Recovery Of Point Defects Research Articles

Strain rate sensitivity: The role of dislocation loop and point defect recovery

The strain rate sensitivity of nominally pure polycrystalline aluminum was precisely measured using a testing system which minimized the machine transient. The strain rate sensitivity vs temperature relation was determined over three decades of strain rate in the temperature range 247–333 K. The scatter in the averaged values was small enough that steps or plateaus in the ascending curve could be detected. The activation energy of 0.53 eV for the initial step is comparable to that of vacancy or divacancy migration. The increasing trend of strain rate sensitivity with temperature and the observed plateaus can be explained if a two obstacle model of discrete barriers is invoked whereby the recovery of the weaker one affects the force-distance relation of the stronger obstacle.

On the derivation of sink strengths in bounded media

Several reasons for the need to extend the rate theory of point defect recovery processes to bounded media are cited. This paper is concerned with the essential first step, the derivation of the strength of sinks near a free surface. The Effective Medium method is used. Sink strengths so derived are shown to be formally the same as those found in an unbounded system. Image effects associated with the surface are proved to be small when the sink is located more than a few sink radii from the surface.

Effects of Hydrogen on Resistivity Recovery of Tantalum Cold-Worked at 4.2 K

High purity tantalum specimens with 15∼50 at.ppm of hydrogen were cold worked at 4.2 K to investigate the recovery of point defects by a resistivity measurement. The resistivity instability, which has been revealed by the resitometric studies of Va metals, was confirmed to be caused by the small amount of hydrogen. A method to controll the state of hydrogen, Precipitated or dispersed, was devised to enable one to perform a stable resistivity measurement at 4.2 K after heat treatments at higher temperatures. The method proved to be useful both for a reproducible resistivity measurement at 4.2 K in hydrogen contaminated Ta and for determining the amount of H in cold worked Ta. Also, the possibility of hydrogen trapping by point defects is suggested.

The annealing of deformed potassium

Abstract Measurements of length-change have been used to study the recovery of high-purity potassium deformed by compression at 1·2 K. There is a large annealing band between 1·2 and about 20 K, the relative length-recovery rate being roughly constant at about 2 × 10−5 K−1, the total length-increase being about 1% of the original plastic compression. The recovery spectrum is quite different from the 15 K point-defect stage observed in resistivity studies on cold-worked potassium. It is shown that the point-defect recovery stage is not affected by dislocation movement involved in the length recovery, which itself reflects a strong temperature dependence of internal stress due to thermal activation over some obstacle to dislocation motion, perhaps the Peierls-Nabarro stress.

〔62〕体心立方金属の照射損傷

Recent development in the study of radiation-induced defects in B.C.C. metals is discussed, with particular reference to the fundamental aspects of the problem. The subjects treated include the rate of point-defect generation and its saturation, point-defect recovery and changes in mechanical properties directly related thereto. A critical study has been made of the stage I defects through internal friction measurements and examination of the resulting data on the Snoek peak and dislocation pinning phenomena, which revealed unexpectedly low values for the energy of free migration of interstitials in W and Mo.

Point defect recovery of shock deformed iron alloys

Point defect recovery of shock deformed iron alloys

Recovery Process in Thin Films of Noble Metals Vacuum-Deposited on Low Temperature Substrate

Recovery of electrical resistivity of pure Au, Ag and Cu thin films deposited in vacuo on a mica sheet kept at liquid helium temperature was studied. Recovery process of Au films consists of several substages below 80°K, which are similar to those found in irradiated Au: The point defect recovery is strongly suggested. In the higher temperature region, both the point defect recovery and grain growth seem to be involved in the recovery process. The activation energies for recovery range from 0.013 eV (at 13°K) to 0.14 eV (at 55°K). The reaction is of the first order at around 25°K. Similar results have been obtained for Ag. In contrast to Au films, Ag and Cu films show remarkable reverse annealing between 20°K and 40°K, which presumably is closely connected with residual gas molecules such as oxygen. Some possible mechanisms of reverse annealing are also discussed.

Low-Temperature Electron and Alpha-Particle Irradiation of Titanium

The recovery of point defects produced by 1.2-MeV electron and 40-MeV $\ensuremath{\alpha}$-particle irradiation has been studied from 50 to 270\ifmmode^\circ\else\textdegree\fi{}K. The recovery spectrum qualitatively resembles that of copper, although shifted to higher temperature. The influence of varying irradiation dose, preirradiation at elevated temperatures, predeformation and dilute alloying were also studied. The recovery below 150\ifmmode^\circ\else\textdegree\fi{}K is dominated by the annihilation of Frenkel pairs, with long-range or uncorrelated migration of the interstitial beginning at approximately 120\ifmmode^\circ\else\textdegree\fi{}K. The recovery of annealed samples irradiated to relatively low dose is characterized by super-recovery. A model involving the migration of interstitial hydrogen impurities from interstitial to substitional sites is proposed to explain this phenomenon.

