Interstitial Impurity Atoms Research Articles

Atomic ordering in ternary alloys with hexagonal-close-packed structures. II

This paper constitutes a theoretical study of the influence of interstitial impurity atoms on the type of order-disorder transition in an alloy of the AB3-Cin type with an hcp structure. Allowance is made for a variety of types of interstices over the range 0 ≤η 1. The free energy is calculated by the Gorskii-Bragg-Williams method. In the presence of an impurity there may be a change in the magnitude of the jump taking place in the degree of long-range order at the transition point, and also a change in the type of order-disorder transition occurring.

Snoek Peak of Neutron-Irradiated Fe-3.48% Ni Alloy

The Snoek peak of a neutron-irradiated Fe-3.48% Ni alloy was measured to determine the resolution of interstitial impurity atoms. The internal friction was measured after isothermal annealing on specimens irradiated at 75° ± 10°C to a dose of ∼3.6 × 1019 nvt. The change of the Snoek peak height, which corresponds to the amount of nitrogen in solution, was analyzed upon assuming the applicability of the rate equation. As a result of calculation, the order of the reaction was found to be about 2.6, and the activation energy for resolution of nitrogen atoms about 2.9 eV.

Elastic interaction between a dilatation centre and split dislocations in b.c.c. metals

The elastic interaction energy between a dilatation centre and 1/2[111] dislocation split on {110} planes in b.c.c. metals is calculated for sessile and planar splittings of screw dislocation and for planar splitting of edge dislocation; the splitting is considered as a model of the dislocation core. It is concluded that the interstitial impurity atom has three or four possible minimum energy positions in the dislocation core. A hypothesis on the impurity jumps in the dislocation core is proposed.

Snoek Peak of Neutron-Irradiated Fe-3.48% Ni Alloy

The Snoek peak of a neutron-irradiated Fe-3.48% Ni alloy was measured to determine the resolution of interstitial impurity atoms. The internal friction was measured after isothermal annealing on specimens irradiated at 75° ± 10°C to a dose of ∼3.6 × 1019 nvt. The change of the Snoek peak height, which corresponds to the amount of nitrogen in solution, was analyzed upon assuming the applicability of the rate equation. As a result of calculation, the order of the reaction was found to be about 2.6, and the activation energy for resolution of nitrogen atoms about 2.9 eV.

Orientation effects in interaction of screw dislocation with interstitial impurity atom in b.c.c. lattice

The cross-slip and pinning of a 1/2a〈111〉 screw dislocation in b.c.c. metals in the vicinity of an interstitial impurity atom are studied in dependence on crystal orientation. To this purpose, the interaction energy between the dislocation and an interstitial atom is calculated in an anisotropic elastic continuum and it is assumed that the screw dislocation moves microscopically on {112} or {110} planes between its stable configuration positions in b.c.c. lattice.

Thermal embrittlement of 18 Ni(350) maraging steel

The embrittlement of as-solutionized 18 Ni(350) Maraging steel was monitored as a function of heat treatment variables by means of Charpy impact tests. The processing parameters of interest were annealing temperatures in the range of 1900° to 2400°F, intermediate holding temperatures in the range of 1300° to 1800°F, and the quenching rate. The changes in fracture mode with heat treatment were characterized by replica and scanning electron microscopy. The severity of thermal embrittlement increases with decreasing cooling rate from the annealing treatment upon direct quenching to room temperature. Intermediate isothermal holding, particularly at 1500° to 1600°F, further accentuates the embrittlement. A large grain size is beneficial to the toughness when rapid direct quenches from the annealing range are imposed but is detrimental upon air cooling or intermediate holding. The major loss in toughness may be associated with the diffusion of interstitial impurity atoms (C+N) to the austenite grain boundaries during cooling or intermediate isothermal holding below 2000°F. An advanced stage of the embrittlement is characterized by the discrete precipitation of Ti(C,N) platelets on these boundaries. Thermal embrittlement is accompanied by change in fracture mode from transgranular dimpled rupture to intergranular quasi-cleavage.

