure, to be of lower energy than a single large bubble due to the elastic interaction between bubbles on the assumption that this is approximately the same in magnitude as for the void lattice.

Argon defect complexes in low energy Ar irradiated molybdenum

Abstract Thermal desorption spectrometry has been used to study the defects created in Mo irradiated along the 〈110〉 direction with Ar ions ranging in energy from 0.1 to 2 keV. In addition to monitoring the release of the implanted Ar, additional information has been obtained by decoration of the defects with low energy helium and subsequent monitoring of the helium release. These studies show evidence that the Ar can be trapped in both substitutional sites and in a configuration in which the Ar is associated with vacancies (ArV n , n ⩾2). Most of the Ar implanted at high energy is released at ≈ 1500 K by thermal vacancy assisted diffusion. Argon trapped closer to the surface is released at lower temperatures via at least three different surface related release mechanisms. Additional results are presented on the interaction of self interstitial atoms (introduced by 100 eV Xe bombardment) with the Ar defects. Substitutional Ar is found to convert to interstitial Ar which seems to be mobile at room temperature. The Ar-vacancy complexes are found to be reduced to substitutional Ar. The results of atomistic calculations of the release mechanisms will also be presented.

NUCLEATION OF HELIUM PRECIPITATES IN NICKEL OBSERVED BY HDS

Abstract Thermal Helium Desorption Spectrometry (HDS) has been used to study the room temperature nucleation of helium precipitates at point defects in Ni(110), notably HeV defects at depth ~ 20 nm below the crystal surface. Helium is injected into the crystal by 50 eV He ion-irradiation which causes no atomic displacements. It has been observed that He n V defects with occupation from n = 2 He to n = 4 He bind helium equally strongly,but weaker than for HeV. For n⩾5 He the binding increases rapidly. The observed behaviour is attributed to helium induced trap mutation and agrees qualitatively with results of atomistic calculations in nickel for this case. Helium precipitation at near surface trapping sites is held responsible for the observed increase of helium release temperatures with helium dose when an undamaged crystal is irradiated. Preliminary TEM observations of Ni specimens irradiated with 50 times higher helium doses than the maximum dose used in the HDS experiments indicated planar clustering of the helium.