A. Seeger

The Flow Stress of Ultra-High-Purity Molybdenum Single Crystals

The paper investigates the dependence of the flow stress σ of ultra-high-purity molybdenum single crystals on temperature T, strain rate, and crystallographic orientation of the crystal axis. The cyclic-deformation technique developed by Mughrabi and Ackermann allowed a complete set of flow-stress data (covering the temperature range 125 to 460 K at 15 different shear strain rates, varying from 5.9 × 10—7 to 1.7 × 10—3 s—1) to be obtained on one and the same specimen. The two crystals investigated, with Schmid factors μ{110} = 0.50 and μ{112} = 0.43, or μ{110} = 0.40 and μ{110} = 0.43, respectively, had residual resistivity ratios exceeding 2.5 × 105, the main impurity being W. The data are in excellent agreement with the theory of flow-stress control by kink-pair formation and kink migration. It is shown that for both orientations the elementary glide steps of the a0 〈111〉/2 screw dislocations controlling the flow stress occur on {112} planes. The formation energy of a pair of isolated kinks on a {112} plane is 1.27 eV, in perfect agreement with the activation energy of the so-called γ-relaxation as determined by internal-friction measurements. The apparent kink mass and the kink diffusivity could be determined, too. The present results leave no doubt that the “hump” observed in the σ–T relationship of high-purity b.c.c. metals is due neither to a change in the glide mechanism nor to a special form of the Peierls potential but is a natural consequence of the dependence of the kink-pair formation enthalpy on the resolved shear stress.

Lattice Vacancies in High-Purity α-Iron

The paper is both a review of the properties of vacancies in high-purity α-iron and an original contribution. In the review part it is shown that self-diffusion in α-iron occurs by the vacancy mechanism, that the sum of the monovacancy formation and migration enthalphies in the magnetically fully ordered state is HF1V + HM1V = (2.91 ± 0.04) eV, and that for the monovacancy formation enthalpy 1.61 eV ≤ HF1V ≤ 1.75 eV holds. The result for the monovacancy migration energy, HM1V = (1.23 ± 0.11) eV, excludes with certainty the attribution of Stage-III recovery to vacancy migration and supports the view that the Stage-III defect is a self-interstitial. It is shown that at elevated temperatures positrons trapped in vacancies in body-centred cubic iron may escape from them. The binding energy of positrons to the vacancies is estimated to be about 1.1 eV. This estimate is in agreement with the analysis of the high-temperature trapping of positive muon by vacancies, which in the present case proved to be the most sensitive technique for detecting vacancies present in thermal equilibrium.

1/f noise as an early indicator of electromigration damage in thin metal films

Electromigration in thin films of aluminium and aluminium alloys is shown to lead to stepwise increases of the electrical 1/f noise. These are attributed to the generation of highly mobile defect configurations by a nucleation‐and‐growth process. It is conjectured that among them may be the defects that are responsible for the eventual failure of VLSI electronic devices by electromigration damage. 1/f noise promises to be an early indicator of this damage.

Anomalous slip - A feature of high-purity body-centred cubic metals

The hitherto poorly understood phenomenon of anomalous slip in body-centred cubic (bcc) metal crystals, constituting a striking violation of Schmids law of resolved shear stresses in limited ranges of temperature and crystallographic orientation of the stress axis, is shown to be a natural consequence of the fact that the slip planes of a 0 /2 screw dislocations change from {110} at low temperatures to {112} at higher temperatures. Above the temperature T of this change, which is responsible for the so-called lower bend of the flow-stress-temperature relationship, in the orientation range of anomalous slip the two slip systems with the largest resolved shear stress have different slip directions. The screw dislocations of these systems may avoid jog formation and generation of atomic defects by cross-slipping to the anomalous {110} plane. This mechanism accounts for the dependence of anomalous slip on temperature, crystallographic orientation, and purity. The variation from metal to metal is shown to be correlated with the elastic anisotropy. Mechanisms that might be responsible for this variation, in particular for the absence of anomalous slip in bcc iron, are discussed.

A review of the magnetic relaxation and its application to the study of atomic defects in α-iron and its diuluted alloys

Abstract This review presents a comprehensive survey on intensive studies performed during the last decades on point defect reactions on α‐iron (α‐Fe) and its diluted alloys. Our intention is to give an actual account of the knowledge accumulated on this subject, as it has been obtained predominantly by means of the magnetic after‐effect (MAE) spectroscopy. After a concise introduction into the theoretical and experimental fundamentals of this technique, the main concern is focused on the presentation and detailed discussion of the MAE spectra arising — after low‐temperature electron (e–)‐ or neutron(n)‐irradiation and subsequent annealing — in: (i) high‐purity α‐Fe and α‐Fe doped with (ii) substitutional solutes (like Ni, V, Al, Cu, Ti, Be, Si, Mn, …) or (iii) interstitial solutes (like O, H, C, N). During the course of systematic annealing treatments, these respective spectra undergo dramatic variations at specific temperatures thereby revealing in great detail the underlying intrinsic reactions of the radiation‐induced defects, i.e., reorientation, migration, clustering, dissolution and finally annihilation. In alloyed Fe systems the corresponding reaction sequences are even multiplied due to additional interactions between defects and solute atoms. Most valuable information concerning formation‐, dissociation‐ and binding enthalpies of small, mixed clusters (of the type CiVk, NiVk; i, k ≥ 1) has been obtained in high‐purity α‐Fe base material which, after charging with C or N, had been e–‐irradiated. Concerning the basic recovery mechanisms in α‐Fe, two complementary results are obtained from the analysis of the various systems: (i) in high‐purity and substitutionally alloyed α‐Fe the recovery in Stage‐III (200 K) is governed by a three‐dimensionally migrating (H M I = 0.56 eV) stable interstitial (dumb‐bell); (ii) following the formation and dissociation kinetics of small clusters (C1Vk, N1Vk) in interstitially alloyed α‐Fe the migration enthalpy of the monovacancy must hold the following relation H M N (0.76 eV) < H M C (0.84 eV) < H M V1. These results are in clear agreement with the so‐called two‐interstitial model (2IM) in α‐Fe – a conclusion being further substantiated by a systematic comparison with the results obtained from nonrelaxational techniques, like i.e. positron annihilation (PA), which by their authors are preferentially interpreted in terms of the one‐interstitial model (1IM).

