Disordered Fcc Structure Research Articles

Elastic constants of equiatomic Fe–Ni Invar alloy single crystal

In a disordered fcc structure, the equiatomic Fe–Ni alloy is of great interest as a soft magnetic Invar alloy with a near zero or low thermal expansion coefficient. In the L10 ordered phase, it is of interest as a potential high-performance permanent magnet alloy. In this work, the first detailed measurements of the elastic constants in a [001]-oriented Fe-50 at. % Ni alloy single crystal under a no magnetic field condition were made using the resonant ultrasound spectroscopy technique. The elastic constants measured allow one to understand the nature of atomic bonding in a material and the mechanical response to applied stress. The single crystal sample having a rectangular parallelepiped shape was prepared from a single crystal grown using the vertical Bridgman technique. The elastic constants C11, C12, and C44 were obtained from the resonant frequencies observed over a frequency range of 100–900 kHz. Using these C11, C12, and C44 values, Young’s modulus, shear modulus, bulk modulus, and Poisson’s ratio were calculated to be 99.1 GPa, 73.4 GPa, 176.7 GPa, and 0.20, respectively. The large anisotropy factor of 3.29 highlights the strong directional dependence of the material’s mechanical behavior.

Mechanical property enhancement of the Ag–tailored Au–Cu–Al shape memory alloy via the ductile phase toughening

The Au–Cu–Al biocompatible shape memory alloys (SMAs) have attracted much attention due to the requirements of biomedical applications. However, brittleness remains a critical issue in the intermetallics of Au–Cu–Al alloys. In this study, to solve the dilemma of embrittlement, Ag was introduced into the Au–Cu–Al alloy to realize the ductile phase toughening (DPT) since the ductile disordered–fcc phase was predicted to precipitate in the brittle parent β phase based on the Ag–Au–Cu phase diagram. Microstructure observations, chemical composition analysis, crystal structure identifications, and thermal analysis revealed that the Au–28Cu–16Al–28Ag (mol.%) alloy was mainly composed of the Ag–rich α1 primary solidification phase with disordered–fcc structure and the eutectic structure of the α1 phase and the β phase with L21 structure. The ductile phase was successfully inserted into the brittle Au–Cu–Al intermetallic while the martensitic transformation temperature and crystal structures remained almost uninterrupted. In the cyclic loading–unloading tensile tests, the alloy mainly composed of the β phase failed in the early elastic region due to its embrittlement; while the alloy mainly composed of the α1 phase performed 29% in the total strain deformation. The Au–28Cu–16Al–28Ag alloy, which is composed of the β phase and the α1 phase, solve the issue of embrittlement; in addition, shape recovery was also found in this alloy during unloading. According to the microstructure observations, cyclic tensile tests, and fracture surface observations, the aforementioned improvement of strengthening and enhancement of ductility were brought from the insertion of the ductile α1 phase into the brittle β/β grain boundaries.

Quantitative description of the role of Cr on the ordering characteristics of a single phase Ni-16 wt.% Mo- 7wt.% Cr alloy

Binary and ternary Ni-Mo based alloys under the service conditions of high temperature show precipitation and chemical ordering which influence their microstructure, mechanical and corrosion properties. In the present work, the phase transformation behaviour of Ni-Mo and Ni-Mo-Cr alloys is studied to identify the role of Cr in altering the stability of brittle ordered intermetallic phases. TEM characterization demonstrates that the addition of Cr destabilizes not only the long-range ordered D1a phase but also the short-range order represented by <1 ½ 0> ordering wave vectors. EXAFS studies on Ni-Mo and Ni-Mo-Cr alloys establish preferential bonding of Cr with Ni as the first nearest neighbour in the fcc lattice replacing Mo. The cluster-expansion based calculations on Ni-Mo, Ni-Cr and Cr-Mo systems show that the nearest neighbour pair and multisite interaction energies are changed in magnitude as well as in sign for Ni-Mo and Ni-Cr systems signifying that the D1a type ordering tendency is reduced with Cr addition and the disordered fcc phase is stabilised. A combination of experimental and theoretical studies unequivocally establishes the stability of the disordered fcc structure, which is crucial for the long-term use of the selected Ni-Mo-Cr alloy at elevated temperatures.

Oxidation of polycrystalline indium nanowires induced by electron bombardment: An in situ TEM study

Thin (d < 10 nm) polycrystalline indium nanowires were prepared by pulsed laser ablation in superfluid helium and studied by means of transmission electron microscope (TEM). Nanowires showed disordered fcc structure unlike bulk indium that has tetragonal structure. Oxidation of indium nanowires induced by probing electron beam and yielding single-crystalline In2O3 nanowires was observed by TEM in situ. The oxidation reaction was found to proceed in a mode frontal propagation along the wire axis.

