Papers by Keyword: High Temperature X-Ray Diffraction

Abstract: The liquid local structure of Au₅₀Cu₅₀ solid solution was detected by high-temperature X-ray diffraction experiment and Reverse Monte Carlo (RMC) simulation. The clusters in the liquid Au₅₀Cu₅₀ alloy comprise the 12-coordinated polyhedron with Au center, which was the same as the clusters in the liquid pure Au. In the case of alloying, there was a high population of Au-Au bonds, and the local structure around Cu atoms was changed. In the case of solidification, the 12-coordinated clusters around Au atoms were preserved into the AuCu alloy, forming the disordered solid solution structure. The strong tendency for Cu-Cu bonds was weakened from 2.35 Å in the liquid to 2.81 Å in the solid solution, and the local structure around Cu atoms rearranges. It is shown that the liquid structure of the Au₅₀Cu₅₀ alloy plays a crucial role in the solid solution. Our findings elucidate that the disordered solid solution structure in AuCu alloy stems from the highly dominated 12-coordinated clusters associated with centered Au atom in the melt.

Abstract: The liquid lead-bismuth eutectic (LBE) alloy is of great interest for applications in future nuclear reactors. The structure and clustering of alloying elements have been investigated on an extended range of temperature (125-720 °C) by high temperature X-ray diffraction (HT-XRD), XPS and scanning photoemission microscopy (SPEM) at the ELETTRA synchrotron in Trieste. After melting the short-range order in liquid metal corresponds to a cuboctahedral arrangement of atoms that progressively evolves towards an icosahedral one as temperature increases. Such process, which is completed around 720 °C, is accompanied by a micro-chemical re-arrangement of atoms with changes of cluster size and composition. At high temperature the atom distribution results to be more homogeneous and the average size of clusters noticeably smaller.

Abstract: Precursor phenomena of melting in pure metals and alloys have been investigated by means of Mechanical Spectroscopy (MS) and High Temperature X-ray Diffraction (HT-XRD). The examined materials were the pure metals In, Sn, Pb and Bi, and some alloys of the systems In-Sn and Pb-Bi with different compositions.MS tests have been carried out by means of a novel method developed by us that permits to operate in resonance conditions and employs hollow reeds of stainless steel containing the liquid metal. In all the metals a sharp drop of dynamic modulus and a Q^-1 maximum were observed in a temperature range ΔT before melting that depends on the specific metal and its structure. Such anelastic behaviour is consistent with an increase of the interstitialcies concentration as predicted by the Granato’s theory.Moreover, HT-XRD evidenced that sudden grain re-orientation, shift and broadening of diffraction peaks occur just before the formation of the first liquid, therefore self-interstitials and vacancies seem to play a synergic role in melting. The increase of self-interstitials over ΔT has the effect of weakening interatomic bonds that favours the successive vacancy avalanche leading to the collapse of crystal lattice (melting).

Abstract: The hexagonal cordierite was synthesized by the reverse coprecipitation-calcination method and characterized by powder X-ray diffraction. The lattice thermal expansion behavior of hexagonal cordierite was investigated by high temperature X-ray diffraction in the temperature range 298-1273 K. The lattice parameters of the hexagonal cordierite at different temperature were calculated by a least squares method. The hexagonal cordierite expressed anisotropic thermal expansion behavior with the average lattice thermal expansion coefficient were 2.13×10^-6 K^-1 along a or b axis and-1.03×10^-6 K^-1 along c axis from room temperature to 1273 K. The crystal structure of hexagonal cordierite at 298 K and 1273 K were refined by Rietveld method. The thermal expansion coefficient of the height of the [MgO₆]-[AlO₄] polyhedral layer is-1.8×10^-6 K^-1. Although the six-member ring expressed the normal positive thermal expansion along arbitrary direction, the height thermal expansion coefficient of the six-member ring is just 0.6×10^-6 K^-1.

