Measuring Single-Crystal Diffuse Neutron Scattering on the Wombat High-Intensity Powder Diffractometer

Single-crystal diffuse scattering was collected on the Wombat high-intensity powder diffractometer at the OPAL reactor at the Bragg Institute. The difficulty in measuring diffuse scattering comes from its relatively low intensity compared to the Bragg peaks, a factor of 10³10^{3} to 10⁴10^{4} smaller. Wombat allows collection of diffuse scattering due to its high intensity and large two-dimensional detector. Diffuse scattering data from yttria-stabilized cubic zirconia (YCSZ, Y₂O₃\hbox{Y}_2\hbox{O}_3 stabilized ZrO₂\hbox{ZrO}_2) and PbZn_1/3Nb_2/3O₃\hbox{PbZn}_{1/3}\hbox{Nb}_{2/3}\hbox{O}_3 (PZN) were successfully collected, the latter at a range of temperatures. The data were processed, normalized, and background subtracted to reconstruct flat reciprocal space sections with a minimum of artifacts. The strategies used to tackle the collection of neutron diffuse scattering and the way in which they are implemented will be discussed. The results show that the neutron powder diffractometer with a continuous detector is capable of collecting high-quality diffuse scattering data.     

Precipitation in Nb-Stabilized Ferritic Stainless Steel Investigated with <Emphasis Type="Italic">in-situ</Emphasis> and <Emphasis Type="Italic">ex-situ</Emphasis> Transmission Electron Microscopy

A Nb-stabilized Fe-15Cr-0.45Nb-0.010C-0.015N ferritic stainless steel is studied with transmission electron microscopy (TEM) to investigate the morphology and kinetics of precipitation. Nb_x(C,N)_y\hbox{Nb}_{x}\hbox{(C,N)}_y and MnS precipitates are present in the steel in the initial condition. Ex-situ TEM analysis is performed on samples heat treated at 973 K, 1073 K, 1173 K, and 1273 K (700 °C, 800 °C, 900 °C, and 1000 °C). Within this temperature range, both Fe₂Nb\hbox{Fe}_2\hbox{Nb} and Fe₃Nb₃X_x\hbox{Fe}_{3}\hbox{Nb}_{3}\hbox{X}_{x} (with X = C or N) precipitates form. Fe₂\hbox{Fe}_2Nb is observed at 1073 K (800 °C).   Fe₃Nb₃X_x\;\hbox{Fe}_{3}\hbox{Nb}_{3}\hbox{X}_{x} precipitates form at the grain boundaries between 973 K and 1273 K (700 °C and 1000 °C). Up to at least 1173 K (900 °C) their fraction increases with time and temperature, but at 1273 K (1000 °C) they lose stability with respect to Nb_x(C,N)_y.\hbox{Nb}_{x}\hbox{(C,N)}_{y}. With in-situ TEM, no phase transition is observed between room temperature and 1243 K (970 °C). At 1243 K (970 °C) the precipitation of Fe₃Nb₃X_x\hbox{Fe}_{3}\hbox{Nb}_{3}\hbox{X}_{x} is observed in the neighborhood of a dissolving Nb₂\hbox{Nb}_2(C,N) precipitate. For sections of grain boundaries where no Nb_x(C,N)_y\hbox{Nb}_x\hbox{(C,N)}_y precipitates are present, Fe₃Nb₃X_x\hbox{Fe}_3\hbox{Nb}_3\hbox{X}_{x} does not form. It is concluded that the precipitation of Fe₃Nb₃X_x\hbox{Fe}_{3}\hbox{Nb}_{3}\hbox{X}_x is directly related to the dissolution of Nb₂\hbox{Nb}_2(C,N) through the redistribution of C or N.     