Point defect recovery in neutron-irradiated tungsten at 0.31 Tm

The results of some resistivity studies of the recovery of defects in neutron-irradiated tungsten provide additional evidence for the recovery of monovacancies at 0.31 T m.

Effect of Plastic Deformation on the Recovery of Stage I after Neutron Irradiation

AbstractThe recovery of point defects in copper after fast neutron irradiation at 15 °K was studied by measurements of residual resistivity. Especially, the effect of deformation before and after low‐temperature irradiation on the recovery spectrum was studied. The amount of stage I recovery was reduced by 20% by deformation after irradiation where the deformation was such that only 2% of atomic planes were swept by dislocations. This result suggests the transformation of some of stage I defects into more stable ones due to deformation and the fairly large distance between the interstitial and vacancy in a close pair of stage I. Also, the recovery spectra observed in the present experiments seem to show the annealing of interstitial clusters in stage II (≈︁ 130 °K recovery peak) in cold‐worked copper.

Recovery of point defects in iron after low-temperature deformation

High-purity iron was severely deformed at 77°K by rolling or drawing under liquid nitrogen. Changes in electrical resistivity were used to measure the recovery during isochronal and isothermal anneals in the temperature range 110–520°K. A sharp recovery stage was observed at 211°K for which the activation energy is 0.55 eV. The recovery followed a kinetic law of the form m (t) m (0) = exp[−(αt) n] where m (t) m (0) is the fraction of defects remaining at time t, and α and n are constants. Between 190 and 220°K, n = 0.43. The annealing stage is attributed to the migration of vacancies (or divacancies) to dislocation sinks. The rate of increase in the mole fraction of these vacancies during straining is estimated to he: ΔN v / Δϵ = 2 × 10 −7 per percent strain.

On the Analysis of Experimental Data on Point Defect Recovery

AbstractA general method is derived for transforming isothermal annealing equations into expressions for the recovery as a result of arbitrary thermal treatment. Three types of treatment are considered in detail: linear continuous heating, linear isochronal pulse heating and hyperbolic isochronal pulse heating. The latter is shown to present some real advantages since in this case the transformation can be made without approximation. It is shown that linear isochronal pulse annealing data may be treated in good approximation by using the expression for continuous linear heating, if a rather severe condition is fulfilled (ΔT ⪅ (k T + 2o/E) and provided that the annealing temperature Tp is replaced by Tp + 1/2 ΔT. By this method the isothermal annealing equations, as derived by Waite for the case of bimolecular diffusion‐controlled reactions between homogeneously‐distributed defects, are adapted for application to isochronal annealing data.

Recovery of point defects in cold worked Au, Ag and Cu

The recovery of the electrical resistivity of polycrystalline, 99.999% pure Au, Ag and Cu, after deformation at −195°C, has been studied throughout the range from −100 to +200°C. The recovery curves after large deformations agree with those in the literature, showing the rather continuous recovery of the stages II, III and IV. However, after strains of a few percent or less, the isochronal curves reveal 4 well-separated stages which we have called a, b, c and d. With increasing strain stages b and c merge, making up stage III of Ag and Cu. In Au this stage is composed of the overlapping stages b, c and d. The migration energy, E, determined by the slope intersection method, increases from 0.4 to 0.8 eV, as recovery proceeds in stage III of Au. For Ag and Cu, the first part of this stage shows increasing E-values (0.2 to 0.5 eV for Ag and 0.5 to 0.7 eV for Gu), whereas the second part reveals a constant energy (0.55 ± 0.03 eV for Ag and 0.72 ± 0.03 eV for Cu). The activation energy for stage IV of Ag is 0.89 ± 0.06 eV. In Cu this stage is too small to determine the energy for the defect migration involved. In addition, it is shown that in the alloys Au + 4.64 at.% Cu, Au + 3.6 at.% Ag, Ag + 15.4 at.% Au and Cu + 3.92 at.% Au, the migration of the defects in the stages and d influences the degree of short range order. The present results are compared with literature data and it is concluded that the c and d defects are di- and single vacancies respectively. The concept of a uniquely activated, second order process for stage III of the noble metals, is criticized.

Effect of Elongation on the Electrical Resistivity of Metals. Part I. Aluminum. Part II. Molybdenum

The changes in the electrical resistivity of metals at 78°K with elongation are investigated. Part I is devoted to the changes introduced in the face-centered cubic metal aluminum upon elongation at 78°K. Part II discusses the changes that occur in the body-centered cubic metal molybdenum upon elongation at 298°K. Both sections describe the relationship between the portion of the resistivity generally attributed to point defects and the resistivity that remains after the point defect recovery has been completed; these resistivities are proportional to each other for both metals. It is noted that for both metals the resistivity caused by point defects and the residual resistivity are directly proportional to the elongation. It is further discovered that the rate of resistivity recovery at 145°C for cold-worked molybdenum is directly proportional to the amount of elongation introduced.