Fcc Region Research Articles

Composition and non-equilibrium crystallization in partially devitrified co-rich soft magnetic nanocomposite alloys

The body-centered cubic (bcc) phase tends to preferentially nucleate during solidification of highly undercooled liquid droplets of binary alloy systems, including Fe–Co, Fe–Ni and Fe–Cr–Ni. We investigate a similar tendency during the partial devitrification of Co-rich amorphous precursors of composition (Co 1− x Fe x ) 88Zr 7B 4Cu 1 by identifying the structure and composition of the nanocrystalline grains. The Co:Fe ratio of the bcc nanocrystals varies linearly with the Co:Fe ratio of the amorphous precursor, and can lie well within the single-phase face-centered cubic (fcc) region of the Fe–Co phase diagram at the crystallization temperature. Classical nucleation theory therefore suggests several potential explanations for the preferential nucleation of bcc phase from an amorphous precursor, including: (i) a reduced amorphous/bcc interface energy as compared to the close-packed phases; (ii) a lower strain of precipitation for bcc nuclei as compared to close-packed fcc and hexagonal close-packed nuclei; and (iii) stabilization of the bcc phase by dissolved glass-formers such as Zr and B.

Phase evolution during crystallization of nanocomposite alloys with Co:Fe ratios in the two-phase region of the binary Fe–Co phase diagram

A series of alloys was prepared to investigate the crystallization of Co-rich HiTPerm-type alloys [(Co1−xFex)88Zr7B4Cu1] with Fe:Co ratios within or near the two-phase (bcc+fcc) region of the binary phase diagram. The goal of this work is to better understand the phase evolution and crystallization of alloys in which the Fe–Co binary phase diagram predicts more than one transition metal rich primary crystalline phase to be present in equilibrium at the primary crystallization temperature. X-ray diffraction, transmission electron microscopy, and high-temperature vibrating-sample magnetometry have been performed to identify the first phase to crystallize and to follow the evolution of phases during crystallization. The bcc phase appears to be the primary crystalline phase that forms first after annealing at 450°C for 1h, in agreement with previous work on Co-rich nanocomposite alloys. We observe that as the Co concentration is increased, the fcc crystalline phase forms at lower annealing temperatures and its volume fraction increases for a given annealing temperature.

Twin nucleation mechanisms at a crack tip in an hcp material: Molecular simulation

Twinning is an important deformation mode in hexagonal close-packed (hcp) materials that can strongly affect fracture toughness. In order to clarify the early stages of twin nucleation, lattice statics simulations of zirconium crystals containing mixed-mode basal cracks with [ 1 ¯ 2 1 ¯ 0 ] and [ 1 1 ¯ 0 0 ] front orientations were carried out using an embedded-atom method potential. The simulations show that crack tip twin nucleation is a two-stage process: (I) initial plastic deformation occurs within a thin layer ahead of the crack, possibly involving basal slip, crack tip blunting by the formation of Frank partials or an hcp-face-centered cubic (fcc) transformation produced by Shockley partials emitted from the crack tip and (II) a twin forms in the surrounding hcp matrix. In this second stage, either a { 1 1 2 ¯ 1 } twin is nucleated homogeneously or a { 1 0 1 ¯ 1 } twin is nucleated heterogeneously by Shockley partials that nucleate inside the fcc region and penetrate the hcp matrix.

Effect of Ni concentration on quantum-well states of the alloy systemAg∕Fe1−xNix: A spin- and angle-resolved photoemission study

We investigate the quantum-well states (QWSs) of Ag on an $\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Ni}$ alloy film grown on a W(110) substrate, using spin- and angle-resolved photoemission spectroscopy (SPARPES). As the Ni content is increased, the $\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Ni}$ alloy undergoes a transition from a bcc (110) structure to a fcc (111) structure at a Ni concentration of 30%. In the bcc (110) region, we found that as the Ni concentration was increased, the binding energy shifted linearly toward higher binding energy in the QWSs of a thin Ag film, with a maximum shift of $0.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for a Ni concentration of 30%. Our SPARPES data and phase accumulation model calculation indicate that this shift is due to electron doping resulting from inserting Ni, which has two more electrons than Fe. In the fcc (111) region, by contrast, the binding energy shift in the QWSs is barely noticeable, indicating that the potential shift caused by electron doping is insufficiently large in this region. Moreover, as the Ni concentration is increased, a decrease in the polarization of QWSs is observed, which can be attributed to the reduced exchange splitting energy.

Do Methanethiol Adsorbates on the Au(111) Surface Dissociate?

