Inelastic neutron scattering on in situ grown bcc single crystals of the group 4 metals Ti, Zr and Hf show a band of low energy and strongly damped phonons. Geometrical considerations show how these damped lattice vibrations achieve the displacements necessary for the two martensitic phase transitions from bcc to ω(under pressure) and from bcc to hcp (upon lowering the temperature). The low energy and temperature dependent phonons are precursor fluctuations of the hcp or ω phase within the bcc phase.

The two-dimensional position sensitive detector at IPNS was used to probe the splitting of a single Bragg peak of the ferroelectric KD2PO4 (DKDP) at temperatures below the Curie point. The (440) peak was split into four components as the crystal itself divided into four domains. When the crystal was subjected to electric fields of sufficient strength and duration, these peaks would change in relative intensity, thus displaying the rearrangement of the domain structure as the crystal changed its level of polarization. Even at temperatures 15–20K below the Curie point, the polarized crystal was observed to relax back to an almost completely unpolarized state upon removal of the applied electric field. This indicates that the true coercive field for the crystal at this temperature was much smaller than values previously reported [l]. Low frequency dielectric constant studies using a capacitance bridge have since been conducted, which confirm and expand on these results.

The structure of a series of homogeneous ErBa2Cu3Ox powder samples (6.1 < x < 7.0) has been determined by elastic neutron scattering using the Rietveld method of profile refinement. All samples were prepared in the same way using a defined absorption-desorption method.

By removing oxygen the c axis expands, whilst the Cu2–Cu2 and the Ba–Ba distances contract. Accordingly, the most significant changes in atomic distances occur between the Cu2–O2,3 planes and the Cul–O4–Cul chains. With decreasing oxygen content the distance between the Cu–O planes and the Cu–O–Cu chains increases and the oxygen (0, 0, z) apex atom O1 moves closer to the Cu–O–Cu chains withdrawing from the Cu–O planes. The O1–plane and O1–chain distances are monotonic functions of the oxygen content, but the x dependence of these bonding lengths seems to be much stronger below x < 6.5. The change of the interatomic distances can be correlated to the suppression of superconductivity which clearly occurs within the orthorhombic phase.

The title compounds are structurally isomorphous and crystallize with the symmetry of space group 14/mmm. The lattice parameters are a = 3.8248(1), c = 19.4286(5)Å for La1.9Ca1.1Cu2O6 and a = 3.8601(1), c = 19.9994(5)Å for La1.9Sr1.1Cu2O6+δ. The structure can be easily derived from that of La2CuO4 by substituting the layers of composition CuO2 present in La2CuO4 with a block of three layers having composition and sequence (CuO2) - (R) - (CuO2), where R is Ca, Sr, and/or La. Although the general structural configuration is the same for the Ca and the Sr compounds, the distribution of the metal atoms is different in the two cases. More specifically, in La1.9Ca1.1Cu2O6 the perovskite-type layers form blocks with sequence (CuO2) - (Ca) - (CuO2), and are separated by rock-salt blocks of two layers (LaO) - (LaO). In La1.9Sr1.1Cu2O6+δ, on the other hand, the perovskite-type blocks have composition and sequence (CuO2) - (R) - (CuO2) and they are separated by blocks (NO) - (NO), where both R and N are about 65%La and 35%Sr.

We have recently reported[l] neutron depolarization measurements on an YBa2Cu3O7−δ composition exhibiting a transition from a low temperature state which caused depolarization (P=0.76+−.02) to one at high temperatures (T=55K) where depolarization was absent. Since this temperature is well below the measured superconducting transition at 86K, we speculated that the transition was a dynamic one corresponding to flux lattice melting[2] or to formation of a glassy state[3]. We have now extended these depolarization measurements to two specimens of Bi2.2Sr1.7CaCu2O8+δ' prepared and characterized[4] at IBM, which showed superconducting transitions at 80K with weak tails extending to 110K. The two samples have critical current densities, as determined by the Bean expression[5], that differ by 50%.

The Argonne Intense Pulsed Neutron Source and General Purpose Powder Diffractometer have been used to study high Tc metal oxide composites composed of yttrium barium copper oxide and silver. Neutron diffraction techniques were applied to composites with 15, 20 and 30% silver content by volume. We have observed that after hot pressing, the 30% Ag specimens contained both orthorhombic high Tc and tetragonal, non-superconducting phases near the center of the specimens but only tetragonal near the surface. The relationship of shifts in Bragg peaks to strains of the constituents is discussed.