The effect of neutron irradiation and oxygen interstitials on the dislocation dynamics in vanadium

The results obtained in the present investigation of neutron irradiated vanadium indicate that it is possible to have an increase, decrease, or no change in the temperature dependence of the yield stress, depending upon a low, intermediate or high interstitial concentration, respectively. From a consideration of the activation volumes of neutron-irradiated high-purity vanadium, the spacing of the irradiation produced defect was found to be small, i.e.≅10b. Also, the strain associated with the defect was found to have a maximum value of 0.48. It was also determined that there is a single rate-controlling mechanism of slip in neutron-irradiated, high-purity vanadium. Another result obtained in this investigation demonstrated the magnitude of interstitial impurity atoms necessary so that the solution hardening is the strengthening mechanism. The results indicate that in the case of annealed vanadium an oxygen concentration of 15–30 wt.-p.p.m. does not affect intrinsic lattice hardening, while 200 wt.-p.p.m. already causes solution effects.

Annealing of point defects inp-type germanium after electron irradiation at liquid nitrogen temperature

Abstract Annealing of radiation induced defects in p-type germanium was studied by measuring Hall coefficient and conductivity. The dopant was gallium or indium. It was concluded that the annealing stage between 80° and 140°K is caused by migration of the vacancy to the sink of an impurity atom. In this stage the vacancy migrates to a substitutional impurity atom and makes an association. The activation energy of the stage was found tO be 0.1 ev ad it is regarded to be that of the vacancy migration. The model for the annealing stage which occurs in the range 220 to 270°K is proposed as follows: An interstitial impurity atom migrates to a substitutional impurity atom and makes an association. From the activation energy of the stage, the migration energy of the interstitial impurity atom was concluded to be about 0.4 eV for gallium and 0.7 eV for indium atoms.

Effect of Hydrostatic Pressure on Excess Carrier Lifetimes in Germanium

The effect of hydrostatic pressure up to 10 000 kg/cm2 on excess carrier lifetime in single-crystal samples of n- and p-type germanium has been studied over a temperature range of 200°–350°K. The results of the experiments are interpreted in terms of existing theories of charge carrier recombination via recombination centers, and expressions for the variation of recombination level energies within the forbidden energy gap as a function of pressure are inferred. These variations are understandable on the basis of a simple Bohr model of defect levels in a dielectric medium whose dielectric constant varies with pressure. Tentative suggestions relating the recombination centers in the experimentally investigated samples to the presence of interstitial and substitutional impurity atoms are advanced.

Peierls-Nabarro hardening in the presence of point obstacles

In light of the controversy that has existed between proponents of Peierls-Nabarro hardening and dispersed barrier hardening with regard to possible low temperature deformation mechanisms in bee and hep metals, this paper examines the effect of point obstacles on Peierls-Nabarro hardening. The statics of double kink nucleation over Peierls barrier on finite dislocation segments are studied in detail. Point obstacles such as interstitial impurity atoms are considered to limit the length of the dislocation segments. Adopting the rate theory approach, temperature dependence of the yield stress is then investigated as a function of the concentration of point obstacles. It is found that point obstacles have little or no effect on double kink nucleation processes, when the applied stress is near the Peierls stress or at low test temperatures. At lower applied stresses or at higher test temperatures, the present results are significantly different from predictions of the Dorn-Rajnak calculation. In the present theory, the applied stress on the dislocation, τ, is found to be finite and independent of test temperature in dilute solid solutions at elevated temperatures where the Dorn-Rajnak theory predicts vanishing τ. This apparent “athermal” component of τ increases linearly with the concentration of point obstacles, whereas temperature dependent part of τ decreases. These predictions are consistent with experimental observations on iron-base alloys.

