Bct Structure Research Articles

Magnetism induced by electrochemical nitriding on an austenitic stainless steel

The surface of a Fe-Ni-Cr Alloy (SUS316L) plate was electrochemically nitrided in molten LiF-KF salt including Li3N at 873K. The crystal structure changed from fcc structure to bct structure with nitrogen introduction. The Nitrogen diffusion layers were predominately formed at nitrogen concentration of 23 at%. The nitriding process drastically also changed its magnetic property from non-magnetic to ferromagnetic. The magnetic field of 20 kOe saturated the magnetic moment with its magnetization of 81 emu/g at 10K. The anisotropic magnetization is ascertained. Based on CrN formation and Cr extraction from the original Fe-Ni-Cr system, the induced ferromagnetism was discussed.

Microstructures and Magnetic Properties of Fe70−x Pd30Ni x High-Temperature Ferromagnetic Shape Memory Alloys

This research paper mainly presents an investigation of the microstructures and magnetic properties of bulk ferromagnetic shape memory (FSM) Fe70−x Pd30Ni x (Ni x = 4, 8 at.%) alloys, by transmission electron microscopy (TEM), a magnetostrictive-meter setup, and a superconducting quantum interference device (SQUID) magnetometer. The FSM alloys were homogenized through hot and cold strain forging (SF) to a ∼38 % reduction in thickness, solution-treated (ST) with annealing recrystallization at 1100 °C or 8 h, quenched in ice brine, and then aged at 500 °C for 100 h (5 HTA). The investigation of the microstructures and magnetic properties indicated that the higher Ni content (N x = 8 at.%) in the Fe62Pd30Ni8 alloy SF and ST reduced the saturation magnetostriction at RT. However, with higher Ni content in the Fe62Pd30Ni8 alloy, the decomposition of L10 + L1m twin phases into stoichiometric L10 + L1m + α bct structures was suppressed after the alloy was ST and aged at 5 HTA, as confirmed by TEM investigation. The result was that the FSM Fe62Pd30Ni8 alloy maintained a high saturation magnetostriction and magnetization after the alloy was ST and aged at 5 HTA. This magnetic property of the Fe62Pd30Ni8 FSM alloy makes it suitable for application in high-temperature (T < 500 °C) and high-frequency environments. However, low Ni content FSM Fe66Pd30Ni4 (N x = 4 at.%) alloy SF, ST, and aged at 5 HTA underwent decomposition of the L10 + L1m twin phases into the stoichiometric L10 + L1m + α bct structures, as confirmed by TEM, leading to a decrease of saturation magnetostriction and magnetization. This magnetic property of the Fe66Pd30Ni4 FSM alloy is not suitable for application in high-temperature (T < 500 °C) environments.

Characterization of Continuous and Discontinuous Precipitation Phases in Pd-Rich Precious Metal Alloys

Aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray diffraction (XRD), electron backscatter diffraction, and electron probe microanalysis were applied to characterize continuous and discontinuous phase formation in precious metal alloys used in electrical contacts. The Pd-rich Paliney® (®Paliney is tradename of Deringer-Ney Inc., Bloomfield, CT) alloys contain Pd, Ag, Cu, Au, Pt (and Zn or Ni). With aging at 755 K (482 °C), nanometer-scale chemistry modulation was observed indicating spinodal decomposition. An ordered body-centered tetragonal (bct) structure was also observed with AC-STEM after the 755 K (482 °C) aging treatment and another phase, tentatively identified as β-Cu3Pd4Zn, was found by microscopy and XRD after prolonged holds at higher temperatures. During slow cooling or isothermal holds at high temperature [755 K to 973 K (482 °C to 700 °C)], a two-phase lamellar structure develops along grain boundaries by discontinuous precipitation. XRD and AC-STEM showed that the lamellar structure was comprised of Ag-rich and Cu-rich fcc phases (α 1 and α 2). The phases are discussed in relation to a pseudo-ternary diagram based on Ag-Cu-Pd, which provides a simplified representation of the discontinuous phase compositions in the multi-component alloy system.