Structural and Magnetic Properties of FePt/SiO2 Core/Shell Nanoparticles

The disorder-order phase transition of FePt/SiO2 core/shell nanoparticles (NPs) induced by rapid thermal annealing was investigated. The FePt/SiO2 NPs were synthesized using FePt cores as seeds. For as-prepared FePt/SiO2 NPs, the average size of the FePt core and the thickness of the SiO2 shell are about 4.8 and 1.2 nm respectively, and the FePt NPs exhibit a disordered fcc structure. The FePt core NPs show obvious phase transformation from a disordered to an ordered structure after they were annealed at 600 °C for 2 min. Moreover, increasing annealing time can promote effectively the formation of the ordered fct-FePt phase. The relatively large coercivity $H_{\mathrm {c}}$ is obtained for FePt NPs with a thicker SiO2 shell. The $H_{\mathrm {c}}$ decreases firstly with decreasing temperature from 300 to 100 K and then increases from 100 to 10 K, which is ascribed to the coexistence of magnetically hard fct-FePt and magnetically soft fcc-FePt in the sample, and the incomplete exchange coupling between the hard and soft magnetic phases.

Enhancement of L1 ordering with the c-axis perpendicular to the substrate in FePt alloy film by using an epitaxial cap-layer

FePt alloy thin films with cap-layers of MgO or C are prepared on MgO(001) single-crystal substrates by using a two-step method consisting of low-temperature deposition at 200 °C followed by high-temperature annealing at 600 °C. The FePt film thickness is fixed at 10 nm, whereas the cap-layer thickness is varied from 1 to 10 nm. The influences of cap-layer material and cap-layer thickness on the variant structure and the L10 ordering are investigated. Single-crystal FePt(001) films with disordered fcc structure (A1) grow epitaxially on the substrates at 200 °C. Single-crystal MgO(001) cap-layers grow epitaxially on the FePt films, whereas the structure of C cap-layers is amorphous. The phase transformation from A1 to L10 occurs when the films are annealed at 600 °C. The FePt films with MgO cap-layers thicker than 2 nm consist of L10(001) variant with the c-axis perpendicular to the substrate surface, whereas those with C cap-layers involve small volumes of L10(100) and (010) variants with the c-axis lying in the film plane. The in-plane and the out-of-plane lattices are respectively more expanded and contracted in the continuous-lattice MgO/FePt/MgO structure due to accommodations of misfits of FePt film with respect to not only the MgO substrate but also the MgO cap-layer. The lattice deformation promotes phase transformation along the perpendicular direction and L10 ordering. The FePt films consisting of only L10(001) variant show strong perpendicular magnetic anisotropies and low in-plane coercivities. The present study shows that an introduction of epitaxial cap-layer is effective in controlling the c-axis perpendicular to the substrate surface.

Magnetic ordering and exchange interactions in structural modifications ofMn3Gaalloys: Interplay of frustration, atomic order, and off-stoichiometry

Mn-Ga alloys close to the $\mathrm{M}{\mathrm{n}}_{3}\mathrm{Ga}$ stoichiometry can be synthesized in three different crystal modifications: hexagonal, tetragonal, and face-centered cubic, both in bulk and in thin-film forms. The magnetic ordering of these modifications is varying from noncollinear antiferromagnetic in the hexagonal case to ferrimagnetic order in the tetragonal one, whereas it is still unknown for the atomically disordered fcc structure. Here we study the onset of magnetic order at finite temperatures in these systems on a first-principles basis calculating the interatomic magnetic exchange interactions in the high-temperature paramagnetic regime. We employ the disordered local moment formalism and the magnetic force theorem within the framework of the local spin-density approximation and Monte Carlo simulations taking also the effects of atomic disorder in fcc alloys into account. In particular we find the origin of the stabilization of the noncollinear $3k$ structure in competition between antiferromagnetic inter- and in-plane couplings of frustrated kagome planes in hexagonal $\mathrm{M}{\mathrm{n}}_{3}\mathrm{Ga}$ and predict the antiferromagnetic-1 collinear order due to frustration in fcc alloys. Special attention is paid to the effects of the off-stoichiometry and the consequences of atomic disorder. We calculate the site-preference energy of Ga antisite atoms in the tetragonal structures in the range of the compositions from $\mathrm{M}{\mathrm{n}}_{3}\mathrm{Ga}$ to $\mathrm{M}{\mathrm{n}}_{2}\mathrm{Ga}$ and slightly beyond and confirm the earlier explanation of the effect of magnetization increase due to Ga preferentially occupying one of the Mn sites.

