| Atomic defects which migrate and trap at impurity probe atoms can be labelled by the changes they induce in the hyperfine interactions of the probe nuclei. Many studies have been made using perturbed gamma-gamma angular correlations (PAC) and the 111In probe because of the excellent resolution of different sites. Identification of the bound states is the key problem in applying hyperfine interactions methods to point defects studies. In this study three structure-sensitive methods are applied to help identify the atomic structures of various multivacancy complexes in Pt and Au: (1) Quadrupole interaction parameters are compared with results of point-charge calculations of electric-field gradients for 20 structures containing 1-4 vacancies in the fcc lattice. (2) Hyperfine interactions induced by decorating vacancy complexes with hydrogen atoms are measured and interpreted with the assistance of point-charge calculations. (3) Transformations between complexes observed by annihilation of vacancies by mobile self-interstitials are used to test the consistency of the identifications. Using these methods in conjunction with analysis of trapping behavior which occurs during annealing of damaged samples, structural models are presented for divacancy (2V), 3V and 4V complexes in Pt, and 3V and 4V complexes in Au. The activation temperatures of the 3V defect in Au and Pt are determined to be 162 K and 390 K, respectively, and activation temperatures of defects in Ni, Cu, Pt and Au are compared. For Pt, trapping of H at 1V and 2V complexes is observed to lead to small changes in the quadrupole interactions, consistent with well-shielded protonic charges. However, trapping at 3V and 4V complexes leads to very large changes which we attributed to atomic restructuring of the defect complexes. Finally, the application of the same methodology to interpret recent experiments on NiAl, an ordered alloy, is described. |
| Atomic difffusion in strain fields near solutes, Steven L. Shropshire and Gary S. Collins, Hyperfine Interactions 79, 755-760 (1993). (7 Mbytes) Annihilation reactions between mobile self-interstitial defects and complexes of vacancies with 111In probe solutes in Au were studied. Measurements were made using the technique of pertubed angular correlations of gamma rays (PAC). Au samples were doped with complexes and plastically deformed at a low temperature to generate fluxes of self-interstitials. Changes in the concentrations of monovacancy (1V) to tetravacancy (4V) complexes induced by annihilation reactions were measured. These are now analysed using a system of coupled first-order equations in order to obtain interstitial annihilation cross sections of the complexes and the fractional amounts of different interstitial clusters in the flux. Relative cross sections obtained for Au are 1.0(1), 3.3(3), 1.2(2) and 7.5(2.5), respectively, for 1V to 4V complexes. The large increase in the cross sections with vacancy number is attributed to a progressive relaxation of the dilatational strain surrounding the oversized In solute as more vacancies are trapped. Also obtained from the analysis are the values 0.34(5), 0.66(7), 0.0(1) and 0.0(2), respectively, for the fractions of mobile 1I to 4I clusters in deformed Au, indicating that di-interstitials are produced more readily than mono-interstitials during plastic deformation. |
| Production and migration of interstitials in deformed metals, Steven L. Shropshire and Gary S. Collins, Hyperfine Interactions 60, 667-670 (1990). (7 Mbytes) An investigation was made of defect production during plastic deformation. Prior to cold-rolling, samples of Au were demaged and preannealed at temperatures in order to maximize site fractions of five different In-vacancy defect complexes, such as In-1V and In-2V. Evolutions of site fractions were monitored by perturbed gamma-gamma angular correlation spectroscopy after additional strains at 77 K. Observed transformations between sites are consistent with defect-antidefect annihilation reactions between the small complexes and mobile interstitials, and indicate a large production of interstitials during deformation. An extended defect complex attributed to a planar faulted loop became glissile under small strain. |
| Diffusion
and trapping of hydrogen in vacancies in platinum studied by PAC,
Gary S. Collins, Steven L. Shropshire and Hwa-Jae Jang, Defect
and
Diffusion Forum 66-69, 335-340 (1989). (1.6
Mbytes)
Interaction of diffusing hydrogen atoms with complexes of vacancies and 111In probe atoms was studied using perturbed gamma-gamma angular correlations (PAC). Trapped H was found to immobilize vacancies. Binding enthalpies of ~0.5 and ~0.3 eV, respectively, were determined for H in divacancy and probable trivacancy complexes. It is proposed that trapped H atoms cause an atomic restructuring of a relaxed trivacancy complex. |
| Point defects in deformed metals studied by perturbed gamma-gamma angular correlations, G.S. Collins, Materials Science Forum 15-18, 783-8 (1987). (0.2 Mbytes) Point defect trapping in deformed fcc metals observed by the PAC technique is reviewed, with attention to the types of traps created, saturation of defect populations with the level of deformation, and enhanced trap formation at anomalously low temperatures. For Al, Ni, Cu and Au, comparison is possible to irradiation data, and it is found that the amount of enhancement increases with the stacking fault energy, with conservative dislocation climb offering a plausible explanation. |