The Embedded Atom Model and large-scale MD simulation of tin under shock loading

The goal of the work was to develop an interatomic potential, that can be used in large-scale classical MD simulations to predict tin properties near the melting curve, the melting curve itself, and the kinetics of melting and solidification when shock and ramp loading. According to phase diagram, shocked tin melts from bcc phase, and since the main objective was to investigate melting, the EAM was parameterized for bcc phase. The EAM was optimized using isothermal compression data (experimental at T=300 K and ab-initio at T=0 K for bcc, fcc, bct structures), experimental and QMD data on the Hugoniot and on the melting at elevated pressures. The Hugoniostat calculations centred at β-tin at ambient conditions showed that the calculated Hugoniot is in good agreement with experimental and QMD data above p-bct transition pressure. Calculations of overcooled liquid in pressure range corresponding to bcc phase showed crystallization into bcc phase. Since the principal Hugoniot of tin originates from the β-tin that is not described by this EAM the special initial state of bcc samples was constructed to perform large-scale MD simulations of shock loading.

Structural phase transition of Ru at high pressure and temperature

The magnetism, stabilities and phase transition of Ru in hcp, fcc, bct and bcc structures are investigated with detailed first-principles calculations based on density-functional theory and quasiharmonic lattice dynamics approximation. Magnetic ground states and stability ranges of various phases are obtained. Calculated results indicate that the non-magnetic (NM)-hcp structure is the most stable in the entire pressure range at zero temperature, and the structural transition cannot be induced by pressure alone. NM-fcc structure is a metastable phase of bulk Ru, while both the NM-bcc and ferromagnetic (FM)-bct structures are dynamically unstable. At high pressure and temperature, a transformation from NM-hcp to NM-fcc structure will occur. Finally, the pressure-temperature phase diagram of Ru is presented.

Effects of Cold Deformation on Precipitation in Fe-18Cr-12Mn-0.48N High-Nitrogen Austenitic Stainless Steel

The precipitation behaviors were investigated during isothermally aging at 700°C and 800°C after cold compressed by 30% in thickness in Fe-18Cr-12Mn-0.48N high-nitrogen austenitic stainless steel by using optical microscopy (OM), laser scanning confocal microscopy (LSCM) and transmission electron microscopy (TEM). The results show that precipitation morphology in cold-deformed sample is the same as non-cold-deformed sample, which also displays discontinuous cellular way. Cold deformation accelerates the precipitation of M2N phase. The precipitation occurs at not only grain boundaries but also twin grain boundaries in the experimental steel. In cold-deformed sample, besides the long-strip M2N precipitates, intermetallic phase-σ phase owning bct structure and lattice parameters of a=0.8800nm and c=0.4544nm were observed along grain boundaries and inside the grain.

First principles calculations of the structure and elastic constants of α, β and γ uranium

This study analyzes structural and elastic properties of five uranium crystal structures: the face centered orthorhombic A20 (α phase), the tetragonal D8b (β phase), body centered tetragonal (bct), body centered cubic (γ phase) and face centered cubic structures. Calculations are performed within the density functional theory framework employing the Projector Augmented Wave method and the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE–GGA) of the exchange correlation. The elastic constants are used to compute polycrystalline elastic moduli, Poisson’s ratio and the Debye temperature for all five structures. The α and γ phase properties are compared with theoretical and experimental results. The complex tetragonal 30atom beta phase is examined in detail. Representation of the β phase by a bct structure is examined; we find that the structure of the β phase is significantly different from the bct phase but exhibits similar elastic properties. This is the first comprehensive investigation into the elastic constants of uranium utilizing the PBE–GGA.

Modulated structure and molecular dissociation of solid chlorine at high pressures

