Formation Of Bcc Research Articles

Strengthening of multicomponent glass-forming alloys by microstructure design

The combination of high strength with high elastic strain renders bulk metallic glasses (BMGs) or nanostructured Zr-base multicomponent alloys attractive candidates as advanced high-strength materials for structural applications. To circumvent the limited ductility of these glassy alloys, we prepared a heterogeneous microstructure combining glassy or nanostructured matrix along with a dendritic second-phase particles. The alloys were designed by adding different amounts of Nb to the Zr-based multicomponent glass-forming alloy system. The refractory metal Nb promotes the formation of a primary bcc β-Zr phase having dendritic morphology while the residual melt solidifies to a nanostructured/amorphous matrix. A correlation between the alloy composition and adopted casting method with evolved microstructures and mechanical properties is revealed. These composites exhibit a unique combination of high fracture strength up to 1922MPa as well as plastic strain over 15.8% under uni-axial compression testing at room temperature. We emphasize the possibilities to tailor such composite microstructures in favor of either strength or ductility, or a combination of both, and also discuss the acquired ability to synthesize such in situ composite microstructures in bulk form through inexpensive processing routes.

Dissolution of the Fe4N nitride in the nitrided layer of iron upon cold deformation by shear under pressure

Mossbauer spectroscopy, X-ray diffraction, and transmission electron microscopy were used to study structural and phase transformations upon cold (300 K) deformation by shear under pressure in thin layers of nitrides Fe4N formed on the surface of bcc iron. Strain-induced dissolution of nitrides in bcc iron with the formation of bcc and fcc solid solutions supersaturated with nitrogen and secondary Fe16N2 and Fe4N nitrides was found. The dispersiveness of nitride phases in the layers deposited on bcc iron determines the accelerated kinetics of cold mechanosynthesis and the possibility of the formation of nanocrystalline iron-based solid solutions supersaturated with nitrogen and containing secondary nitrides.

Phase behavior and microstructural length scales of a diblock copolymer in the presence of a selective solvent

We employ self-consistent mean-field (SCMF) theory in studying the phase behavior as well as the microstructural domain sizes for a diblock copolymer in the presence of a selective solvent. First we examine the effects of solvent addition on the formation of fcc and bcc packed spheres. As has been found in experiments, the so-called “normal” spheres, i.e., formed by the minority blocks, tend to pack into the bcc array, while the “inverted” spheres formed by the majority blocks favor the fcc packing. Upon increasing the solvent selectivity and/or solvent amount, the formed inverted spheres tend to pack from bcc to fcc. This thermotropic transition of bcc → fcc upon increasing the solvent selectivity is induced by the fact that the intermicellar interactions vary from long-range to short-range via a combination of the solvent exclusion from the cores and an increase in the aggregation number. In analyzing the effects of solvent addition on the microstructural sizes, the SCMF results have successfully captured the crossover behavior of characteristic domain spacing from decreasing with added solvent to increasing by increasing the solvent selectivity. Further, the variation of the characteristic domain spacing when the systems transform to a more curved structure changes from a discontinuous decreasing behavior to even a discontinuous increasing behavior upon increasing the solvent selectivity and/or the formation of inverted structures.

Magnetostriction and other properties of [formula omitted] alloys

The aim of this work was to present experimental results on the investigation of structure and magnetic properties of amorphous and nanocrystalline ( Fe 1 - x Co x ) 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloys with special attention to the evolution of magnetostriction. It has been proved that cobalt takes part in the primary crystallization and causes formation of BCC α -(FeCo)Si phase and for higher Co content FCC β -Co phase after annealing treatment. It was observed that Fe/Co ratio in α -(FeCo)Si phase and the crystallization of β -Co phase influenced the evolution of magnetostriction.

Structure and magnetism of bcc(1 1 0)-like Fe films grown on Cu(0 0 1) via ion beam triangulation and spin-polarized electron capture

The structure and magnetism of thin epitaxial Fe layers grown on Cu(0 0 1) is investigated by grazing scattering of fast H and He atoms. Information on the atomic structure of the film and substrate surfaces is obtained by making use of ion beam triangulation with protons. The magnetic behavior is studied via the polarization of light emitted after capture of spin-polarized electrons into excited atomic terms during scattering of He atoms. For the formation of bcc(1 1 0)-like Fe films at higher coverages, we detect differences in structural and magnetic properties for room and low temperature growth. We suggest that the crystalline structure depends on the film morphology and that Cu impurities affect the magnetic properties.