Analysis of high resolution neutron powder diffraction data of ferroelastic LaNbO4 indicates a monoclinic to tetragonal structural phase transition that is accompanied by anomalous line broadening. The origins of this line broadening have been investigated using a novel modification of the Rietveld technique. The results show that strain broadening is associated with the ferroelastic phase transition both above and below the transition temperature. This anomalous ‘microstrain’ is well-described by a second rank strain tensor, the principal axes of which are collinear with the spontaneous lattice strain. This phenomenon may be a general feature of proper ferroelastic phase transitions.

For O in Nb the experimental scattering distribution can be described by a so-called 3-force model. The scattering pattern exhibits close similarity to the one observed for the ω-phase in some bcc alloys. It is shown that the lattice distortion calculated from the proposed 3-force model contains static fluctuation of the ω-phase. The ω-phase is closely related to the minimum in the LA 2/3 (111) phonon dispersion branch. With decreasing temperature the frequency of this phonon decreases, whereas the elastic diffuse scattering at the same position increases. This “softening” of the lattice, together with an increasing “central peak”, gives an equivalent description of the situation.

Quenched equilibrium states of Cu–31.1 at.% Zn and Cu–22.4 at.% Zn single crystals (prepared with the Cu-65 isotope) were investigated by elastic diffuse neutron scattering. The diffuse intensity showed maxima which are attributed to the flat portions of the Fermi surface in the <110> directions. Short–range order parameters and linear displacement parameters were obtained from a fit to the measured data. Pair interaction energies were determined based on the inverse Monte–Carlo method. An ordered low-temperature phase Cu3Zn with the DO23 structure is suggested.

Recently, measurements of short-range order (SRO) diffuse neutron scattering intensity have been performed on quenched Cu-Zn alloys with 22.4 to 31.1 atomic percent (a/o) Zn, and pair interactions were obtained by Inverse Monte Carlo simulation [1]. These results are compared to SRO intensities and effective pair interactions obtained from first-principles electronic structure calculations. The theoretical SRO intensities were calculated with the Cluster Variation Method (CVM) in the tetrahedron-octahedron approximation with first-principles pair interactions as input. More generally, phase stability in the Cu-Zn alloy system is discussed, using ab-initio energetic properties.

Recently, short range order (SRO) parameters have been extracted from the neutron diffuse scattering intensities on Ni-Cr alloys with 11 to 25 at. pct. chromium. These data were analyzed using inverse-Monte Carlo simulations to extract atomic interactions and the results suggested the existence of a well defined, flat sheet of Fermi surface normal to the [110] direction. In this paper, this suggestion is investigated with a series of calculations which are based on the first-principles Korringa-Kohn-Rostoker-coherent-potential-approximation (KKRCPA). We calculate the electronic structure and the Bloch spectral function at the Fermi energy. We then proceed to calculate the effective cluster interactions using the generalized perturbation method (GPM) and compare them with the above-mentioned experimentally deduced interactions. In addition, we present preliminary results of two-dimensional angular correlation of positron annihilation radiation (2D-ACPAR) measurements, which probe the fermiology of this alloy system. The positron experiments demonstrate the existence of well-defined features in the Fermi surface of Ni89Cr11 but our calculations still have not yet established the origin of SRO in this system.

We have made detailed measurements of phonon frequencies along all high-symmetry directions on a large single crystal of Cu0.84Al0.16 at room temperature. Phonon frequencies were ascertained to better than ±0.03 meV. Inter-atomic force constants and vibrational density of states were calculated by performing a Born-von Karman analysis on the complete set of phonon dispersion curves. In contrast to the case of pure Cu, no evident Kohn anomaly (neither in the phonon dispersion itself nor in the derivatives) was observed near the expected wave vector q=2kF. The absence of Kohn anomalies in the present system could be due either to a smeared out Fermi surface or to the possibility that the electron-electron interaction which screens the inter-ionic potential is not the dominant interaction in the system; i.e. the existence of Kohn anomalies in these alloys may depend mainly on the details of the electron-phonon interaction.