Among diatomic molecular halogen solids, high pressure structures of solid chlorine (Cl(2)) remain elusive and least studied. We here report first-principles structural search on solid Cl(2) at high pressures through our developed particle-swarm optimization algorithm. We successfully reproduced the known molecular Cmca phase (phase I) at low pressure and found that it remains stable up to a high pressure 142 GPa. At 150 GPa, our structural searches identified several energetically competitive, structurally similar, and modulated structures. Analysis of the structural results and their similarity with those in solid Br(2) and I(2), it was suggested that solid Cl(2) adopts an incommensurate modulated structure with a modulation wave close to 2∕7 in a narrow pressure range 142-157 GPa. Eventually, our simulations at >157 GPa were able to predict the molecular dissociation of solid Cl(2) into monatomic phases having body centered orthorhombic (bco) and face-centered cubic (fcc) structures, respectively. One unique monatomic structural feature of solid Cl(2) is the absence of intermediate body centered tetragonal (bct) structure during the bco → fcc transition, which however has been observed or theoretically predicted in solid Br(2) and I(2). Electron-phonon coupling calculations revealed that solid Cl(2) becomes superconductors within bco and fcc phases possessing a highest superconducting temperature of 13.03 K at 380 GPa. We further probed the molecular Cmca → incommensurate phase transition mechanism and found that the softening of the A(g) vibrational (rotational) Raman mode in the Cmca phase might be the driving force to initiate the transition.

High coercivity of Alnico thin films: effect of Si substrate and the emergence of a novel magnetic phase

In our research work to study the spinodal decomposition of Alnico thin films prepared by sputtering on Si substrates, it has been discovered that Si diffuses into the films and gives rise to a new magnetically hard phase with T C = 305 °C. As-made thin films with the “α phase” of the Alnico were heat treated by annealing at different temperatures in the range of 600–900 °C. The heat treatment gave a room temperature coercivity of 6.5 kOe after annealing at 900 °C. The maximum coercivity observed is approximately ten times larger than the bulk Alnico V value. Planar electron diffraction patterns can be mostly indexed to an FCC spinel phase with a = 7.79 A. On the other hand, 80 nm deep inside the film a separate layer is observed with smaller grains having BCT structure with lattice constants a = 2.65 and c = 3.19 A. We strongly believe that this phase is related to the observed high coercivities in the samples. This discovery is very important in view of the recent “rare earth problem” and may lead to alternative to rare earth materials for the development of high performance magnets.

Martensitic transformation and magnetic properties of ferromagnetic shape memory Fe 66Pd 30Rh 4 alloys

This study mainly shows that in the Fe 66Pd 30Rh 4 (at%) alloys, the L1 0 phase plays an important role in magnetostriction due to the interplay of L1 0 martensitic twins with magnetic domains. The L1 0 martensitic twin structure exhibits a strong magnetocrystalline anisotropy energy constant ( K u =1.27–2.84×10 6 (ergs/cm 3)) along the tetragonal c axis direction. In addition, the L1 0 tetragonal martensitic twin structure shows both a perfect shape memory and a reversible shape memory effect; therefore, it is expected to be applicable in magneto-mechanical applications (such as microactuators or springs). However, in this study, we discover that solution treatment (ST) and aging heat treatments of Fe 66Pd 30Rh 4 ferromagnetic shape memory alloys influence the behavior of the martensitic transition, which is associated with the change in magnetic properties. The process of a thermoelastic L1 0+L1 m twin phase decomposition→non-thermoelastic L1 0+L1 m + α bct structure in Fe 66Pd 30Rh 4 alloys during solution treatment and aging at 400–550 °C for various times is studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The relation of phase separation morphology to the magnetic property change is examined with a superconducting quantum interference device (SQUID) magnetometer, and magnetostriction measurement is performed with a strain gage method and magnetostrictive meter setup. The results indicate that the process of martensitic transformation during aging leads to an increase in coercivity and a decrease in magnetostriction, simultaneously.

Reversible Solvent Vapor-Mediated Phase Changes in Nanocrystal Superlattices

Colloidal nanocrystals are being explored for use in a variety of applications, from solar cells to transistors to medical diagnostics and therapy. Ordered assemblies of nanocrystals, or superlattices, are one particularly interesting class of these materials, in which the nanocrystals serve as modular building blocks to construct nanostructures by self-assembly with spatial and temporal complexity and unique properties. From a fundamental perspective, the nanocrystals are simple molecular models that can be manipulated and studied to test statistical mechanical and thermodynamic models of crystallization and disorder. An article by Bian et al. in this issue of ACS Nano reports surprising new phase behavior in semiconductor nanocrystal superlattices: reversible transitions between non-close-packed body-centered cubic (bcc) and body-centered tetragonal (bct) structures, and close-packed face-centered cubic (fcc) structures, observed by real-time in situ grazing incidence small-angle X-ray scattering (GISAXS) measurements, upon solvent vapor exposure and increased interparticle separation. These studies offer new insight and raise new questions about superlattice structure and the forces that control self-assembly. Accompanying computer simulations show that ligand-ligand interactions are important. Furthermore, it appears that ligand-coated nanocrystals have more in common with soft microphase-separated materials, like diblock copolymers and surfactant assemblies, than previously realized.

