B-rich Phase Research Articles

Structural and thermodynamic properties of interfaces between coexisting phases in polymer blends: a Monte Carlo simulation

A large-scale Monte Carlo investigation of the interface between the A-rich and B-rich phase of a partially incompatible binary (AB) symmetric polymer mixture is presented. The bond fluctuation model on the simple cubic lattice at a volume fraction ϕ= 0.5 of occupied lattice sites and chain lengths NA=NB=N= 32 is used, assuming an interaction IµAA=IµBB=–IµAB=–Iµ between effective monomers, with an interaction range of √6 lattice spacings. The temperature range studied, 0.144 < T/Tc < 0.759, encompasses the whole range from the strongly segregated regime at low temperatures to the weak segregation regime near the critical temperature Tc(the latter has already been estimated very accurately from a finite-size scaling analysis in previous work). The temperature dependence of both interfacial Helmholtz energy and interfacial width, w, are presented, and it is shown that the former is compatible with current theory, while w exceeds the theoretical predictions by about a factor of two. Varying the linear dimension L parallel to the interface from L= 64 to L= 512 we can show that broadening of the concentration profile due to capillary waves cannot explain this discrepancy (note that the large size of our systems, up to ca. 16 × 106 lattice sites, needed the use of a massively parallel CRAY-T3D system). We present evidence for the enrichment of chain ends and of ‘free volume’ at the interface and show that chains near the interface are oriented and slightly contracted in the strong segregation limit. We suggest that the ‘effective’ Flory–Huggins χ parameter is reduced near the interface by these effects.

Detonation coating processes: V. Kadyrov et al (Academy of Sciences, Kiev, Ukraine), Trans. PM Assoc. of India, Vol 20, 1993, 1–5

In this work, alloys from the hypoeutectic iron-rich region of the iron–carbon–boron (Fe–C–B) system were investigated with respect to the solidification and the phase formation. Laboratory melts with a constant carbon content of 0.6 mass% and boron contents of 0.2 mass%, 0.6 mass%, and 1.8 mass% were fabricated and metallographically examined. In addition the microstructures were investigated by CALPHAD method in the state of equilibrium and by multiphase-field (MPF) method to reproduce the non-equilibrium process of the technical solidification. The results were analyzed with respect to the effect of boron on the solidification paths, microstructural crystallization processes as well as the morphological and chemical characteristics of the solidified phases.The investigated alloys undergo primary crystallization of austenite (γ-Fe). Due to the low solubility of B in the primary phase γ-Fe, B is strongly segregated in the melt and the solidification paths are deviated to high B contents. Therefore, as the B content increases, the eutectic solidification sequence starts with the B-rich Fe2B phase and continues with the formation of the B-rich Fe3(B,C) phase in the latter process. The B content of the melt thus decreases during the eutectic reaction, and the eutectic Fe3(B,C) phase exhibits a decreasing B gradient in the direction of growth. Consequently, the low-melting phase of the Fe–C–B system is the Fe3(B,C) phase with a low B content and a composition closest to its low-melting B content of 14.10 at.% B. Increasing B/(C + B) ratios of the alloy composition raise the average B content of the Fe3(B,C) phase (up to >20 at.% B) and hence at the same time increase the solidus temperature of the alloy. These findings revealed consistency with experimental results for chemical composition (WDX), phase analysis (diffraction with synchrotron radiation, EBSD), and thermal analysis (DTA).

A Mössbauer study of the permanent magnets Nd15(Fe,Co)77B8 with Dy and Al additives

Nd−(Fe, Co)−B alloys with Dy and Dy−Al additives have been studied by the Mossbauer effect and X-ray diffraction. The results indicate that the alloys consist of a tetragonal phase, a B-rich phase and a Nd-rich phase. The average magnetic moment of Fe atoms in the tetragonal phase has been determined. The variation of remanence due to Dy and Dy−Al additives has been derived from the Mossbauer data and found to agree with the results of magnetic measurements. The site substitutions of Dy and Al in the alloys are also discussed.

Correlation between al addition and microstructural changes in Nd-Fe-B magnets

Depending on the kind of Al addition (Al, Al 2O 3), two Al-containing phases and an Nd-rich, Fe-poor phase occur besides the hardmagnetic phase Nd 2Fe 14B, the Nd-rich fcc phase and the B-rich phase Nd 5Fe 2B 6. Al addition nearly halves the wetting angle of the liquid phase during liquid phase sintering.

ELECTRON DIFFRACTION AND HIGH RESOLUTION MICRO-SCOPY STUDY ON INCOMMENSURATE MODULATED STRUCTURE IN Ce1+εFe4B4 ALLOY

The permanent magnetic alloy Ce1+εFe4B4(ε= 0.1, B-rich phase) is studied by means of electron diffraction and high resolution electron microscopy. It has been determined that the Ce1+εFe4B4 alloy has a one-dimensional incommensurate modulated structure of Chimney-Ladder type consisting of two overlapped substructures fomed by Ce atoms and Fe-B clusters respectively. The unit cell parameters of Ce substructure are a = 7.08?, cCe= 3.51-3.60?, and the possible space groups are I4,I4,I4mm, I4/m, I4/mmm. The unit cell parameters of Fe-B substructure are a = 7.08?, cFe= 3.91?, and the space group is P/42/mcm. All of the major and satellite diffraction spots are indexed by using of four indexes method. The high resolution electron microscopic image shows contrast modulation bands corresponding to the modulated structure. The fluctuation of the period of Ce substructure has been observed and discussed. The orientational anormaly caused by the possible deviation of Ce atomic planes from the (a, b) plane is also discused.