Abstract: Ni-Cr-Fe based alloy Inconel 690 is widely used in power plant, marine, chemical and nuclear applications due to its excellent mechanical properties, resistance to thermal creep deformation, good thermal stability and resistance to corrosive and oxidizing environments. In order to study the microstructure of the alloy and the precipitates formation during thermal exposure, the alloy was subjected to in-situ high temperature X-ray diffraction technique (HT-XRD) in the temperature range 298-1273K. Results of high temperature XRD patterns show (111), (200), (220) and (311) reflections confirming the stability of fcc structure in the temperature of investigation. With increase in the temperature, a shift in peak positions towards lower 2θ values due to lattice expansion was noticed. The average thermal expansion coefficient (TEC) of the alloy increased from 1.33 x 10^-5 K^-1 to 1.53 x 10^-5 K^-1 in the temperature range 298-1273 K. Scanning electron microscopy indicates austenitic grains of sizes in the range 100-150μm and chromium carbide precipitate at grain boundaries after the HT-XRD heat treatment.

Abstract: W is a promising material to use as protection for thermal shields in future nuclear fusion reactors, however the joining to other metals is really challenging. For realizing such joints plasma spraying (PS) has been used for its simplicity, the possibility to cover complex and extended surfaces and the relatively low cost. An appropriate interlayer must be optimized to increase the adhesion of W on the substrates and to provide a soft interface for better thermo-mechanical compatibility.The present work demonstrates that high-temperature X-ray diffraction (HT-XRD) permits to quickly assess the reliability and quality of the coating-interlayer-substrate system by measuring the strain of coating. This is very useful to orientate the work for optimizing the structure and composition of the interlayer and the parameters of deposition process.

Abstract: This paper has put forward a high-temperature quantitative X-ray powder diffraction analysis method for the determination of an isothermal section of a ternary system in comparison with a conventional method. In a three-phase region of the isothermal section at 1150 °C of Cu₂O(CuO)-Al₂O₃-SiO₂ pseudo-ternary system, the compositions of the solid phase points of three system points are determined according to the quantitative analysis of the crystalline phases in the samples carried out by Rietveld method. Then the liquid phase point of the three-phase region is determined according to the crosspoints of the tie lines of every pair of system point and solid phase point. The precisions of the analytical results have reached to be 0.1 ~ 5.0 %. By comparison, a good result is obtained for the determination of the liquid phase point of the three-phase region in the isothermal section at 1150 °C when the analytical results of high-temperature RQA analysis are used in determination of the isothermal section of the pseudo-ternary system.

Abstract: Microwave sintered Si3N4-MgO system that contains 2, 4 and 10 wt% of ZrO2 as secondary particulates were investigated with respect to phase transformation and microstructure development. The experimental results of microwave sintered samples were compared with conventional methods. Complete α to β phase transformation was observed in the case of microwave sintered samples due to the volumetric nature of microwave heating. High temperature X-ray diffraction (HTXRD) analysis was performed to study in-situ the oxidation behavior of Si3N4 specimens. Si3N4 specimens with 10 wt % ZrO2 were exposed to air at temperature between 25°C and 900°C for up to 24 hours. Microwave sintered sample were structurally stable in air 25°C and 900°C for up to 24 hours of testing.

Abstract: High temperature phase transformations in EUROFER reduced activation ferritic martensitic (RAFM) steel were studied in-situ by means of X-ray diffraction. Results show that, during slow cooling, the austenite to ferrite transformation takes place around 755 oC. Full transformation of the austenitic phase into pure martensite is observed for cooling above 5 oC/min. This transformation was found in samples annealed at 950 oC for 3 h and quenched in liquid nitrogen. TEM analyses reveal a high concentration of carbides along the grain boundaries of the martensitic structure. The thermal expansion coefficient derived from the measurements was 12.7x10-6 K-1.

Abstract: In order to evaluate potential of La1-xCaxCrO3-δ as a material for interconnector of solid oxide fuel cells, reduction expansion was measured by using high temperature X-ray diffraction under various P(O2). Obtained reduction expansion increased with increasing temperature and Ca content. With combination of obtained lattice constants and oxygen nonstoichiometry data, the effect of oxide ion vacancy on crystal lattice and their thermodynamic behavior have been deduced. It was proposed that expansion behavior by δ of La0.9Ca0.1CrO3-δ can be explained assuming ideal solution model on oxide ion vacancies, however, deviation from ideal solution model was observed in reduction expansion behavior of La0.8Ca0.2CrO3-δ and La0.7Ca0.3CrO3-δ with δ region larger than 0.03 and 0.05, respectively. It was revealed that the crystal system approached to cubic with increase of δ in the region where the deviation from ideal solution model was observed.