G. Vogl

The nature of point defects in plastically deformed aluminium

Abstract Abstract In order to find out which types of point defect are produced by plastic deformation, 57Co-doped aluminium specimens were cold worked at various tempera tures (4·2, 45, 55 and 80 K) by unidirectional tensile deformation or cyclic torsion. The specimens were then annealed at increasing temperatures, and the trapping of defects at the 57Co probes was studied by Mossbauer spectroscopy. In none of the experiments wore interstitials found, although it is known that in aluminium 57Co probes readily trap interstitialsaftor low-temperature particle irradiation. However, vacancy trapping was found in two temperature ranges corresponding to two different migration energies of vacancies: in dislocations E m d≃ 0·2 to 0·3eV and in the lattice E m≃ 0·6 eV. The consequences of the results on models of point defect production by cold working are discussed.

A quasi-elastic neutron scattering (QNS) study

Abstract Quasi-elastic line broadening of neutrons scattered from 2at.%Co alloyed in β-Zr shows that at about 1100°C all Co atoms, not merely a small fraction, participate in fast diffusion of Co in Zr within a time interval of 10−9s. In the measured temperature range of 950 to 1380°C the deduced jump frequency is about 20 times faster than that of Zr in Zr-2 at.% Co but 8 times smaller than the Co jump frequency deduced from tracer measurements. This discrepancy, an anomalous decrease in intensity, and quasi-elastic neutron measurements at larger energy transfer indicate that a second jump frequency 30 times faster than the first one exists. From a comparison of all available experimental data it is inferred that the slower jump is either via vacancies between substitutional sites or a release jump from a trap, and it is concluded that the faster jump is between interstitial sites.

Direct evidence for several impurity-interstitial configurations in molybdenum

Abstract 3 MeV electron irradiations on Mo57Co Mossbauer samples were performed with various fluences at 100 K irradiation temperature. Using the Mossbauer technique the trapping of interstitials, mobile at irradiation temperature, in different configurations around the 57Co impurities, could be directly observed from the appearance of additional absorption lines in the Mossbauer spectra. Analysing the irradiation dose dependent population of these defect lines, four different stable sites for trapped single interstitials and two di-interstitial sites were found. They are characterized by a nearly unique isomer shift of +0.46 mm/s relative to a substitutional lattice site and large quadrupole splittings ranging from 1.82 to 0.81 mm/s. At higher irradiation doses interstitial clustering at the 57Co impurities was found, resulting in a defect line with 0.39 mm/s quadrupole splitting and a broadened line width. The general trapping behaviour of the Mo57Co system could be described by the unsaturable trap mod...

The nature of point defects produced by cold working of metals studied with Mössbauer spectroscopy and perturbed γ-γ angular correlation

We report on the production of point defects by cold working of Al, Cu, Ag, Pt and Au doped with the Mössbauer impurity57Co. In our experiments no mobile interstitials were observed, while regarding vacancies the defect patterns are similar to those found after quenching and irradiation. The same observations for these hosts were made in recent experiments in which the PAC probe111In was used.

Anisotropy of the Diffusional Broadening of the Mössbauer Resonance in Al 57Co/57Fe

For the first time experimental results give evidence for an anisotropy of the diffusional broadening of the Mossbauer line by long range diffusion. Measurements have been performed on an Al57Co/57Fe single crystal at 918K. Because of the diffusive jumps of the 57Fe-atoms into discrete lattice directions the Mossbauer line broadening shows an anisotropy. The experimentally determined anisotropy is in agreement with the tendency predicted by calculations by Bender and Schroeder for diffusion via nearest neighbour vacancies.

Quasielastic Neutron Scattering Study of Fast Diffusion of Co in β -Zr

As a common feature,group IVb elements — Zr, Ti, Hf — exhibit a martensitic phase transition from the closed packed hcp structure (stable at low temperature) to the more open bcc structure. Diffusion properties in these metals are dominated by this phase transition. For instance the Zr self-diffusion increases by five orders of magnitude at the phase transition at about 840°C. A similar behaviour is known for most of the metal impurities in Zr of which the diffusivities are in the same order of magnitude as for Zr self-diffusion.

RESTRICTED DIFFUSION OF IRON IN ALUMINIUM AFTER ELECTRON IRRADIATION

In earlier work on polycrystalline samples [I] interstitials have been produced by irradiation with 3 MeV electrons above defect annealing stage I. The highly mobile interstitials were trapped at the 57Co impurities and gave rise to a new M~ssbauer line (defect line). From the sudden decrease of the apparent f-factor by a factor of more than four at T>13K it was concluded that the 57Fe atom was performing a restricted diffusion motion in a cage within one A1 fcc cell.