Simulations of the effects of 2-D interstitial diffusion on void lattice formation during irradiation

This paper describes the use of simulation techniques to examine some of the processes involved in the alignment of voids under the influence of 2-dimensional self-interstitial atom (2-d SIA) transport. It follows an earlier paper in which the effects of 1-d SIA transport were investigated and uses similar methodology. In contrast to the 1-d SIA results, with 2-d SIA transport there is no difficulty in simulating the formation of simple cubic void lattices. In addition the work has been extended to demonstrate the formation of perfect bcc and fcc void lattices. One important feature was that lattice formation took place some 100 times faster than found experimentally. The possibility that this might be due to a diluting effect of out of plane jumps was investigated but seemed very unlikely. This led to the alternative conclusion that the 2-d diffusing defect could not be the basic single interstitial but instead was a larger interstitial defect, such as a di-interstitial, present in far lower concentrations. One consequence of the new approach is that the phenomenon of void swelling would be essentially unaffected by the defect responsible for void lattice formation. This crucially avoids the fast swelling which, as suggested recently, might otherwise occur as void lattices form, and which would conflict with the often quoted association of void swelling saturation and void lattice formation. This association is now more likely to be controlled by a common factor such as high void density than by any direct mechanism. A possible explanation for the influence of impurities on void lattice formation in ion irradiated Nb, Nb-1%Zr, Ta, and Mo is discussed.

A critical test of ab initio and CALPHAD methods: The structural energy difference between bcc and hcp molybdenum

Ab initio calculations of the enthalpy of formation of bcc, fcc, and hcp Ru–Mo alloys have been performed for random, ordered, and partially ordered structures. The lattice stability of the bcc and hcp forms of Mo is isolated in order to compare the hcp–bcc difference calculated by ab initio and CALPHAD methods with experimental measurements of the enthalpy of formation of Ru–Mo alloys. The significance of this comparison in calculating the Mo–Ru phase diagram is illustrated. The results of these considerations suggest a rational method for coupling ab initio and CALPHAD techniques might be utilization of the ab initio methods for calculation of the isostructural energies of formation for binary bcc, hcp, and fcc solutions while retaining the CALPHAD lattice stabilities in the calculation of phase diagrams.

Soft magnetic properties of bulk nanocrystalline Fe–Co–B–Si–Nb–Cu alloy with high saturated magnetization of 1.35 T

Bulk nanocrystalline body-centered cubic- (bcc) (Fe,Co) alloy with high saturated magnetization and good soft magnetic properties was synthesized by the simple process of casting and annealing for the glass-type Fe62.8Co10B13.5Si10Nb3Cu0.7 alloy. It crystallizes through two exothermic reactions. The cylindrical glassy rod with the diameter of 1.5 mm was produced by copper mold casting. The subsequent annealing at the temperature higher than that of the first exothermic peak causes the formation of bcc-(Fe,Co) nanocrystalline with particle sizes between 10 and 15 nm. The bcc-(Fe,Co) alloy rods exhibit good soft magnetic properties of 1.35 T for saturation magnetization and 5 A/m for coercive force.

Nucleation and phase-selection in undercooled melts

Abstract Electromagnetic levitation technique is applied to undercool bulk melts of metals and alloys. A large undercooling range becomes accessible by avoiding heterogeneous nucleation on container walls and processing the melts under high purity environment. A freely suspended drop is produced with the extra benefit that it becomes accessible for direct observation and diagnostics. Temperature–time profiles (ttp) are measured during the melting and solidification process. The ttp profiles give insight into the thermodynamics of phase-selection processes. The levitation technique is combined with the diagnostic means at European Synchrotron Radiation Facility (ESRF Grenoble) to record time-resolved diffraction spectra during rapid crystallization of deeply undercooled melts. In such a way, the sequence of solidification of different crystallographic phases and (partly) remelting of primarily formed phases during recalescence is unambiguously determined. All these investigations allow for the construction of nucleation phase-selection diagrams examples of which are presented for Fe-based, Ni–V, and Nd–Fe–B alloys with a competition of phase formation of bcc, fcc, and some intermetallic phases, respectively. They give the conditions for the formation of metastable solids of different crystallographic structures as a function of concentration and undercooling achieved prior to solidification.

