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, T-1p, in pure NaCl single crystals as a function of temperature (from ambient to about 900 K) and strain-rate (to approximate 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

Dynamical <i>in situ</i> 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 µm s–1, maximum deformation speed: 3×10^5 µm s–1; deformation stroke: digitally controlled between 1 and 8×10^3 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

In-Situ Nuclear Magnetic Resonance Investigation of Strain, Temperature, and Strain-Rate Variations of Deformation-Induced Vacancy Concentration in Aluminum

Critical strain to serrated flow in solid solution alloys exhibiting dynamic strain aging (DSA) or Portevin–LeChatelier effect is due to the strain-induced vacancy production. Nuclear magnetic resonance (NMR) techniques can be used to monitor in situ the dynamical behavior of point and line defects in materials during deformation, and these techniques are nondestructive and noninvasive. The new CUT-sequence pulse method allowed an accurate evaluation of the strain-enhanced vacancy diffusion and, thus, the excess vacancy concentration during deformation as a function of strain, strain rate, and temperature. Due to skin effect problems in metals at high frequencies, thin foils of Al were used and experimental results correlated with models based on vacancy production through mechanical work (vs thermal jogs), while in situ annealing of excess vacancies is noted at high temperatures. These correlations made it feasible to obtain explicit dependencies of the strain-induced vacancy concentration on test variables such as the strain, strain rate, and temperature. These studies clearly reveal the power and utility of these NMR techniques in the determination of deformation-induced vacancies in situ in a noninvasive fashion.

The problem of a metal impurity in an oxide: ab-initio study of electronic and structural properties of Cd in Rutile TiO2

In this work we undertake the problem of a transition metal impurity in an oxide. We present an ab-initio study of the relaxations introduced in TiO2 when a Cd impurity replaces substitutionally a Ti atom. Using the Full-Potential Linearized-Augmented-Plane-Wave method we obtain relaxed structures for different charge states of the impurity and computed the electric-field gradients (EFGs) at the Cd site. We find that EFGs, and also relaxations, are dependent on the charge state of the impurity. This dependence is very remarkable in the case of the EFG and is explained analyzing the electronic structure of the studied system. We predict fairly anisotropic relaxations for the nearest oxygen neighbors of the Cd impurity. The experimental confirmation of this prediction and a brief report of these calculations have recently been presented [P.R.L. 89, 55503 (2002)]. Our results for relaxations and EFGs are in clear contradiction with previous studies of this system that assumed isotropic relaxations and point out that no simple model is viable to describe relaxations and the EFG at Cd in TiO2 even approximately.Comment: 11 pages, 8 figures, Revtex 4, published in Physical Review

On the role of dislocations in heavily strained materials

As a tribute to professor Hans Weertman upon the occasion of his 70th birthday this paper deals with the role of dislocations in materials deformed at very high strain rates. In contrast to classical crystal deformation experiments which occur typically at strain rates of 10(-5) s(-1) to 10(-1) s(-1), ballistic deformation experiments involve strain rates of 10(4) s(-1) to 10(6) s(-1). These high strain rates yield completely different deformation mechanisms since defects can not move the same way at high rates as they do at low rates. Although the main focus will be on shock wave compaction of high T-c ceramic mater:als some comparison will be made also to metals and other cere lies deformed at intermediate high strain rates of the order of 1. s(-1). Because of the brittle nature of the high T-c superconducting ceramic materials plastic deformation of these materials is difficult. Its brittle character may however be suppressed by performing shock loading techniques. This paper describes TEM and HREM study of the dynamic compacted samples at various E/M values, i.e. the ratio between the mass density of the explosives and the mass density of the material to be compacted, ranging between 0.7 and 2.1. Apart from the well established {100} glide system, the role of a novel [110](1-1 0) and [010](1 0 0) glide systems are studied. All glide systems are found to interact with the ferroelastic domains of the material, each in a different way. Because a higher dislocation density is produced by dynamic compaction, one would expect that this technique also contributes to an increment in the pinning capacity of high T-c superconducting properties in an external magnetic field due to the generation of point defects. Here a comparison is made with in-situ NMR experiments on the deformation-induced generation of point defects in ceramic materials and metals. The spin-lattice relaxation rate provides information about the production rate of point defects during deformatio.</p

Nuclear magnetic resonance of atomic motions in vanadium.

Atomic motions in solid vanadium have been investigated by means of NMR. For that purpose, the Zeeman spin-lattice relaxation time, the rotating-frame spin-lattice relaxation time, and the Knight shift of V in polycrystalline vanadium have been measured as a function of temperature in the range between 300 and 2000 K. At all temperatures, the relaxation time is mainly determined by electronic contributions leading to a temperature-dependent expression. In contrast to the data, two pronounced peaks in the relaxation rate at about 750 and 1600 K, respectively, are observed. The peak at 750 K arises from fluctuations in the nuclear quadrupolar interaction between vanadium and interstitially migrating oxygen whereas the maximum at 1600 K is caused by fluctuation in the nuclear dipole interaction due to atomic self-diffusion. From the low-temperature data, the diffusivity of oxygen is determined

In situ NMR study of the two-phase equilibrium in Au-Al alloys.

It is shown that the Al-27 NMR spectrum in alpha-Au-xAl-alloys (x smaller than 15 at%) consists of two lines in the solidus - liquidus range. The analysis of the data leads to a new solidus line in the phase diagram of Au-Al