Interpretation of positron-annihilation experiments on electron-irradiated α-iron in terms of self-interstitial migration in stage III

Abstract A recent interpretation of positron annihilation in electron-irradiated α-iron in terms of free migration of monovacancies in Stage III is shown to be internally inconsistent and incompatible with well-established experimental results, in particular with the monovacancy migration enthalpy deduced from high-temperature equilibrium data. The positron-annihilation experiments find a natural explanation in terms of self-interstitial migration in Stage III. This interpretation accounts not only for the gross features but also for fine details of the positron-annihilation data. It is shown to be in excellent agreement with the results of internal-friction studies on irradiated a-iron. The present analysis supports strongly the previous suggestion that in α-iron two self-interstitial configurations exist, one migrating freely in Stage IE and the other one in Stage III. A further conclusion is that positrons in α-iron can be trapped not only by vacancy-type defects and dislocations but also by clusters o...

The interpretation of X-ray-scattering and electron-microscopy observations on electron-irradiated copper

Abstract The paper examines critically the diffuse, Huang, and small-angle scattering of X-rays from Cu single crystals that were electron-irradiated at low temperatures and subsequently annealed up to about 550 K. It is shown that the experimental results are grossly incompatible with the hypothesis that in recovery stage Id/Ie all self-interstitials are mobile and that above this stage self-interstitials can only be retained as clusters. Since this hypothesis is the very basis of the one-interstitial model of radiation damage of metals, this result constitutes a serious falsification of this model. A consistent interpretation of the X-ray data is possible if they are considered together with the results of electron-microscopy observations pertaining to similar irradiation and annealing conditions. This leads to the conclusion that only a small fraction of the self-interstitials become mobile in the recovery stage I d /I E and that up to temperatures close to Stage III the majority of the self-interstiti...

High-resolution transmission electron microscopy study of nanostructured metals

Abstract High-Resolution Transmission Electron Microscopy (HRTEM) studies of gas-phase prepared nanocrystalline palladium (nc-Pd) and molybdenum (nc-Mo) are presented. In nc-Pd where structural relaxation and initial grain growth may occur the crystallites are found to be slightly strained with a few dislocations as well as stacking faults and twins. In nc-Mo, in addition to some narrow grain boundaries extended transition regions between slightly misoriented crystallites are observed. For a comprehensive study of the interface structure of nc solids by HRTEM the determination of the orientational correlation between adjacent crystallites is pivotal. This is facilitated by a controlled tilting procedure developed in order to align an individual crystallite with the electron beam, e.g., along a zone-axis orientation.

Investigation of muon states in silicon and germanium by field-quenching and RFμSR

The temperature dependence of the three states of positive muons in the semiconductors with diamond structure (μ+ in diamagnetic statesμd and paramagnetic muonium Mu and Mu*) have been investigated on six Si (pure, B and P doped) and four Ge (ultrapure, CZ-grown undoped, Ga and Sb doped) single crystals by longitudinal field-quenching and radio-frequencyμ+SR. Clear evidence for the transition Mu* →μd is found. The influence of light-induced charge-carriers is shown to be quite different in p- and n-type material.

Investigation of muon-state dynamics in silicon by longitudinal field-quenching and radio-frequency μ+ spin resonance

The two paramagnetic muon states-normal and anomalous muonium-and the diamagnetic muon states have been investigated in different monocrystalline silicon samples (intrinsic, boron-, phosphorus-, or arsenic-doped, float-zone or Czochralski-grown) between 6K and 800K by means of longitudinal field-quenching (LFQ) and radio-frequency μ + spin resonance (RFμSR). The LFQ data can be described consistently by coupled equations of motion for the muonium spin systems if spin exchange processes as well as transitions between different muon states are taken into account. It is shown that the initial formation probabilities of the different muon states, the ionization rate of the anomalous muonium, and the electron spin exchange rates depend strongly on the charge carrier densities. These results are in agreement with the RFμSR data obtained on the same samples if the different time scales of RFμSR and LFQ experiments are taken into account. At temperatures above 300 K both RFμSR and longitudinal relaxation results appear to indicate reversible ionization of normal muonium.