Soft Magnetic Properties of Nanocrystalline Fe–Co–B–Si–Nb–Cu Alloys in Ribbon and Bulk Forms

Ribbon and bulk nanocrystalline body-centered-cubic (bcc) (Fe,Co) alloys exhibiting good soft magnetic properties were synthesized in Fe71.5-xCoxB13.5Si10Nb4Cu1 system by the simple production processes of melt-spinning or casting and annealing. The glass-type alloys were formed in the Co content range below 30 at.%. These glassy alloys crystallized through two exothermic reactions. The first stage was due to the precipitation of nanoscale bcc-(Fe,Co) phase with a grain size of about 10 nm, and the second stage resulted from the decomposition of the remaining amorphous phase to α–(Fe,Co), (Fe,Co)2B, (Fe,Co)23B6, (Fe,Co)3Si, and (Fe,Co)2Nb phases. The glass transition temperature increased from 820 to 827 K with increasing Co content from 5 to 20 at.%, while the supercooled liquid region decreased slightly from 37 to 30 K because of the nearly constant crystallization temperature. By choosing the 10 at.% Co-containing alloy, we produced cylindrical glassy alloy rods 1.0 and 1.5 mm in diameter by copper mold casting. The subsequent annealing for 300 s at 883 K corresponding to the temperature just above the first exothermic peak caused the formation of nanoscale bcc-(Fe,Co) structure. The bcc-(Fe,Co) alloy rods exhibited good soft magnetic properties of 1.26 T for saturation magnetization and 5.0 A/m for coercive force, which were comparable to those for the corresponding bcc-(Fe,Co) alloy ribbon. The nanocrystalline alloy in a bulk form is encouraging for future use as a new type of soft magnetic material that requires three-dimensional shapes.

Phase stability of the Hf-Nb system: From first-principles to CALPHAD

Hf-Nb is representative of a large number of alloy systems where the phase diagram is known but experimentally measured thermodynamic data is unavailable. This motivated us to investigate solid-state phase stability of this system employing ab-initio techniques based on electronic density functional theory (DFT). The total energy calculations are performed at the generalized gradient approximation level to account for non-locality of the exchange-correlation functional within DFT. The zero-temperature heat of formation of bcc-, hcp-and fcc-based ordered phases (virtual) and the finite temperature heat of mixing of bcc, hcp and fcc solid solutions are calculated. The latter properties are obtained by the cluster expansion technique coupled with Monte-Carlo simulations. In an effort to build multicomponent thermodynamic and kinetic databases containing Hf and Nb, we demonstrate a hybrid approach to integrate the ab-initio results with CALPHAD formalism to obtain thermodynamic data for the Hf-Nb system. The similarities and differences between previous CALPHAD assessment and our results are discussed.

Magnetic properties of nanocrystalline Fe86.7Zr3.3B4Ag6 thin film

The changes of magnetic properties with annealing temperature were studied in the amorphous Fe86.7Zr3.3B4Ag6 thin film. The thin films were deposited by a DC magnetron sputtering method, annealed at 300–700°C for 1h in vacuum under a field of 1.5kOe parallel to the film plane, and then furnace-cooled. As a result, it has been found that the Ag addition to Fe–Zr–B amorphous thin films resulted in the decrease of crystallization temperature to 400°C due to promoted crystallization ability. Also, it gave rise to formation of fine BCC α-Fe crystalline precipitates with a grain size smaller than 10nm in the amorphous matrix near 400°C, and led to prominent enhancement in the magnetic properties of the Fe86.7Zr3.3B4Ag6 thin films. Significantly, excellent magnetic properties such as a saturation magnetization of 1.7T, a coercive force of 1Oe and a permeability of 7800 at 50MHz were obtained in the amorphous Fe86.7Zr3.3B4Ag6 thin film containing 7.2nm-size BCC α-Fe, which was annealed at 400°C. Also, core loss of 1.4Wcm−3 (Bm=0.1T) at 1MHz in the thin film was obtained, and it is a much lower value than had been obtained in any existing soft magnetic materials. Such excellent properties are inferred to originate from the uniform dispersion of nano-size BCC α-Fe in the amorphous matrix.