Photoconductivity Studies of Defects inp-Type Silicon: Boron Interstitial and Aluminum Interstitial Defects

Two defects introduced in $p$-type silicon by 1.5-MeV electron irradiation are studied by means of infrared photoconductivity, including the measurement of the stress-induced dichroism. They are identified as being dopant atoms in interstitial positions produced by the silicon interstitial-impurity atom replacement mechanism proposed by Watkins. They are introduced by room-temperature irradiations as well as by irradiations performed at 77, 20.4, and 4.2\ifmmode^\circ\else\textdegree\fi{}K. They disappear during annealing at temperatures \ensuremath{\sim}250-300\ifmmode^\circ\else\textdegree\fi{}C. The symmetry of these defects ${C}_{3v}$ is deduced from the low-temperature stress-induced dichroism of the photoconductivity which is associated with electronic reorientation among different configurations. This ${C}_{3v}$ symmetry can be explained by distortion of a possible Jahn-Teller type of a configuration in which the dopant atom was originally in a tetrahedral position. The defect response to the stress is determined by the value of the term in the piezospectroscopic defect tensor which characterizes the relative change in defect energy per unit strain. This value is \ensuremath{\simeq} -12 eV/(unit strain). Numerical values of the dichroic ratios show that the photoconductivity transition which is observed corresponds to a distribution of dipole moments which is an ellipsoid of rotation about the trigonal axis of the defect. They also allow the determination of this distribution.

On the mechanism of interstitial-impurity-induced cross-slip in iron deformed near 175°K

A mechanism is proposed whereby the elastic dipole distortion of stationary interstitial impurity atoms in lattice solution promotes nucleation and stabilization of double-kink-size screw dislocation segments on cross-slip planes. Stress relaxation accompanying interstitial-impurity-induced cross-slip can account for the experimental observations that small atom fractions of nitrogen in lattice solution in iron deformed at temperatures near 175°K decrease yield and flow stresses and increase ductility.

Point defects in vanadium

Abstract After either plastic deformation or fast neutron irradiation, electron beam zone-refined vanadium exhibits an electrical resistivity recovery peak at about 170°C. The activation enthalpy associated with the peak, which is approximately 0·8 ev, is significantly different to the activation enthalpies for the diffusion of carbon, nitrogen, oxygen and hydrogen in vanadium. Thus it is concluded that the peak is probably due to the annihilation of some species of lattice point defect rather than to the redistribution of interstitial impurity atoms.

Surface Superconductivity in Tantalum

The superconducting behavior of tantalum wire in a longitudinal magnetic field was studied by means of electrical-resistance measurements. The results were compared with the predictions of the Saint-James and DeGennes theory. Under certain conditions the critical field for surface superconductivity as well as a higher critical field were observed. The persistence of superconductivity in fields higher than the critical field for surface superconductivity was attributed to the presence of clusters of interstitial impurity atoms in the material. The effects of strain on these critical fields were also explained on the same basis.

The Kinetics of Migration of Impurities to Small Dislocation Loops

The initial kinetics for the migration of interstitial impurity atoms to a distribution of small dislocation loops has been evaluated and compared with the measured kinetics of Stage III recovery in neutron-irradiated niobium containing oxygen impurity. Both the theoretical calculation and the relevant electrical-resistivity measurements are presented. The agreement is very good and provides strong support for the assumption that the Stage III recovery is due to the migration of oxygen to the dislocation loops introduced by the irradiation.

Defects in neutron irradiated molybdenum

Molybdenum of purity 99.965% has been examined by transmission electron microscopy and field ion microscopy before and after neutron irradiation to 1 × 10 19 n.v.t. at reactor ambient temperature. The structure after 1 hr anneal at 760°C following irradiation was also investigated. Dark field strain contrast analysis has shown that the defects induced by irradiation are in the form of small (~100 Å dia., grown to ~200 Å dia. after the anneal) perfect prismatic dislocation loops of the {110} a 2 〈111〉 type. The loops are identified, within the limits of strain contrast theory, to be mostly interstitial in character. Field ion micrographs taken of specimens made from foils previously examined by electron microscopy, failed to reveal any vacancies, although images characteristic of interstitials and/or interstitial impurity atoms were obtained. The results are discussed in terms of the known irradiation behavior of molybdenum.

