T.M. Holden

Intergranular Stresses in Incoloy-800

Abstract The generation of intergranular residual strains under uniaxial loading conditions in the plastic regime has been measured in detail by neutron diffraction in Incoloy-800. A relatively simple theory, based on the Taylor model, gives a good semiquantitative account of the magnitudes of the strains. The results clarify the interpretation of measurements made earlier on Incoloy-800 steam generator tubes.

Neutron-diffraction measurements of stress

Abstract Experiments on bent steam-generator tubing have shown that different diffraction peaks, (1 1 1) or (0 0 2), give different results for the sign and magnitude of the stress and strain. From an engineering standpoint, the macroscopic stress field cannot be both positive and negative in the same volume, so this difference must be due to intergranular effects superposed on the macroscopic stress field. Uniaxial tensile test experiments with applied stresses beyond the 0.2% offset yield stress, help to understand this anomaly, by demonstrating the different strain response to applied stress along different crystallographic axes. When Zr-alloys are cooled from elevated temperatures, thermal stresses always develop, so that it is difficult to obtain a stress-free lattice spacing from which residual strains may be derived. From measurements of the temperature dependence of lattice spacing, the temperature at which the thermal stresses vanish may be found. From the lattice spacing at this temperature the stress-free lattice spacings at room temperature can be obtained readily. To interpret the measured strains in terms of macroscopic stress fields it is necessary to know the diffraction elastic constants. Neutron diffraction measurements of the diffraction elastic constants in a ferritic steel for the [1 1 0], [0 0 2] and [2 2 2] crystallographic axes, in directions parallel and perpendicular to the applied stress are compared with theoretical diffraction elastic constants.

Neutron diffraction study of magnetic phases in polycrystalline Mn2SiS4

Abstract A narrow region of spontaneous magnetization was found to occur in the synthetic olivine Mn 2 SiS 4 between 83 and 86 K. Neutron diffraction analysis shows that for the manganese ions there is antiferromagnetic spin alignment along the crystallographic a -axis, with opposing ferromagnetic c -components on two sites of different symmetries, inversion and mirror. Below 83 K the analysis shows canted antiferromagnetism progressing to collinear alignment on the b -axis at 4.2 K.

Discovery of τ=2/9 lock‐in in holmium

The complex magnetic behavior of holmium is a well‐established experimental fact and has been studied by a variety of experimental techniques, including magnetization, x‐ray diffraction, thermal expansion, and heat capacity. Our recent studies have focused on using neutron diffraction to study the temperature and field dependence of the helimagnetic structure. The pitch of the helix is described by τ, the spiral wave vector. τ usually varies smoothly with temperature but tends to lock in at various values that are commensurate with the lattice when a field is applied. These lock‐ins provide clues to the nature of the different magnetic phases and, up to now, all the lock‐ins have corresponded to features of the magnetic phase diagram obtained from magnetization measurements. However, recent experiments have revealed a previously unobserved lock‐in at τ=2/9 rlu in a 1.4 T b‐axis field. It shares the same general features of other lock‐ins except that it does not correspond to any feature of the magnetic ph...

Industrial research and development and the role of neutron diffraction

Abstract The property of thermal neutrons that makes them valuable for studying industrial structures is their high penetration through most materials. This means that neutron diffraction can be used as a non-destructive probe for obtaining information about material properties deep inside engineering components. to a large extent we have developed the techniques in response to pressing problems of industry such as the need to measure residual stress fields, the need to understand the grain-to-grain interaction stresses that drive initial growth transients in nuclear reactor materials, and the need to measure temperature and stresses non-invasively in operating engines.

INTERGRANULAR STRAINS IN INCONEL-600 STEAM GENERATOR MATERIALS

Abstract Engineering components are inhomogeneous on the scale of the grain size since the grains generally have different elastic and plastic properties depending on their crystallographic orientations. Constitutive equations describing the mechanical properties of materials in terms of the deformation state of the grains within it are needed to describe the behaviour of structures using the finite element method. Neutron diffraction provides a test of such constitutive models as the elasto-plastic self-consistent (EPSC) model, because the elastic strains in all grain orientations inside a sample can be measured independently. In this paper the intergranular strains measured in tensile test coupons after uniaxial plastic deformation of +5.6% and −6.0% are presented as strain pole-figures for the (1 1 1), (0 0 2), (2 2 0) and (1 1 3) reflections of the FCC structure. The results are compared with the predictions of the EPSC model, which has no adjustable parameters.

Magnetic structure of erbium in field

Abstract The magnetic structure of erbium in a field up to 2.8 T along the c -axis has been studied by eleastic neutron scattering. At low temperatures the conical structure with q = 5/21 persist field. The sequence of ionics-in antiferromagnetic phases at higher temperatures in zero field is destroyed by the field. Larger fields stabilize the conical phase, though at temperatures above 20 K there is no longer a lock-in at q = 5/21. For fields between 2.1 and 2.8 T we observe a new conical phase with a lock-in at q = 1/4. This phase is stable between 34 and 39 K.

The effect of 0.2 at.% V impurity on the dynamic magnetic susceptibility of chromium

Abstract Inelastic neutron scattering above the Neel transition in Cr is strongly affected by fractional per cent V impurity. The cross-section decreases markedly, while at low energies the commensurate fluctuations decrease relative to the incommensurate. These effects may correspond to pronounced changes seen also in magnetoelastic, transport, and magnetic properties of the paramagnetic phase.

Anisotropic spin-wave dispersion in

The first measurements have been made of the spin-wave dispersion in the tetragonal intermetallic compound by inelastic neutron scattering. The experiments were performed at temperatures T between 4.2 K and 300 K in the collinear antiferromagnetic phase (T < 263 K) and in the incommensurate phase (263 < T < 280 K). There is no energy gap at zero wavevector. Along the a axis the spin-wave dispersion curve rises to a maximum frequency of 6.4 THz at the zone boundary. Along the c axis the frequency rises more rapidly with wavevector and reaches 11 THz approximately a sixth of the way to the Brillouin zone boundary. Measurements in the incommensurate phase indicate that the spin waves are overdamped along the a axis.

Magnetic excitations in a single-domain FCT γ-Mn(10%Fe, 3%Cu) alloy

Abstract The spin waves in a single-domain fct γ-Mn alloy have been measured by inelastic neutron scattering. The measurements were performed at 4.2 K for [001], [110] and [111] directions and energies up to 100 meV. Data analysis gives a spin-wave velocity, v = (290±15) meV A, an energy gap, E g = (12±0.5) meV, and a damping parameter, Γ, having about 30% of the excitation energy. Within the experimental accuracy the spin-wave velocity was independent of direction.