Real-time detection of metastable phases in Zr-based bulk glasses during fast heating in a synchrotron beam

We have studied in situ crystallization from the glassy state and solidification from the liquid state in Zr-based bulk glass-forming alloy. The experiments were performed with a high intensity, high-energy monochromatic synchrotron beam. Samples of amorphous Zr 55Cu 20Al 10Ni 10Pd 5 alloy with rectangular section and 2 mm thickness, sealed under vacuum in specially designed quartz tubes were studied. Diffraction in transmission was used to minimize surface crystallization effects. The choice of the sample thickness and the monochromatic beam energy allow an acquisition time of 2 s per full pattern. The heating rate was around 20 K/s nearly 20 times greater than that usually used in DSC calorimetry and close to those of cooling rates for glass formation. In Zr 55Cu 30Al 10Ni 5, crystallization occurs via initial nucleation of a metastable phase that remains only a few seconds before transformation to the equilibrium phase, tetragonal Zr 2Cu. The increase of Zr content to 69% increases the stability of the metastable phase, which remains until melting. It was found that the Pd addition to the quaternary alloy with Zr 55 at.% also makes the metastable phase stable up to a temperature near melting. We have also found that, contrary to the Zr addition, the Pd addition eliminates the formation of bcc or hexagonal solid solution phase at high temperatures.

(FeCoNi)–ZrBCu base alloys, the influence of transition metal composition and heat treatment on structure and magnetic properties

Magnetic properties of amorphous and partially crystallized (Fe : Co : Ni) 86Zr 7B 6Cu 1 alloys were investigated. Primary formation of BCC as well as FCC nanocrystals takes place at the first stage of crystallization. Low coercivities (H c <10 A/m) and low magnetostrictions are observed only in the Fe-rich corner. Maximum value of saturation polarization J S =1.63–1.67 T are obtained in the partially crystallized (Fe : Co) 86Zr 7B 6Cu 1 alloys with 30–35 Co-parts.

Monte Carlo study of structural ordering in charged colloids using a long-range attractive interaction

Monte Carlo simulations have been carried out for aqueous charged colloidal suspensions interacting via an effective pair potential ${U}_{s}(r),$ which has the long-range attractive term in addition to the usual screened Coulomb repulsion. Simulations are performed over four orders of magnitude of particle volume fractions $(\ensuremath{\varphi})$ under salt free as well as added salt conditions. The computed pair correlation functions $g(r)$ at high values of $\ensuremath{\varphi}$ show a face centered cubic (fcc) crystalline order, which is found to transform to a body centered cubic (bcc) crystalline order upon lowering of $\ensuremath{\varphi}.$ The crystalline order is found to melt into a liquidlike order upon the addition of salt. A purely repulsive potential based on Derjaguin-Landau-Vervey-Overbeek theory was earlier claimed to be responsible for the formation of bcc and fcc phases and the associated order-disorder transitions. It is clearly shown here that ${U}_{s}(r)$ also explains equally well such phenomena and the results are shown to be in close agreement with experimental observations. Simulations in the dilute regime show a vapor-liquid transition upon variation of $\ensuremath{\varphi}$ and the coexistence of ordered and disordered regions with voids upon variation of the charge on the particles. These results explain the reported experimental observations, which suggested the existence of a long-range attraction in the interparticle interaction. For very low volume fractions calculated pair correlation functions show only a single peak and are found to be independent of $\ensuremath{\varphi}.$ The reported results on the direct measurement of the pair potential are discussed in the light of the present results.