The interaction of methanethiol molecules CH3SH with the Au(111) surface is investigated, and it is found for the first time that the S-H bond remains intact when the methanethiol molecules are adsorbed on the regular Au(111) surface. However, it breaks if defects are present in the Au(111) surface. At low coverage, the fcc region is favored for S atom adsorption, but at saturated coverage the adsorption energies at various sites are almost isoenergetic. The presented calculations show that a methanethiol layer on the regular Au(111) surface does not dimerize.

Adhesive wear behavior of Al xCoCrCuFeNi high-entropy alloys as a function of aluminum content

The Al x CoCrCuFeNi alloys with different aluminum contents prepared by arc melting were investigated on their adhesive wear behaviors. With increasing aluminum content, both the volume fraction of BCC phase and the hardness value increase, and thus the wear coefficient decreases. Moreover, the wear mechanism changes from delamination wear to oxidative wear. For low aluminum content, x = 0.5, the microstructure is of simple ductile FCC phase and the worn surface is deeply grooved and undergoes a periodic delamination which produces big debris. For medium aluminum content, x = 1.0, the microstructure is a mixture of FCC and BCC phases, and the worn surface is deeply grooved in FCC region but smooth in BCC region. Delamination wear is still dominant although oxidative wear occurs in the smooth region. For high aluminum content, x = 2.0, the microstructure is of BCC phase and the worn surface is smooth and yields fine debris with high oxygen content. The high aluminum content gives a large improvement in wear resistance. This improvement is attributed to its high hardness, which not only resists plastic deformation and delamination, but also brings about the oxidative wear in which oxide film could assist the wear resistance.

Atomic-scale simulations of the interaction between a moving dislocation and a bcc/fcc phase boundary

In the past, molecular dynamics (MD) simulations have shown that grain boundaries are very strong obstacles to dislocation motion. Whereas such studies typically have been performed for boundaries between grains with the same crystal structure, in the present study the interaction of a moving edge dislocation and a boundary between a bcc (Fe) and an fcc (Cu) phase with almost identical lattice distances in the glide planes has been studied using MD. It is found that an edge dislocation can be transmitted from the bcc- into the fcc-regime, but not vice versa. This may be due to the splitting of the edge dislocation into Shockley partials inside the fcc region.

Molecular dynamic simulation of a homogeneousbcc→hcptransition

We have performed molecular dynamic simulations of a Martensitic bcc->hcp transformation in a homogeneous system. The system evolves into three Martensitic variants, sharing a common nearest neighbor vector along a bcc <111> direction, plus an fcc region. Nucleation occurs locally, followed by subsequent growth. We monitor the time-dependent scattering S(q,t) during the transformation, and find anomalous, Brillouin zone-dependent scattering similar to that observed experimentally in a number of systems above the transformation temperature. This scattering is shown to be related to the elastic strain associated with the transformation, and is not directly related to the phonon response.

Structural analysis of Fe/Ni(001) films by photoelectron diffraction

The structure of Fe films, epitaxially grown on Ni(001), has been studied in the 0--14 ML coverage range by means of photoelectron diffraction (PD) in the forward scattering regime. Quantitative analysis by a multiple scattering approach has been performed on Fe films at a coverage of 3 and 7 ML. Analysis of the 3-ML data showed that growth was not layer-by-layer but rather occurred through islands nucleation and that transition from the pseudomorphic fcc to the bcc phase was located in this early stage of growth. In fact, best fit was obtained by calculations on a 2 ML bcc(110)/3 ML fcc(001) Fe film with the $\mathrm{bcc}〈111〉\ensuremath{\parallel}\mathrm{fcc}〈110〉$ in-plane orientation. Interlayer spacings of $2.05\ifmmode\pm\else\textpm\fi{}0.06\AA{},$ $2.01\ifmmode\pm\else\textpm\fi{}0.03\AA{},$ and $1.85\ifmmode\pm\else\textpm\fi{}0.03\AA{}$ were found in the bcc region, between bcc and fcc layers and in the fcc region, respectively. Best-fit in-plane nearest-neighbors (n-n) distance was $2.49\ifmmode\pm\else\textpm\fi{}0.02\AA{},$ in registry with that of the Ni substrate. To analyze the 7-ML data a 4 ML bcc(110)/3 ML fcc(001) film was employed, varying the fitting parameters in the bcc region only. Best fit was obtained for an interlayer spacing of $2.04\ifmmode\pm\else\textpm\fi{}0.04\AA{}$ and in plane n-n distance of $2.47\ifmmode\pm\else\textpm\fi{}0.01\AA{}.$ At 14 ML the PD pattern collected over a 94\ifmmode^\circ\else\textdegree\fi{} azimuthal range displayed symmetry around the [110] substrate direction, which was explained by the equipopulation of the 4 bcc(110) domains satisfying the $\mathrm{bcc}〈111〉\ensuremath{\parallel}\mathrm{fcc}〈110〉$ alignment.