Shape-Anisotropy Driven Symmetry Transformations in Nanocrystal Superlattice Polymorphs

Despite intense research efforts by research groups worldwide, the potential of self-assembled nanocrystal superlattices (NCSLs) has not been realized due to an incomplete understanding of the fundamental molecular interactions governing the self-assembly process. Because NCSLs reside naturally at length-scales between atomic crystals and colloidal assemblies, synthetic control over the properties of constituent nanocrystal (NC) building blocks and their coupling in ordered assemblies is expected to yield a new class of materials with remarkable optical, electronic, and vibrational characteristics. Progress toward the formation of suitable test structures and subsequent development of NCSL-based technologies has been held back by the limited control over superlattice spacing and symmetry. Here we show that NCSL symmetry can be controlled by manipulating molecular interactions between ligands bound to the NC surface and the surrounding solvent. Specifically, we demonstrate solvent vapor-mediated NCSL symmetry transformations that are driven by the orientational ordering of NCs within the lattice. The assembly of various superlattice polymorphs, including face-centered cubic (fcc), body-centered cubic (bcc), and body-centered tetragonal (bct) structures, is studied in real time using in situ grazing incidence small-angle X-ray scattering (GISAXS) under controlled solvent vapor exposure. This approach provides quantitative insights into the molecular level physics that controls solvent-ligand interactions and assembly of NCSLs. Computer simulations based on all-atom molecular dynamics techniques confirm several key insights gained from experiment.

Organic mediator-induced structural transformation in superlattices of monolayer-protected gold nanoparticles

N-acetylglutathione (NAG)-protected gold nanoparticles self-assemble into three-dimensional (3D) face-centered cubic ( fcc)-type superlattices at an air/water interface under highly acidic conditions. To prepare the well-defined superlattices, 1 month’s incubation is at least necessary since the size growth of the as-prepared nanoparticles is essential. Addition of 4-pyridinecarboxyic acid (PyC), a bifunctional hydrogen-bonding mediator, promotes the formation of the superlattices, which are created for about 2 weeks’ storage. Interestingly, PyC-induced nanoparticle superlattices are in a body-centered tetragonal ( bct) structure. The fcc-to- bct phase transformation would be due to stronger interaction between NAG and PyC than that between NAG molecules on the gold nanoparticle surfaces.

Hydrogenation properties of Fe–Ti–V bcc alloys

The hydrogenation properties of Fe–Ti–V bcc alloys have been studied by the Sieverts method. Starting from a fully determined ternary phase diagram, a large number of compositions have been synthesized and pressure–composition–isotherms have been measured at different temperatures both in the low and high hydrogen concentration regions. The enthalpies of absorption and desorption have been found to vary linearly as a function of composition in the ternary domain and depend on the ones of the pure elements. The absorption capacity is mainly dependent on the iron concentration. Additionally, the crystal structure of the hydrides has been studied. For the first time a progressive and continuous distortion from body centered cubic (bcc) to body centered tetragonal (bct) structure has been evidenced.

Electronic and optical properties of body-centered-tetragonal Si and Ge

We present a first-principles calculation of the quasiparticle and optical excitation spectra of recently predicted silicon and germanium polytypes in the body-centered-tetragonal (bct) structure. The quasiparticle spectra calculated within the $GW$ approximation predict that both silicon and germanium in the bct structure are small band gap materials with indirect gaps of 0.86 and 0.38 eV, respectively. The optical spectra is evaluated by solving the Bethe-Salpeter equation taking into account electron-hole interactions. We then make comparison to the cubic phases of Si and Ge which suggest the possible utility of the silicon bct phase in photovoltaic applications.