Al-Ni-Fe 3元合金の急冷凝固組織とAlのアルカリリーチングによる非平衡相の生成

A new approach for the metastable phase formation based on chemical reaction was proposed. Various ternary Al3(NixFe1−x) alloys were rapidly solidified and then aluminum atoms were leached from the alloys by the hot alkaline solution. The structure of the as-leached and heat treated specimens were examined by SEM and X-ray diffraction. The results are as follows.(1)In the as-leached state, the crystal structure varied with the value of x.In the range of x=0 to 0.33, a metastable bcc structure (supersaturated bcc in Ni) and magnetite were formed. Fcc and magnetite were formed between 0.33 and 0.5 of x. At a high values of x (more than 0.5), only the fcc phase was observed.(2)X-ray diffraction patterns of the fcc phase were very broad, and it was estimated that the grain size of the leached specimens were in the range of 5 and 10 nm. Since the specific surface area of the as-leached specimens was quite large, each grain might have a gap (free space) at the grain boundary.(3)The leached specimens showed high activity and sometimes ignited spontaneously in atomosphere due to their high specific surface area.(4)Rapidly solidified alloys were nonmagnetic, but after leaching they showed magnetism due to the formation of magnetite and ferromagnetic bcc and fcc phases.

Effect of Prior Deformation of Austenite on the &amp;gamma;&amp;rarr;&amp;epsilon; Martensitic Transformation in Fe&amp;ndash;Mn Alloys

The influence of prior deformation of austenite (γ) on the formation of hcp (∈) martensite in Fe-Mn binary alloys during subsequent cooling was investigated. The formation of bcc (α') lath martensite in deformed austenite was also examined for comparison. In both of the martensitic transformations to α'in the Fe-9%Mn alloy and to ∈ in the Fe-16%Mn and Fe-24%Mn alloys, the transformation start temperature (M s ) monotonously decreases with increasing the reduction of austenite at 773 K

Fe-Ti metastable-phase formation by ion-beam mixing.

The glass-forming ability of the Fe-Ti system by ion-beam mixing of multilayers at 300 K is investigated at three compositions: ${\mathrm{Fe}}_{83}$${\mathrm{Ti}}_{17}$ (close to eutectic), and ${\mathrm{Fe}}_{2}$Ti and FeTi (compositions of the equilibrium compounds). The formation of bcc ${\mathrm{Fe}}_{83}$${\mathrm{Ti}}_{17}$ is shown whereas an amorphous state is obtained at ${\mathrm{Fe}}_{2}$Ti and FeTi compositions. A good qualitative interpretation of the experimental results is given in the framework of metastable enthalpy curves and correlations with the equilibrium phase diagram are discussed. Rutherford-backscattering spectrometry of \ensuremath{\alpha} particles has been used to characterize the mixing efficiency. X-ray diffraction at glancing incidence and conversion-electron M\"ossbauer spectroscopy have been performed to investigate the short-range order in the glassy state.

Nonequilibrium Crystalline and Amorphous Fe&amp;ndash;Cu&amp;ndash;Ag Ternary Alloys Produced by Vapor Quenching

Fe1−y(Cu1−xAgx)y alloys sputter-deposited on liquid nitrogen-cooled substrates have been investigated by X-ray diffraction and differential scanning calorimetry. Although this ternary alloy system is typically immiscible in the equilibrium state, a single bcc phase is formed at the Fe-rich corner and a single fcc phase at the Cu- and Ag-rich corners, while an amorphous phase is formed in the central concentration region. The formation of the single bcc and fcc phases is consistent with the results of the vapor quenched binary Fe–Cu, Fe–Ag and Cu–Ag alloys. However, the formation of the single amorphous phase is a new aspect for the vapor quenched ternary Fe–Cu–Ag alloys. The crystallization temperature of amorphous alloys is about 400 K, being insensitive to the alloy concentration.

Comparison of pair potential and thermochemical models of the heat of formation of BCC and FCC alloys

Data bases covering the thermochemical properties of solid solution phases are being assembled for a wide variety of applications. Such activities are often hampered by limitations on available experimental data and fundamental methods for estimating missing information. To bridge this gap the pair potential model has been developed and refined and is compared here with results drawn from thermochemical and phase diagram analysis of the heat of formation of bcc and fcc alloys. A computer program is presented providing a means for calculating the heat of formation for binary systems composed of twenty-seven elements. The results are compared with thermochemical values for sixty-six binary systems composed of all combinations of aluminum, cobalt, chromium, copper, iron, manganese, molybdenum, niobium, nickel, silicon, titanium and tungsten. The level of agreement provides a good measure of confidence in the usefulness of this pair potential model to obtain estimates for data which is otherwise available.