The mechanical properties of polycrystalline materials are largely determined by the kinetics of the phase transformations during the production process. Progress in x-ray diffraction instrumentation at synchrotron sources has created an opportunity to study the transformation kinetics at the level of individual grains. Our measurements show that the activation energy for grain nucleation is at least two orders of magnitude smaller than that predicted by thermodynamic models. The observed growth curves of the newly formed grains confirm the parabolic growth model but also show three fundamentally different types of growth. Insight into the grain nucleation and growth mechanisms during phase transformations contributes to the development of materials with optimal mechanical properties.

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We describe two spin-echo instruments for neutron small angle scattering, which have been installed at the reactor institute in Delft. The first setup is using a monochromatic beam and magnetized foils as spin flippers, while the second uses resonant spin flippers in a pulsed neutron beam. The components that play an essential role for operation are described in some detail. Each setup has specific advantages in its range of spin-echo lengths that covers the range of correlation lengths that could be measured. This is demonstrated in a comparative measurement, the setup with magnetized foils measuring at spin-echo-lengths up to 20 μm and the setup with resonant flippers measuring in the range up to 0.5 μm.

Spin-echo small-angle neutron scattering (SESANS) is a novel real-space scattering technique. SESANS measures a correlation-like functionG(Z), the meaning of which was unknown until now. Here a direct real-space interpretation ofG(Z) through the particle scattering density and pair correlation function is given. One-dimensional and two-dimensional SESANS are compared. The case of non-interacting particles is considered in detail with an explicit geometrical interpretation. General methods for the calculation of structural parameters, such as the total scattering length and the radius of gyration, are developed. Analytical expressions ofG(Z) for non-interacting solid spheres, hollow spheres and Gaussian coils are derived. The case of solid spheres is compared with experimental data.

The in¯uence of the microsegregation of Mn, Si, and Cr on the austenite decomposition during isothermal transformations in hot rolled medium carbon steel has been studied by neutron depolarisation, electron probe microanalysis (EPMA), and optical microscopy. Eight specimens of the same alloy were held at 1173 K for 30 min and were rapidly cooled to different isothermal transformation temperatures. Two-dimensional EPMA maps of the specimen annealed at 1013 K showed that microsegregation of alloying elements in hot rolled steel is strongly related to the ferrite/pearlite band formation. The local variations in alloying element concentration lead to variations in local transition temperatures, which were calculated with the thermodynamic database MTDATA. Similar EPMA maps for the specimen transformed at 953 K demonstrate the presence of microchemical bands, while optical microscopy reveals the absence of microstructural bands. It is shown that the formation of microchemical bands is a prerequisite for band formation, but that the kinetics of the phase transformation determines the actual formation of microstructural bands. A quantitative model has been developed, which describes the observations in terms of the relative difference between ferrite nucleation rates in regions with a high and low local undercooling and the subsequent growth of the ferrite. The isothermal transformation experiments have led to generalised nucleation and growth criteria for the formation of microstructural bands.MST/5150

The validity of nonmagnetic grain-boundary (NMGB) models for the initial permeability of polycrystalline ferrites is examined. The domain size in a series of wet-chemically prepared polycrystalline MnZn ferrites, in the demagnetized state, has been determined by neutron depolarization. A transition in the intragranular domain structure from mono- to two domain is observed at grain size D≊4 μm. An expression for this transition has been derived for a dense magnetic material. The grain size dependence of the initial permeability of the ferrites studied, particularly for monodomain grains, is consistent with the NMGB model. The grain-boundary width and composition have been determined with a nanoprobe (1.5 nm resolution). Composition variations extend ≊10 nm into the grain. The findings suggest that, although the NMGB model considers ‘‘nonmagnetic’’ grain boundaries, in practice the grain boundary may be hard magnetic.

Hard magnetic properties of Mn-Ga melt-spun ribbons J. Appl. Phys. 112, 083901 (2012) Influence of chemical pressure in Sn-substituted Ni2MnGa Heusler alloy: Experimental and theoretical studies J. Appl. Phys. 112, 073921 (2012) Hysteresis effects in the inverse magnetocaloric effect in martensitic Ni-Mn-In and Ni-Mn-Sn J. Appl. Phys. 112, 073914 (2012) Magnetic and electronic properties of Fe and Ni codoped SnO2

A method of carrying out high-precision measurements of crystal lattice parameters, at least an order of magnitude more sensitive than conventional neutron techniques using Larmor precession is presented. We show that the method can be used for absolute and relative measurements of the lattice constants and mosaicity and report some new results demonstrating the power of the method.Introduction. -Recently one of us (MTR) [1], has proposed the use of an existing neutron spin echo (NSE) instrument with tilted coils for high-precision diffraction measurements. This Larmor precession (LP) method can be applied to diffraction in 2 modes, the first, similar to the usual applications of NSE, is sensitive to the mosaic spread of a crystal. The second, called the "parallel" mode, is independent of the mosaic spread to first order, and can be used to measure changes in the lattice constant to high precision. (The method will allow measurements at least an order of magnitude more sensitive than usual neutron methods comparable to what can be achieved with X-rays. See for example the HRPD instrument at the Rutherford Laboratory http://www.nd.rl.ac.uk/crystallography/hrpd.htm.) This mode is highly unusual in that it works by summing the Larmor precession phases in the two arms of the spectrometer, as opposed to the other mode and usual NSE applications which work with the phase difference (echo). In this sense the new method is not a spin-echo, although it uses the same precession coils as the existing spin echo technique with tilted coils. In this work we present some general expressions for the Larmor precession angles.Neutron spin echo. -Neutron spin echo, first introduced in the 70's by Mezei [2] has proven to be a very powerful method of high-resolution neutron scattering spectroscopy. It is based on using Larmor precession in two coils, one before, and one after the sample. In a given coil the neutron spin will precess through an angle(1)