O. Kanert

Dislocation Dynamics in Al-Li Alloys. Mean Jump Distance and Activation Length of Moving Dislocations

Pulsed nuclear magnetic resonance proved to be a complementary new technique for the study of moving dislocations in Al-Li alloys. The NMR technique, in combination with transmission electron microscopy and strain-rate change experiments have been applied to study dislocation motion in Al-2.2 wt% Li alloys, aged at 215°C (1 h) and 245°C (115 h). These heat treatments were chosen in order to obtain considerable differences in particle sizes which influence the mechanical properties. From the motion-induced part of the spin lattice relaxation rate, T1ρ^-1 of 27Al the mean jump distance of mobile dislocations has been measured as a function of strain. Transmission electron microscopic observations of the mean planar diameter of δ’ precipitates, together with the NMR data, predicted the increase in yield stress of these alloys compared to ultrapure Al in agreement with experiments. In alloys aged 1 h at 215°C the precipitates are believed to be shearable. After aging 115 h at 245°C NMR and TEM observations indicated that the particles were not sheared. It was found that the activation length, obtained from mechanical strain-rate change experiments have different values compared to the values of the mean jump distance determined by NMR. Reasons for the mean jump distance being different from the activation length have been given. Nevertheless, there exists an internal consistency. namely: both mean jump distance and activation length have been found to decrease with strain hardening more rapidly in Al-Li containing nonshearable precipitates than in Al-Li containing shearable precipitates.

Solution Hardening in Al-Zn Alloys. Mean Jump Distance and Activation Length of Moving Dislocations

Abstract Pulsed nuclear magnetic resonance proved to be a complementary new technique for the study of moving dislocations in Alue5f8Zn alloys. The NMR technique, in combination with strain-rate change experiments and transmission electron microscopy have been applied to study dislocation dynamics in Alue5f8Zn alloys (1–2 at.% Zn). Spin-lattice relaxation measurements clearly indicate that fluctuations in the quadrupolar field caused by moving dislocations in Alue5f8Zn are different compared to those in ultra-pure Al. From the motion induced part of the spin-lattice relaxation rate the mean jump distance of mobile dislocations has been measured as a function of strain. Based on the NMR data and data obtained from strain-rate change experiments it could be concluded that moving dislocations advance over a number of solute atoms (order of 10) as described by Mott-Nabarros model and interact with forest dislocations as predicted by Friedels model. The strain rate change experiments confirm the linear additivity of flow stresses and the additivity of inverse activation length.

DISLOCATION DYNAMICS IN ALUMINIUM AND IN ALUMINIUM-COPPER ALLOYS: A NUCLEAR MAGNETIC RESONANCE AND TRANSMISSION ELECTRON MICROSCOPIC STUDY

Abstract Pulsed nuclear magnetic resonance techniques as well as transmission electron microscopy have been applied to study dislocation motion in ultrapure aluminium and aluminium-copper alloys ( Al : xCu with x max = 1 at.%). The spin-lattice relaxation rate in the rotating frame. T 1 ρ −1 of 27 Al has been measured as a function of the plastic strain rate ϵϵ at 77 K. For finite strain rates ϵϵ, dislocations induces an additional relaxation rate arising from time fluctuations in the nuclear quadrupole interaction. From this motion-induced part of the relaxation rate the mean free path L of mobile dislocations can be calculated which is determined by the distribution of lattice defects acting as obstacles for moving dislocations. The NMR experiments are combined with transmission electron microscopic investigations to reveal the static structure of defects in the samples. Correlations between the in situ observed mean free path L of mobile dislocations and between the microscopic defect structure arising from the ageing process (θ′ phase, solid solution) and the degree of plastic deformation ϵ are shown. It turns out that in the θ′ phase at small strains L is determined by the microstructure and is equal to the mean separation between the precipitates. For large strains L is determined by the statistical distribution of the dislocation loops lying between the precipitates. On the other hand, in ultrapure aluminium L is determined by the dislocation cell structure.

Dynamical in situ nuclear‐magnetic‐resonance tensile apparatus

A combination of a servohydraulic tensile machine and NMR pulse spectrometer is described enabling nuclear‐spin relaxation rates to be recorded simultaneously with stress‐strain data incorporating tension as well as compression of nonmetallic as well as of metallic samples. The data of the mechanical system are as follows: Maximum load: 5000 N; minimum deformation speed: 10 μmu2009s−1, maximum deformation speed: 3×105 μmu2009s−1; deformation stroke: digitally controlled between 1 and 8×103 μm; bandwidth: dc to 1 kHz; resolution: 2–4 μm; temperature conditions of the sample: from 80 to 570 K. The operation and performance of the system is described by means of experiments observing nuclear‐spin relaxation rates which are induced by the movement of dislocations due to the finite deformation rate of the sample.

Solution hardening in aluminium-magnesium alloys: A nuclear magnetic resonance and transmission electron microscopic study

Pulsed nuclear magnetic resonance techniques as well as transmission electron microscopy have been applied to study dislocation motion in aluminium magnesium alloys (0.2–1.6 at.% Mg). The spin lattice relaxation rate in the rotating frame of 27A1 has been been measured at 77 K as a function of strain at constant plastic strain rate ϵ. For finite strain rates, the movement of dislocations induces an additional relaxation rate arising from time fluctuations in the nuclear quadrupole interactions. From the motion-induced part of the relaxation rate the mean free path of mobile dislocations can be calculated. The NMR experiments are combined with transmission electron microscopic investigations to reveal the static structure of defects in the samples. The NMR measurements clearly indicate that fluctuations in the quadrupolar field caused by moving dislocations in Alue5f8Mg are different compared to those in ultra pure Al. From the NMR data it could be concluded that moving dislocations advance over a number of solute atoms (order of 7) as described by Mott-Nabarros model. On the other hand, Mott-Nabarros model does not predict the effective solute spacing as a function of the concentration of solute atoms in accordance with NMR experiments.