First-Principles Calculations on Stabilization of Iron Carbides (Fe3C, Fe5C2, and η-Fe2C) in Steels by Common Alloying Elements

The control of carbide formation is crucial for the development of advanced low-alloy steels. Hence, it is of great practical use to know the (de)stabilization of carbides by commonly used alloying elements. Here, we use ab initio density functional theory (DFT) calculations to calculate the stabilization offered by common alloying elements (Al, Si, P, S, Ti, V, Cr, Mn, Ni, Co, Cu, Nb, Mo, and W) to carbides relevant to low-alloy steels, namely cementite $(\hbox{Fe}_{3}\hbox{C}),$ H?gg $(\hbox{Fe}_{5}\hbox{C}_{2}),$ and eta-carbide $(\eta{\text{-}}\hbox{Fe}_{2}\hbox{C})$ . All alloying elements are considered on the Fe sites of the carbides, whereas Al, Si, P, and S are also considered on the C sites. To consider the effect of larger supercell size on the results of (de)stabilization, we use both 1?×?1?×?1 and 2?×?2?×?2 supercells in the case of $\hbox{Fe}_{3}\hbox{C}.$      

Inverse Magnetocaloric Effect Related to the Magnetostructural Phase Transition in Quaternary Ni–Co–Mn–Al Alloy

The inverse magnetocaloric effect of Ni–Co–Mn–Al quaternary alloy with the relatively low material cost is achieved firstly in a theoretical study (V. Sokolovskiy et al.: J. Appl. Phys., 2020, vol. 127, p. 163901). To investigate and prove this study, the exact composition of \(\hbox{Ni}_{{40}}\hbox{Co}_{{10}}\hbox{Mn}_{{36}}\hbox{Al}_{{14}}\) alloy is selected and and explored by the combination of X-ray diffraction, scanning electron microscopy, resistivity, and magnetic studies. The quaternary alloy reveals that the main phase is associated with a martensitic L1₀ phase structure with some austenitic B2 phase in the vicinity of room temperature. The results show that the alloy maintains both Austenite and Martensite phases and has a grand scale change in magnetization of approximately 95 emu \(\hbox{g}^{-1}\) around the Martensitic phase transition (in the range of 20 K) that exhibits a first-order magnetic transition from ferromagnetic to non-ferromagnetic state. The alloy reveals the inverse magnetic entropy change of about 12 and 8 J \(\hbox{kg}^{-1}\,\hbox{K}^{-1}\) and the relative cooling power of 125 and 76 J kg⁻¹ under only 15 and 10 kOe, respectively. Likewise, the MR value of 11.5 pct obtains in the external magnetic field source of 10 kOe in the heating direction. The experimental results support the referenced theoretical study and make this material prominent in future magnetocaloric and magnetoresistivity studies.

     

Diffuse Scattering and Monte Carlo Studies of Relaxor Ferroelectrics

A renewed interest in the field of ferroelectricity has taken place in recent years since the finding of exceptional piezoelectric properties in the lead-oxide class of relaxor ferroelectric materials typified by the disordered perovskite PbZn_1/3Nb_2/3O₃ (PZN). Although PZN and numerous related materials have been extensively studied over a long period, a detailed understanding of the exact nature of their polar nanostructure has still not emerged. In this article, we describe the development of Monte Carlo computer models, which seek to account for the detailed three-dimensional (3-D) diffuse neutron scattering data that have been recorded from a single crystal of PZN. It has been established that the observed diffuse patterns are due to planar nanodomains oriented normal to the six directions, but there is still some uncertainty concerning the direction of the local Pb ionic shifts, which remains an area of controversy. It is argued that further detailed analysis and experiments in which data are recorded with the crystal in an applied field should allow these remaining issues to be resolved. This article is based on a presentation given in the symposium entitled “Neutron and X-Ray Studies for Probing Materials Behavior,” which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee.     