Effect of hydrogen on the structure of alloys formed in the Ti-Zr-Hf-H system

Investigated was the crystal structure of the Ti-Zr, Ti-Hf, and Ti-Zr-Hf alloys prepared by the novel method of “hydride cycling” suggested by us recently. The XRD data showed formation of the ω-phase in alloys with hydrides ratios TiH2/ZrH2 (TiH2/HfH2) ≈ 2, 1, 0.44: Ti0.67Zr0.33 (Ti2Zr), Ti0.67Hf0.34 (Ti2Hf), etc. The ω-phase is unusual for binary Ti-Zr and Ti-Hf alloys at normal pressure. The SHS reaction of α- and ω-alloys with hydrogen was found to yield complex hydrogen-rich hydrides with a fcc or bct structure such as Ti0.9Zr0.1H1.9, Ti0.677Zr0.33H1.81, Ti0.5Zr0.5H1.37, and Ti0.3Zr0.7H1.96.

The uniaxially aperiodic structure of a thin Cu film on fivefoldi-Al–Pd–Mn

A thin film of copper on the fivefold surface of Al–Pd–Mn forms a structure that is uniaxiallycommensurate with the aperiodic structure of the substrate. This structure has been analyzedusing low-energy electron diffraction and is found to consist of a vicinal surface of abody-centered tetragonal (bct) (100) structure. This bct(100) structure has lattice parameters ofa = 2.88 Å,b = 2.55 Å andc = 2.88 Å, with the vicinal surfacemaking an angle α of 13.28° relativeto the a–b plane. This structure provides an explanation for the delayed ordering observed during thegrowth of the film. Simple conditions are derived for which the growth of orderedone-dimensionally quasiperiodic thin films on quasicrystals may be favorable. This findingis relevant to the use of quasicrystals as a means of matching interfaces in thin filmsystems.

Effects of Si addition on the microstructure and the hydrogen storage properties of Ti 26.5V 45Fe 8.5Cr 20Ce 0.5 BCC solid solution alloys

The microstructure and the hydrogen storage properties of Ti 26.5(V 0.45Fe 0.085) 100− x Cr 20Ce 0.5Si x ( x = 0 and 1) have been investigated by EPMA, XRD, in situ temperature XRD, neutron diffraction and P-C isotherm. Si addition results in the precipitation of a TiFe 2-type Laves phase and produces chemical heterogeneity in the BCC phase. As a consequence, Si-added alloy exhibits a lower hydrogen capacity and both a higher plateau pressure and slope factor as compared to Si-free alloy. Si enters in both Laves and BCC phases with a higher preference for the former phase. For both alloys, all metal atoms (Ti, V, Fe and Cr) are supposed to be randomly distributed in the 2 a sites of the BCC phase and deuterium atoms occupy the 8 c sites on fully charged deuterides. Si has no significant influence on the hydrogen occupation. Two hydrides are observed during the desorption process for Ti 26.5(V 0.45Fe 0.085) 100Cr 20Ce 0.5 alloy, a hydrogen rich one with distorted FCC structure (space group: P4/ mmm) and a hydrogen poor one with BCT structure (space group: I4/ mmm).

Fcc-to-bct phase transformation of aluminum under triaxial stresses: an ab initio constant pressure study

We have carried out constant pressure ab initio simulations within a generalized gradient approximation to investigate the response of aluminum to triaxial stresses and found that aluminum undergoes a phase transformation from the face-centered cubic structure (fcc) to a body centered tetragonal (bct) structure having the space group of I4/mmm. The critical stress for the fcc-to-bct transformation increases as the ratio of the triaxial stress increases but such a phase transition never occurs under hydrostatic compression. The bct phase is elastically unstable.

Structural phase transition, elastic properties and electronic properties of chalcopyrite CuAlX2 (X=S, Se, Te)

Abstract The density functional theory calculations of structural phase transition, elastic constants and electronic structure of the CuAlX 2 (X = S, Se, Te) have been reported using the full potential linear muffin-tin orbital (FP-LMTO). In this approach, the local density approximation is used for the exchange-correlation potential. Results are given for lattice constant, bulk modulus, and its pressure derivative, band structure, density of states and pressure dependence of energy gaps. We note that these compounds are semiconductors with direct energy gaps. The valence-band maximum and conduction-band minimum are located at Γ . Our results are compared with previous theoretical calculations and experimental data. The elastic constants at equilibrium in BCT structure are also determined.