Chemical ordering in PtNi nanocrystals

We investigated the chemical ordering in PtNi nanocrystals fabricated on sapphire substrate using in-situ synchrotron X-ray scattering. Nanocrystals with composition close to 1:1 were ordered in the tetragonal L10 structure at low temperatures. The transition to disordered FCC structure occurred at around 640 °C and substantial hysteresis of about 50 K was observed. Nanocrystals of smaller sizes fabricated under the same conditions were Ni rich and ordered into Cu3Au type L12 structure. Significantly higher degree of chemical ordering was observed in L12 structure than in L10 structure.

Size Controlling of L10-FePt Nanoparticles During High Temperature Annealing on the Surface of Carbon Nanotubes

Mono-size FePt nanoparticles with particles size about 2.5 nm have been prepared by polyol method on the surface of carbon nanotubes (CNTs). The CNTs functinalization time and the mass ratio of nanoparticles to CNTs affects on the CNTs surface coating. The as-synthesis nanocomposites have a superparamagnetic behavior with chemically disordered fcc structure at room temperature and they can be transformed into chemically ordered fct structure after thermal annealing above 600 °C. Their magnetic behavior changes from the superparamagnetic to the ferromagnetic with a large coercivity up to 0.83 T for the nanocomposites which annealed at 800 °C. The CNTs surfaces as a substrate prevent the agglomeration of nanoparticles during high temperature annealing and the FePt nanoparticles after annealing at 800 °C have finite size with an average about 10 nm. The structure, composition and magnetic properties of nanocomposite were characterized by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy and vibrating sample magnetometer.

Adsorbate-induced structural changes in 1-3 nm platinum nanoparticles.

We investigated changes in the Pt-Pt bond distance, particle size, crystallinity, and coordination of Pt nanoparticles as a function of particle size (1-3 nm) and adsorbate (H2, CO) using synchrotron radiation pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) measurements. The ∼1 nm Pt nanoparticles showed a Pt-Pt bond distance contraction of ∼1.4%. The adsorption of H2 and CO at room temperature relaxed the Pt-Pt bond distance contraction to a value close to that of bulk fcc Pt. The adsorption of H2 improved the crystallinity of the small Pt nanoparticles. However, CO adsorption generated a more disordered fcc structure for the 1-3 nm Pt nanoparticles compared to the H2 adsorption Pt nanoparticles. In situ XANES measurements revealed that this disorder results from the electron back-donation of the Pt nanoparticles to CO, leading to a higher degree of rehybridization of the metal orbitals in the Pt-adsorbate system.

Mechanical properties of ground state structures in substitutional ordered alloys: High strength, high ductility and high thermal stability

We have studied the effect of atom arrangements in the ground state structures of substitutional ordered alloys on their mechanical properties using nickel–molybdenum-based alloys as model systems. Three alloys with nominal compositions of Ni–19.43at% Mo, Ni–18.53at% Mo–15.21at% Cr and Ni–18.72at% Mo–6.14at% Nb are included in the study. In agreement with theoretical predictions, the closely related Pt2Mo-type, DO22 and D1a superlattices with similar energies are identified by electron diffraction of ground state structures, which can directly be derived from the parent disordered fcc structure by minor atom rearrangements on {420}fcc planes. The three superlattices are observed to coexist during the disorder–order transformation at 700°C with the most stable superlattice being determined by the exact chemical composition. Although most of the slip systems in the parent disordered fcc structure are suppressed, many of the twinning systems remain operative in the superlattices favoring deformation by twinning, which leads to considerable strengthening while maintaining high ductility levels. Both the Pt2Mo-type and DO22 superlattices are distinguished by high strength and high ductility due to their nanoscale microstructures, which have high thermal stability. However, the D1a superlattice is found to exhibit poor thermal stability leading to considerable loss of ductility, which has been correlated with self-induced recrystallization by migration of grain boundaries.