Scanning Tunneling Microscopy Observation of an Electronic Superlattice at the Surface of Clean Gold

We have used scanning tunneling spectroscopy to spatially resolve the electronic structure of clean Au(111) at low temperature. We find that the long-range herringbone reconstruction on Au(111) acts as a superlattice for surface-state electrons, creating a new band structure and modulated electronic density. Low energy electrons respond to the superlattice by localizing in the hexagonal-close-packed (hcp) region of the reconstruction, while higher energy electrons reverse this trend, shifting density back to the adjacent face-centered-cubic (fcc) region. These observations are quantitatively explained by an extended Kronig-Penney model, from which we estimate the well-depth of the reconstruction-induced surface superlattice.

Investigation of face-centered-cubic Fe thin films using wedged samples

The structural and magnetic properties of metastable face-centered-cubic (fcc) Fe thin films on fcc Co(100) substrate were studied using wedged samples. fcc Co(100) was chosen for the substrate because it is structurally very similar to Cu(100) but is ferromagnetic at room temperature. Reflection high energy electron diffraction and low energy electron diffraction characterizations confirm that epitaxially grown (MBE) Fe on Co(100) is structurally very similar to Fe on Cu(100): face-centered-tetragonal (fct) for dFe&amp;lt;6 ML, fcc for 6 ML&amp;lt;dFe&amp;lt;11 ML, bcc for dFe≳11 ML. In situ surface magneto-optic Kerr effect measurements show that at room temperature the fct and bcc regions are ferromagnetic, while the fcc region is nonferromagnetic with some magnetic live layers. All magnetizations are in-plane. Oxygen absorption experiments suggest that these live layers are at the Fe/Co interface. Low temperature growth Fe/Co(100) shows a Kerr signal that increases linearly with dFe and suggests that the magnetic moments for fcc Fe and bcc Fe are the same. To further study the magnetic properties of the nonferromagnetic ‘‘fcc’’ phase, we used metastable fcc Fe as a spacer layer between two Co layers. The Co/fcc Fe/Co on Cu(100) sandwiches exhibit ferromagnetic coupling, strong antiferromagnetic coupling (AFC) and weak AFC. An oscillation in the strong AFC was found by artificially lengthening the thickness range of the nonferromagnetic fcc phase.

Magnetic properties and microstructures of CoNiCr and CoFeCr thin films

The large difference in coercivities between CoNiCr (800 Oe) and CoFeCr (200 Oe) thin films, which were deposited on Cr-coated NiP/Al substrates under the same conditions, is related to their differences in microstructures. The magnetic properties were measured with a vibrating sample magnetometer and the microstructures were studied by transmission electron microscopy. It is found that both the CoNiCr and the CoFeCr films are hcp in phase, and of similar grain sizes. However, the stacking fault density (fcc region) is higher in CoNiCr films than in CoFeCr films, which may account for the difference in their coercivities. The presence of stacking faults is related to composition effects on stacking fault energy and their generation to release residual stresses.

炭素綱マルテンサイト中の炭素

The atomic arrangements in Fe- C, especially the interstitial carbon positions have been discussed utilizing 57Fe Mossbauer spect roscopy and 13C and 57 Fe spin-echo NMR measurements. Domain wall enhanced spin-echo of 13C and 57Fe have been observed in ferromagnet is Fe-1-6.9 at. % C martensite which were prepared by quenching from 1373K, the stable austenite (fcc) region. 57Fe Mossbauer measurements have been done on the same specimens.13C spectra at 4.2K show two intense resonance lines and a few clear satellite lines in the frequency range between 30 to 44 MHz corresponding to the hyperfine fields from 2.8 to 4.1 T. Their intensities depend on the carbon concentration and the aging period at 323 K. Especially, the satellite lines gradually rise during the aging at the sacrifice of the main line, suggesting the jumps of carbon atoms to form a local ordered arrangement like as Fe8C.