The cold-work damping peak in alpha-iron

The cold-work damping peak in poly- and single-crystal iron specimens was studied. The essential problem on the mechanism of the cold-work peak is whether an anelasticity arises from reorientations of interstitial impurity atoms or from motions of dislocations. Models based on the former seem to disagree with results of this investigation, which show no significant orientation dependence of the height of the cold-work peak, and also disagree with those of measurements of interstitial impurity dependence. We proposed two complements to Schoeck's model by taking into account the effect of internal stress due to dislocation interaction and clustering of impurities. Thus various experimental results of the cold-work peak can be explained.

The influence of interstitial impurity atoms on the recovery behaviour of cold-worked vanadium

Vanadium containing oxygen and nitrogen shows, after deformation at room temperature by rolling, two distinct recovery stages at annealing temperatures of about 450 and 550° K. Measurements of electrical resistivity and of low frequency damping were made. By variation of the gas content of the samples, it was established that the stage situated at 450° K is caused by the migration of oxygen atoms to dislocations, whereas at 550° K nitrogen atoms migrate to dislocations. Two small steps occurring at 340 and 400° K can be attributed to ordering of oxygen and nitrogen atoms in the dislocation stress fields.

Neutron irradiation damage in molybdenum

Abstract The technique of transmission electron microscopy has been used to study neutron irradiation damage and its behaviour on annealing in molybdenum. Micro-hardness tests were carried out on all specimens before thinning to obtain some indication of the relation between structural effects and the mechanical properties. The results show that part of the damage in molybdenum neutron irradiated to a dose of 2 × 1022 n cm−2 (fission) is in the form of point defect clusters which are deduced to be interstitial. The interstitial point defects must therefore be sufficiently mobile during irradiation at 60°c to form into these clusters. However, the experimentally estimated point defect density only accounts for a small proportion of the theoretical number suggesting that most of the interstitials are present singly or as sub-microscopic clusters. On annealing there is little change in the observable damage up to 800°c but the micro-hardness shows a peak at 170°c followed by steady recovery. Above 800°c the loops grow rapidly and finally join up to form dislocation networks. This stage is accompanied by an increase in the rate of recovery of the micro-hardness. The loops are shown to be interstitial in nature with Burgers vector a/2〈111〉, though during the early stages of network formation some 〈100〉 type dislocations form through the reaction:a/2[111]+a/2[11 1]→a[100]. These results are discussed in conjunction with other more indirect recovery data and a model is proposed for the annealing of irradiation damage in molybdenum above room temperature. The general conclusion is that the initial damage structure and its subsequent behaviour on annealing are largely governed by the distribution and mobility of interstitial impurity atoms during irradiation.

Stress relaxation prior to the yield point in niobium and molybdenum

Stress relaxation, i.e. the drop in load due to plastic flow at constant strain, has been used as a technique to study the phenomenon of plastic strain in the pre-yield region of the load-elongation curve. Results from experiments on niobium and molybdenum have been interpreted to mean that at room temperature some dislocations are free to move at very low stresses. The amount of plastic strain due to the movement of these free dislocations at any given stress level, below the upper yield stress, was found to be a function of loading-rate. The movement of the free dislocations was inhibited by the presence of interstitial impurity atoms, defects produced by irradiation damage and by debris and dislocation tangles produced by cold work. It was also concluded that the defects produced by irradiation damage, which anneal out in the temperature range 140–200°C, form clusters on annealing rather than condensing onto dislocation lines. It was inferred from these experiments that the yield point phenomenon results from dislocation multiplication rather than unpinning from impurity atmospheres.