Order–disorder effects on the elastic properties of CuMPt6 (M=Cr and Co) compounds

The elastic properties of CuMPt6 (M=Cr and Co) in disordered face-centered cubic (fcc) structure and ordered Cu3Au-type structure are studied with lattice inversion embedded-atom method. The calculated lattice constant and Debye temperature agree quite well with the comparable experimental data. The obtained formation enthalpy demonstrates that the Cu3Au-type structure is energetically more favorable. Numerical estimates of the elastic constants, bulk/shear modulus, Young's modulus, Poisson's ratio, elastic anisotropy, and Debye temperature for both compounds are performed, and the results suggest that the disordered fcc structure is much softer than the ordered Cu3Au-type structure.

Templated assembly of Co–Pt nanoparticlesvia thermal and laser-induced dewetting of bilayer metal films

Templated dewetting of a Co/Pt metal bilayer film on a topographic substrate was used to assemble arrays of Co-Pt alloy nanoparticles, with highly uniform particle size, shape and notably composition compared to nanoparticles formed on an untemplated substrate. Solid-state and liquid-state dewetting processes, using furnace annealing and laser irradiation respectively, were compared. Liquid state dewetting produced more uniform, conformal nanoparticles but they had a polycrystalline disordered fcc structure and relatively low magnetic coercivity. In contrast, solid state dewetting enabled formation of magnetically hard, ordered L1(0) Co-Pt single-crystal particles with coercivity >12 kOe. Furnace annealing converted the nanoparticles formed by liquid state dewetting into the L1(0) phase.

The effect of thermal treatment on structure and surface composition of PtCo electro-catalysts for application in PEMFCs operating under automotive conditions

Structure and surface characteristics of carbon-supported PtCo cathode electro-catalysts were investigated to evaluate their performance and resistance to degradation under high temperature (∼110°C) operation in a polymer electrolyte membrane fuel cell (PEMFC). Two different thermal treatments were investigated, i.e. 600°C and 800°C causing the occurrence of a disordered face-centered cubic (fcc) structure and a primitive cubic ordered (L12) phase. A specific colloidal preparation route and a carbothermal reduction allowed to obtain a similar mean crystallite size, i.e. 2.9 and 3.3nm for the catalysts after the treatment at 600°C and 800°C, as well as a suitable degree of alloying. Both electrocatalysts were subjected to the same pre-leaching procedure to modulate the surface characteristics. The surface properties were investigated by X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LE-ISS, 3He+ at 1kV). A Pt segregation in the outermost surface layers and similar electronic properties for the materials were observed. Both catalysts showed good performance under PEMFC operation; however, the catalyst characterised by the disordered fcc structure performed slightly better at low temperature (80°C) and full humidification; whereas, the primitive cubic ordered structure catalyst showed superior characteristics both in terms of performance and stability at high temperature (110°C) and low R.H. These operating conditions are more relevant for automotive applications. The enhanced stability of the catalyst characterised by primitive cubic ordered structure was attributed to the growth of a stable Pt-oxide layer during operation at high temperature and low R.H. hindering sintering and dissolution processes at the catalyst surface.

Consolidation of FePd nanoparticles by spark plasma sintering

Nanoparticles of FePd alloys in the size range of 5–8nm have been processed in gram quantities using the polyol process and were further consolidated into bulk magnets by spark plasma sintering (SPS). The as-prepared FePd nanoparticles showed disordered fcc structure; while their SPSed counterparts displayed typical L10-ordered fct structure with interesting magnetic properties.

Synthesis and Characterization of CoPt Nanoparticles Prepared by Room Temperature Chemical Reduction with PAMAM Dendrimer as Template

We describe an approach to synthesize monodisperse CoPt nanoparticles with dendrimer as template by a simple chemical reduction method in aqueous solution using NaBH4 as reducing agent at room temperature. The as-made CoPt nanoparticles buried in the dendrimer matrix have the chemically disordered fcc structure and can be transformed to the fct phase after annealing at 700 degrees C. This is the first report of dendrimer-mediated room temperature synthesis of monodisperse magnetic nanoparticles in aqueous solution.

Thickness-Dependent Perpendicular Magnetic Anisotropy of CoPt Top Layer on CoPt/AlN Multilayer

The magnetic anisotropy of the CoPt top layer on a CoPt/AlN multilayer has been studied. The layered structures were sputter deposited on fused quartz substrates and subsequently annealed at 500 C in a vacuum. CoPt layers are of a disordered fcc structure and highly (111) textured. It has been found that the CoPt top layer is perpendicular magnetic anisotropic and shows an enhanced coercivity in comparison with the bottom CoPt/AlN multilayer. A critical (maximum) thickness of 6 nm is found for the perpendicular anisotropy in the CoPt top layer. The structural results indicate that the CoPt top layer has experienced a tensile strain under a certain thickness. However, when the thickness of the CoPt top layer is above 6 nm, the inplane CoPt lattice parameter begins to decrease as the thickness increases, indicating that the CoPt top layer cannot tolerate the elastic energy and shrinks to release this energy. The simultaneous changes of the magnetic anisotropy and the inplane lattice parameter with the thickness strongly suggest that the thickness-dependent magnetic anisotropy of the CoPt top layer is correlated with the elastic strain through the magnetoelastic effect.

