R. Serve

Bio: R. Serve is an academic researcher. The author has contributed to research in topics: Neutron scattering & Small-angle neutron scattering. The author has an hindex of 1, co-authored 1 publications receiving 12 citations.

A combined furnace for crystal growth and neutron scattering

Abstract: A furnace for growing single crystals of metals and metal alloys by the floating zone technique has been constructed. This makes it possible to grow single crystals of materials with a phase transition in their high temperature phase in situ on a neutron scattering instrument. Without the crystals being cooled below their transition temperature they can be oriented and a neutron scattering experiment can be performed. The typical crystal size is about 5 cm 3 . The furnace is designed for continuous operation up to 2000 K under ultrahigh vacuum or high purity gas atmospheres. It gives access to a 360° neutron scattering plane with a maximum angle of inclination of ±20°.

Phonons at martensitic phase transitions of bcc-Ti, bcc-Zr and bcc-Hf

Abstract: 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.

X-ray and Neutron Scattering

Abstract: This chapter describes the use of neutrons and X-rays as probes in the study of structural and dynamic properties of metallic materials. Crystalline materials are characterized by their diffraction peaks related to their average crystallographic structure. In real crystals, locally displaced atoms and chemically (or isotopically for neutrons) different species may lead not only to changes of peak shapes and positions, but also to additional (diffuse) scattering between Bragg peak, including scattering around the primary beam (small-angle scattering). All these features can be used to extract information about the state of a sample, its compositional and structural variations on a scale depending on the scattering, in static and time-resolved kinetic studies. Energy-resolved scattering also offers an insight into solid-state dynamics on a microscopic scale. Some of the most important methods will be described and illustrated by instructive examples. The presentation offers a combined view of neutron and X-ray scattering, with the necessary simplifications dictated by space limitations. The special properties of thermal neutrons and of hard X-rays (now widely available at synchrotron radiation sources), their mutual combination, and combinations with other methods, in particular electron microscopy, offer ample opportunity to better understand and control materials properties. After a brief introduction to scattering from real crystals and some general ideas about long-range strains and Bragg peaks, the vicinity of Bragg peaks (displacement scattering at large scattering angles), the scattering far away from Bragg peaks (chemical heterogeneities, short-range order), and, in greater detail, small-angle scattering (which is not sensitive to the extent of crystallinity, but to nanoscale variations of chemical composition and of magnetization, precipitation) will be described, along with classical and more recent applications related to short-range ordering and precipitation in bulk and nanostructured alloys. Some other fields are only briefly addressed (grazing-incidence studies of surfaces, radiography, absorption spectroscopies, coherent X-rays). The final section offers some information on the influence of defects on lattice dynamics and on (slow) diffusive motion in materials.

Lattice dynamics and related diffusion properties of intermetallics: I. Fe3Si

Abstract: The phonon dispersion of iron-rich Fe-Si alloys of D03 structure (space group Fm-3m) has been studied by inelastic neutron scattering. The measurements were carried out on two crystals of different composition: Fe75Si25 at 20 and 930 degrees C, and Fe80Si20 at 20, 930 and 1100 degrees C. The respective degree of order was determined by powder diffraction measurements. A general decrease of phonon frequencies with increasing temperature is found, the temperature dependence being strongest for transverse phonons with ( xi xi 0) propagation. With decreasing order, phonon gaps close and the highest optical band degenerates into a broad distribution of inelastic intensity due to disorder scattering. The dependence of the dispersion on the alloy composition is not very pronounced. The migration enthalpies as well as several other thermodynamic quantities have been calculated from the densities of states. Although the low frequencies of the transverse phonons indicate particularly low migration barriers, they cannot explain the pronounced change of the diffusivity with composition. This composition dependence is tentatively explained by high vacancy concentrations.

Design and operating characteristic of a vacuum furnace for time-of-flight inelastic neutron scattering measurements.

Abstract: We present the design and operating characteristics of a vacuum furnace used for inelastic neutron scattering experiments on a time-of-flight chopper spectrometer. The device is an actively water cooled radiant heating furnace capable of performing experiments up to 1873 K. Inelastic neutron scattering studies performed with this furnace include studies of phonon dynamics and metallic liquids. We describe the design, control, characterization, and limitations of the equipment. Further, we provide comparisons of the neutron performance of our device with commercially available options. Finally we consider upgrade paths to improve performance and reliability.