In situ nuclear magnetic resonance study of defect dynamics during deformation of materials

Nuclear magnetic resonance techniques can be used to monitor in situ the dynamical behaviour of point and line defects in materials during deformation. These techniques are non-destructive and non-invasive. We report here the atomic transport, in particular the enhanced diffusion during deformation by evaluating the spin lattice relaxation time in the rotating frame, T1ϱ,in pure NaCl single crystals as a function of temperature (from ambient to about 900K) and strain-rate (to ≈ 1.0s−1) in situ during deformation. The strain-induced excess vacancy concentration increased with the strain-rate while in situ annealing of these excess defects is noted at high temperatures. Contributions due to phonons or paramagnetic impurities dominated at lower temperatures in the undeformed material. During deformation, however, the dislocation contribution became predominant at these low temperatures. The dislocation jump distances were noted to decrease with increase in temperature leading to a reduced contribution to the overall spin relaxation as temperature is increased. Similar tests with an improved pulse sequence (CUT-sequence), performed on ultra-pure NaCl and NaF single crystals revealed slightly different results; however, strain-enhanced vacancy concentrations were observed. The applicability of these techniques to metallic systems will be outlined taking thin aluminium foils as an example.

DISLOCATION DYNAMICS IN AL-MG-ZN ALLOYS - A NUCLEAR MAGNETIC-RESONANCE AND TRANSMISSION ELECTRON-MICROSCOPIC STUDY

Pulsed nuclear magnetic resonance (NMR) proved to be a complementary new technique for the study of moving dislocations in Al-Mg-Zn alloys. The NMR technique, in combination with transmission electron microscopy (TEM), has been applied to study dislocation motion in Al-0.6 at. % Mg-1 at. % Zn and Al-1.2 at. % Mg-2.5 at. % Zn. Spin-lattice relaxation measurements clearly indicate that fluctuations in the nuclear quadrupolar interactions caused by moving dislocations in Al-Mg-Zn are different compared to those in ultra pure Al. From the motion induced part of the spin-lattice relaxation rate the mean jump distance of mobile dislocations has been determined as a function of strain. From the NMR data it is concluded that moving dislocations advance over a number of solute atoms in these alloys as described by Mott-Nabarros model. At large strains there exists a striking difference between the mean jump distances in Al-0.6 at. % Mg-1 at. % Zn and in Al-1.2 at. % Mg-2.5 at. % Zn. The latter is about five times smaller than the former one. This is consistent with TEM observations that show dislocation cell formation only in Al-0.6 at. % Mg-1 at. % Zn and the macroscopic stress-strain dependences of these alloys.

Dislocation Dynamics in Vanadium : A Nuclear Magnetic Resonance and Transmission Electron Microscopic Study

Pulsed nuclear magnetic resonance proved to be a complementary new technique for the study of moving dislocations in b.c.c. metals. From the motion induced part of the spin-lattice relaxation rate the mean jump distance of mobile dislocations has been measured in Vanadium as a function of temperature. The NMR experiments are combined with transmission electron microscopic investigations to reveal the static structure of defects in the samples. The NMR experiments show that the mean jump distance is nearly constant below 230 K whereas it decreases substantially above 230 to 300 K indicating a transition that marks two different mechanisms. NMR observations in combination with TEM support the physical picture that above the transition temperature dislocation segments are stopped between localized obstacles whereas below Tc the lattice friction controls the plastic behaviour.

A Nuclear Magnetic Resonance and Transmission Electron Microscopic Studyof Moving Dislocations in Ternary Al-Base Alloys

Pulsed nuclear magnetic resonance proved to be a complementary new technique for the study of moving dislocations in Al-Mg-Zn alloys. The NMR technique, in combination with transmission electron microscopy have been applied to study dislocation motion in A1-0.6 at% Mg-1 at% Zn and Al-1.2 at% Mg-2.5 at% Zn. From the motion induced part of the spin-lattice relaxation rate the mean jump distance of mobile dislocations has been determined as a function of strain. From the NMR data it is concluded that moving dislocations advance over a number of solute atoms in these alloys as described by Mott-Nabarros model.

Mean Jump Distance and Activation Length of Moving Dislocations in Al-Li Alloys

ABSTRACT Pulsed nuclear magnetic resonance techniques, in combination with transmission electron microscopy and strain-rate change experiments have been applied to study dislocation motion in Al-2.2 wt% Li alloys, aged at 215 °C (1 h) and 245 °C (115 h). These heat treatments were chosen in order to obtain considerable differences in particle sizes which influence the mechanical properties. From the motion-induced part of the spin lattice relaxation rate, T −1 10 of 27 Al the mean jump distance of mobile dislocations has been measured as a function of strain. It was found that the activation length obtained from mechanical strain-rate change experiments have different values compared to the values of the mean jump distance determined by NMR. Reasons for the mean jump distance being different from the activation length have been given. The mean jump distance as well as the activation length have been found to decrease with strain hardening more rapidly in Al-Li containing non-shearable precipitates than in Al-Li containing shearable precipitates.