Composition and film thickness effects on microstructure and magnetic properties of ordered L1 0-structured Fe 100− xPt x films

Fe 100− x Pt x films with different Pt atomic fractions ( x) and film thicknesses ( t f) were fabricated by magnetron sputtering at room temperature and subsequently annealed at 650 °C. Their structure and magnetic properties have been investigated systematically. All the as-deposited Fe 100− x Pt x films with disordered face-centered-cubic (fcc) structure are soft ferromagnetic. The annealed Fe 100− x Pt x films evolve from Fe 3Pt, FePt to FePt 3 with increasing x. High-temperature annealing makes the Fe 100− x Pt x films transform from the disordered fcc structure to the ordered face-centered-tetragonal (fct) L1 0 structure as x is in the range of 40–60. The grain size of the annealed Fe 52Pt 48 films increases with increasing t f. All the annealed Fe 100− x Pt x films are hard ferromagnetic. The coercivity of the annealed Fe 100− x Pt x films first increases, and decreases latterly with increasing x. Meanwhile, the coercivity of the annealed Fe 52Pt 48 films decreases with increasing temperature.

Magnetic Properties of Fe&lt;SUB&gt;&lt;I&gt;x&lt;/I&gt;&lt;/SUB&gt;Pt&lt;SUB&gt;&lt;I&gt;y&lt;/I&gt;&lt;/SUB&gt;Au&lt;SUB&gt;100−&lt;I&gt;x&lt;/I&gt;−&lt;I&gt;y&lt;/I&gt;&lt;/SUB&gt; Nanoparticles

Fe(x)Pt(y)Au(100-x-y) nanoparticles of size 3.5 nm were prepared by polyol reduction of platinum acetylacetonate and gold acetate and the thermal decomposition of iron pentacarbonyl. The as-synthesized nanoparticles with disordered fcc structure were then heat treated to transform to the L1(0) structure with high magnetocrystalline anisotropy. By tuning the stoichiometry of the Fe(x)Pt(y)Au(100-x-y) nanoparticles, the phase transition temperature was reduced by more than 200 degrees C. After the annealing 500 degrees C, for instance, the highest coercivity of 18 kOe was obtained from the Fe51Pt36Au13 nanoparticles which is substantially higher compared to 2 kOe for Fe51Pt49 nanoparticles annealed at the same temperature. In addition to the high coercivity, the saturation magnetization value obtained from Fe51Pt36Au13 nanoparticles was 47 emu/g which is similar to that for the Fe51Pt49 nanoparticles, indicating that there is no trade-off between the coercivity and the saturation magnetization upon Au doping.

Exchange bias in epitaxial Fe/Ir0.2Mn0.8 bilayers grown on MgO (0 0 1)

Exchange bias has been studied in epitaxial Fe/Ir0.2Mn0.8 (IrMn) bilayers grown by molecular beam epitaxy on MgO (0 0 1) substrates. The IrMn layer has a chemically disordered fcc structure with an epitaxial relationship of MgO(0 0 1)[1 1 0]//Fe(0 0 1)[1 0 0]//IrMn(0 0 1) [1 1 0]. In this system, exchange bias is induced during growth at room temperature without post-annealing, by applying a small magnetic field, HA, along the bcc Fe [1 0 0] easy axis. The temperature dependence of the exchange bias field and the coercivity along the Fe [1 0 0] easy axis, measured up to approximately half the Néel temperature, shows an exponential decay of these parameters with increasing temperature. The magnetization measured as a function of applied field along the Fe [1 0 0] easy axis results in a negatively shifted hysteresis loop, typical for exchange bias, while when the magnetic field is applied along the Fe [0 1 0] easy axis (perpendicular to HA), a symmetric double-shifted loop is observed. In both cases, the magnetization reversal occurs through two successive events involving nucleation and propagation of domains in which the magnetic moments are perpendicular to each other. We demonstrate that due to the epitaxial growth of the IrMn layer, the unidirectional anisotropy induced by exchange bias is aligned with one of the Fe easy axis directions and, in effect, is added to the four-fold cubic anisotropy of the Fe layer.