The order or segregation properties of compounds or solid solutions are important ingredients of the phase diagrams. If the order can be described as an atomic distribution on an underlying lattice, and if the interactions can be expressed in terms of pairs and other multiplet potentials between the atomic species, phase diagrams should be deducible from these potentials, along with other properties, such as antiphase boundaries, core structures of the dislocations in ordered compounds&; This approach, i.e. the very existence of the potentials, is legitimated by electronic structure calculations for or alloys of normal [1] and transition metals [2]. The G.P.M. (General perturbation Method) allows indeed to develop the order energy in terms of interatomic potentials, the reference state, namely the random alloy, being calculated within the C.P.A. (Coherent Potential Approximation).

Diffuse neutron scattering experiments have been performed on a single crystal Fe0.8Al0.2at T-1013K. The data are analysed in terms of short-range order and lattice displacements. Effective pair-interactions have been determined by the inverse Monte Carlo method.Further measurements of the temperature dependence of the diffuse scattering reveal a long-range ordered phase with the B32 structure below T∼ 650K. This new phase is related to the influence of the magnetism on the chemical ordering in the, iron-aluminum alloys.

The effect of lattice mismatch δ between the γ-matrix and γ'-precipitates on the decomposition process of Ni-Al-Mo alloys has been investigated. The results show that for δ = 0 spherical precipitates are formed without any orientational correlation. For δ > 0 non-spherical particles, aligned along <100>, are produced. The kinetics of the γ-γ' decomposition becomes slower as δ is reduced to zero.

Neutron powder diffraction data have been obtained on Ti-8.6wt.% Al alloys with three different oxygen contents ( 500, 1000, and 2000 ppm) in the solution-treated and aged condition ( containing α2) to investigate possible oxygen and aluminum ordering and lattice parameter variations. The lattice parameters “a” and “c” were found to depend on the oxygen content and heat-treatment condition. Consistent with the literature lattice parameter data on pure Ti, the “a” lattice parameter of the solution-treated Ti-8.6Al material is relatively insensitive to the oxygen content while the “c” parameter increases with increasing oxygen. Following aging to precipitate the ordered α2, the “c” lattice parameter in the Ti-8.6Al alloys still exhibits a positive dependence on oxygen content with aging and a slight “c” increase over the solution-treated condition. The “a” lattice parameter following aging is observed to be lower than the solution-treated value, particularly for the 1000 and 2000ppm alloys. The diffraction data will be discussed in light of parallel electron microscopy results and literature data.

Phase separation of quenched supersaturated Ni-(9.5-12.0)at.% Ti single crystals was followed by small-angle neutron scattering during in-beam aging at 773, 813 and 853 K. With the incident beam along <110>, scattering maxima in <100> directions were observed from the earliest stages of decomposition. While the results obtained at 773 K may be compatible with the general features of spinodal decomposition, a two-phase model with homogeneous precipitation in a depleted matrix is more appropriate at 813 and 853 K. Though scaling may be shown to hold, the aging kinetics is considerably slower than expected, which is related to a stabilizing effect of elastic coherency stresses.

Small angle neutron scattering (SANS) measurements have been made on deformed polycrystal palladium samples with and without deuterium dissolved in the solution phase (a) at room temperature. Concentrations were held constant during SANS experiments by an equilibrium gas pressure cell. The difference scattering cross section for the same sample with and without deuterium loading has a 1/Q behavior (Q=4Φ/λsinθ/2) at intermediate values of Q. At very low values of Q the dependence is much stronger than 1/Q. The 1/Q behavior is attributed to deuterium trapping close to long dislocation cores forming rod-like scattering structures.

The use of neutrons for the measurement of stress is complementary to and extends traditional x-ray diffraction methods to new types of problems. This is due to the lower absorption of neutrons compared to x-rays by most engineering materials, which increases the sampling depth from microns to millimeters. It is particularly suitable for triaxial macrostress gradients through the depth of engineering components and volumetric microstresses in composites. In addition, applied stress studies may also be performed. This paper briefly describes the nature of residual stresses, the use of diffraction for stress measurements, experimental aspects of the use of neutrons, and illustrative applications.

Energy-dispersive neutron diffraction has been developed at the NIST reactor as a probe of sub- and near-surface residual stresses in technological samples. Application of the technique has been made to a variety of metallurgical specimens which includes the determination of tri-axial stresses as a function of depth in a number of uranium-3/4wt%Ti samples with different thermo-mechanical histories, and in two types of 7075-T6 aluminum “ogives”- of interest to the Army. Preliminary results have been obtained for an induction-hardened steel shaft, a fatigue lifetime test specimen for the SAE.