Diffusion of Au in the Intermetallic Compound Ti<Subscript>3</Subscript>Al

The Au diffusion in the Ti₃Al compound was investigated at six compositions from 25 to 35 at. pct Al by using the diffusion couples (Ti-X at. pct Al/Ti-X at. pct Al-2 at. pct Au; X = 25, 27, 29, 31, 32, and 35) at 1273 to 1423 K. The diffusion coefficients of Au in Ti₃Al ( D_\textAu^{\textTi₃ \textAl} ) \left( {D_{\text{Au}}^{{{\text{Ti}}_{3} {\text{Al}}}} } \right) are relatively close to those of Ti. The D_\textAu^{\textTi₃ \textAl} \texts {D}_{\text{Au}}^{{{\text{Ti}}_{3} {\text{Al}}}} {\text{s}} slightly increase with Al concentration within the same order of magnitude. The activation energies of Au diffusion, Q_\textAu^{\textTi₃ \textAl} \texts, Q_{\text{Au}}^{{{\text{Ti}}_{3} {\text{Al}}}} {\text{s}}, evaluated from the Arrhenius plots were relatively close to those of Ti diffusion, Q_\textTi^{\textTi₃ \textAl} \texts, Q_{\text{Ti}}^{{{\text{Ti}}_{3} {\text{Al}}}} {\text{s}}, rather than those of Al diffusion, Q_\textAl^{\textTi₃ \textAl} \texts; {Q}_{\text{Al}}^{{{\text{Ti}}_{3} {\text{Al}}}} {\text{s}}; therefore, it was suggested that Au atoms diffuse by the sublattice diffusion mechanism in which Au atoms substitute for Ti sites preferentially in Ti₃Al and diffuse by vacancy mechanism on Ti sublattice. The influence of the D0₁₉ ordered structure (hcp base) of Ti₃Al on diffusion of Au and other elements is discussed by comparing the diffusivities in Ti₃Al and α-Ti.     

Hydrogen Solubility in Pd/V2O5 Composites Formed by Internal Oxidization and Hydrogen Isotherms in Un-oxidized Pd-V Alloys

Pd-V alloys were internally oxidized (IOed) resulting in composites of nano-particle V₂O₅ precipitates within Pd matrices. These composites were found to interact with H₂ to form hydrogen bronzes, H _x V₂O₅, within the Pd matrix where x can vary between 1.65 and 2.20. Relative partial molar enthalpies for H intercalation into the H-bronze within the Pd/V₂O₅ composite were measured calorimetrically as a function of the H content of the bronze, and these molar enthalpies decrease in magnitude from about ?75 to ?20 kJ/mol H as the H content increases. H₂ isotherms have also been measured in disordered, fcc Pd_0.96V_0.04, Pd_0.945V_0.055, and Pd_0.93V_0.07 alloys from 273 K to 343 K (0 °C to 70 °C). Thermodynamic data have been derived from these isotherms. The relative partial molar enthalpies at infinite dilution of H, $\Updelta H_{\hbox{H}}^\circ,$ increase with atom fraction V, X $_{\hbox{V}},$ while the corresponding standard partial molar entropies, $\Updelta \hbox{S}_{\hbox{H}}^\circ,$ decrease with $\hbox{X}_{\hbox{V}}.$ The first-order term, g₁, in a polynomial expansion of the excess or non-ideal chemical potential of H in r = H-to-metal, mol ratio, decreases in magnitude with $\hbox{X}_{\hbox{V}}$ at a given temperature.     

Calculation of the product phase grain boundary area during solid state transformations

A general formal expression is derived for the calculation of product phase grain boundary area per unit volumeA _v at any time during solid state transformations occurring by nucleation and growth process. It is assumed that the spatial distribution of the product phase particles is random and dis-crete product phase particles have a spherical shape. The analysis is applicable to any arbitrary nu-cleation and growth kinetics. For the sizeindependent growth rate,A _v is given by: whereV _{V ex} is the extended volume fraction of the product phase, andS _{V ex} is the total extended product phase-matrix interfacial area per unit volume. If the growth rate depends onparticle size or particle size and time, then, The results are applicable to any arbitrary functional form of nucleation rate. The result for size de-pendent growth rate is approximate; however, the error involved in this approximation is less than ±10 pct. The analysis demonstrates thatA _v , and also the grain size of transformed structure, are basically determined by thepath of microstructural evolution described by the variation of product phase-matrix interfacial area per unit volume with the product phase volume fraction, and do not explicitly depend on any other variables. The analysis is also applicable to nonisothermal and con-tinuous cooling transformations. On leave from the Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur-208016, India