Interdiffusion in the Ni-Cr-Co-Mo system at 1300 °C

Interdiffusion was investigated with solid-solid diffusion couples in theα (fcc) region of the quaternary Ni-Cr-Co-Mo system at 1300 °C for the determination of diffusion paths and diffusional interactions among the components. The concentration profiles for a given couple exhibited a common cross-over composition, Yc, which reflected the relative depths of diffusion in the terminal alloys. Interdiffusion fluxes were calculated directly from the concentration profiles, and the quaternary interdiffusion coefficients were calculated at selected compositions. Ni and Co exhibited uphill diffusion against their individual concentration gradients in a direction opposite to the interdiffusion of Cr. Quaternary diffusion paths were presented as a set of partial diffusion paths on the basis of relative concentration variables.

Quaternary diffusion in the Cu−Ni−Zn−Mn system at 775°C

Quaternary diffusion was investigated in the α (fcc) region of the Cu−Ni−Zn−Mn system at 775°C with solid-solid diffusion couples assembled with alloys characterized by similar concentrations of one of the components. The concentration profiles of the quaternary couples were examined on the basis of a relative concentration variable for each component; the profiles for any given couple exhibited a common cross-over composition. The interdiffusion fluxes of the components were calculated directly from the concentration profiles. Interdiffusion of each component up its own concentration gradient was observed in several couples, while the zero-flux planes for the individual components were identified in selected couples. The common cross-over composition reflected the relative diffusion depths in the terminal alloys of the single-phase quaternary diffusion couples. The quaternary diffusion paths were presented on the basis of relative concentration variables for the individual components.

Surface interactions in the iron-nitrogen system

The Fe-N system has been used as an example case to examine the apparent enhancement of the solubilities of interstitial elements due to surface trapping phenomena. Solubilities have been measured using pure iron foils subjected to a series of differing surface finishing processes. The iron foils were equilibrated with N2-gas at atmospheric pressure and a series of differing temperatures. It was found that, at lower temperatures in the bcc range, trapping effects for N-atoms are predominantly due to surface defects introduced by deformation or abrasive mechanical treatments. On the other hand, at high temperatures in the fcc region, the predominant trapping mechanism giving rise to enhanced spurious solubilities is related to the interaction between N-atoms and O-atoms occluded at or in the surface.

Invar anomalies of Fe-Pd alloys

The Mössbauer spectra and the magnetostriction have been observed for Fe 1− x Pd x alloys. The internal field of Fe atoms is 330±10 kOe and is almost constant in the Invar region 0.45⩾ x⩾0.3. The magnetostriction constants as well as forced magnetostriction (∂ω/∂ H) increase with decreasing temperature and with decreasing x. Observed large values of magnetostriction constants are related to a lattice instability of the fcc structure, which is confirmed by the measurement of elastic constants. The magnitude of ∂ω/∂ H is about one-fifth of Fe-Ni Invar. The thermal expansion of Fe 1− x Pd x alloys has also been measured in the whole concentration range. The Invar-like behavior observed in the region 0.28⩾ x#620 originates from an annealing effect of the martensite bcc phase. The spontaneous volume magnetostriction in the fcc region increases with decreasing x. The thermal expansion coefficients (α exp) of fcc alloys are analyzed from the Grüneisen equation. The high-temperature anomaly of α exp is discussed.

Measurements of Temperature Dependence of 55Mn NMR Frequency in Transition-Metal Alloys with Various Compositions

Temperature dependence of the 55 Mn nuclear resonance frequency in Fe–Co, Fe–Ni, Ni–Co and Ni–Cu alloys containing 3 at %Mn has been measured by spin echo technique over a range from 4.2 K to room temperature in zero external field, and the dependence has been studied as a function of e / a . Here, e is the number of electrons in host, and a is the number of host atoms. The temperature dependence in the bcc phase of 8.4≤ e / a ≤8.8, is found to be slow compared with that of spontaneous magnetization. On the other hand, the temperature dependence in the fcc phase of 8.9≤ e / a ≤10.0, is found to be rapid. In this fcc region, the deviation of the reduced frequency from the reduced magnetization at a given reduced temperature increases monotonically with decreasing e / a .

Electrical resistivity of Gd-Th alloys

Electrical-resistivity measurements have been made over the 4.2–360 K temperature range on polycrystalline Gd x Th 1- x samples with x varying from 0.10 up to 0.99. In the region of the hexagonal structures ( x = 0.99-0.60) the upper magnetic-ordering temperature decreases rapidly with increasing Th content. In the fcc region ( x ⩽ 0.50) no indications of magnetic ordering could be detected. It was impossible to describe the results with the Dekker formulas for the electrical resistivity of binary heavy rare-earth alloys.