THE 1-DIMENSIONAL COMPOSITION INCOMMENSURATE STRUCTURE OF Ce 1+ε Fe 4 B 4 *

In this article the crystal of a B-rich phase Ce_(1+e)Fe_4B_4 in the ternary alloy system of Ce-Fe-B has been studied by means of X-ray powder diffraction as well as electron diffraction. It is found that the of this phase is a kind of the so-called chimeny-ladder structure consisting of 1-dlmensional composition incommensurate which is seldom observed in common alloys. The of this compound is composed of two interpenetrating structures, one being the (Fe-B) substructure and the other Ce-substructure, and both possessing tetragonal symmetry with the same lattice parameter α, but without the lowest common multiple relation between c_(Fe-B) and c_(Ce) parameters. The lattice parameters of Ce_(1+e) Fe_4B_4 are as follows: α=0.7068 nm, C_(Fe-B)=0.3902 nm, c_(Ce)=0.3440 nm at room temperature and α=0.7065 nm, c_(Fe-B)=0.3887 nm and c_(Ce)=0.3563 nm after quenching from 950℃. The space groups of the substructure of Fe-B and that of Ce are P4_2/ncm and I4/mmm respectively.

Large Barkhausen jumps observed in Nd-Fe-B sintered magnets at very low temperatures

The demagnetization process for Nd/sub 15/Fe/sub 77/B/sub 8/ sintered magnets was studied by precise loop measurements below 50 K. A zigzag variation of coercivity with temperature was observed below about 18 K. In this temperature range, the magnetization reverses with large discontinuous magnetization jumps. The origin of the jumps, which are caused by the sudden growth of residual reversed magnetic domains pinned at some defects, was investigated by measuring hysteresis loops at very low temperatures and analyzing them from the viewpoint of domain-wall nucleation and propagation. The temperature of 18 K coincides with the Curie temperature of the B-rich Nd/sub 1.11/Fe/sub 4/B/sub 4/ phase segregated in the Nd/sub 15/Fe/sub 77/B/sub 8/ magnet. This behavior may be related to the soft magnetic properties of the B-rich phase and the demagnetizing field in the magnet. >

The Mössbauer study of Nd−Fe−Co−B permanent magnetic alloys

Sintered Nd17 (Fe1−x Cox)75B8 permanent magnetic alloys have been studied by Mossbauer effect, X-ray diffraction and electron probe micro-analysis, The results show that the alloys are composed of the tetragonal phase Nd2(Fe,Co)14B, the B-rich phase Nd111(Fe,Co)4B4 and the Nd-rich phase Nd80(Fe,Co)19B. In the tetragonal phase, Co atoms occupy preferentially the k2 and j2 sites, and Fe atoms occupy randomly the k1 site and preferentially the j1 site while the e and c sites seem to be completely occupied by Fe atoms.

Spectroscopic investigations of Mn 2+ in sodium borosilicate glasses

The decay times and excitation spectra for the two characteristic luminescence bands of Mn 2+ are reported, and analysed. The analysis of the decay times indicates that the high energy emission band arises mainly from Mn 2+ centers in the Si-rich phase, while the low energy emission is principally due to Mn 2+ in the B-rich phase. An analysis of the excitation spectra results in the determination of crystal field parameters for each of the two bands. From these parameters it is concluded that in these glasses Mn 2+ occupies two sites, one octahedral and the other tetrahedral. This together with the conclusions drawn from the decay times, implies that the high energy band arises from tetrahedral manganese mainly in the Si-rich phase while the low energy emission is due to octahedrally coordinated manganese located primarily in the B-rich phase.

Permanent magnet materials based on the rare earth-iron-boron tetragonal compounds

Structural and metallographic studies were carried out on the Nd-Fe-B alloy system as well as the Nd-Fe-B tetragonal compound on which record high energy magnets have been developed using a powder metallurgical technique. The study on the new magnet has also been extended to other R-Fe-B componds containing various rare earths (R) and to R-Fe-Co-B alloys. The results are as follows; (1) The sintered Nd-Fe-B magnet is composed of mainly three phases, the Nd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</inf> B matrix phase plus Nd-rich phase and B-rich phase ∼ Nd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</inf> B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> ) as minor phases. (2) Nd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</inf> B has the space group of P4 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> /mnm. The crystal structure of this phase can be described as a layer structure with alternate stacking sequence of a Nd-rich layer and a sheet formed only by Fe atoms. The sheet of Fe atoms has a structure similar to the σ-phase found in Fe-Cr and Fe-Mo systems. (3) The Nd-rich phase containing more than 95 at.% Nd, 3∼5 at.% Fe and a trace of B has fcc structure with a=0.52 nm. This phase is formed around grain boundaries of the matrix phase. Nd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</inf> B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> phase has an one-dimentional incommensurate structure with a=a <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</inf> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c\simeq8</tex> C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</inf> , based on a tetragonal structure with a <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</inf> =0.716 nm and c <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</inf> =0.391 nm. (4) In the as sintered Nd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">77</inf> B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</inf> alloy periodic strain contrasts are observed along grain boundaries, which disappear after annealing at 870K. This may be related to the enhancement of the intrinsic coercivity of the sintered magnet by post sintering heat treatment. (5) Stable R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</inf> B phases are formed by various rare earths except La. Of all the R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</inf> B compounds, Nd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</inf> B has the maximum saturation magnetization as high as 1.57 T. Dy and Tb form R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</inf> B phases with the highest anisotropies. Small additions of these elements greatly enhance the coercive force of the Nd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</inf> B base magnet. (6) Partial replacement of Fe by Co raises the Curie temperature of the Nd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</inf> B compound, which improves the temperature coefficient of the remanence of the magnet. But the intrinsic coercive force is decreased by the Co addition.

Kinetics of a first-order phase transition: computer simulations and theory

We make a quantitative comparison between the predictions of the Becker-Doring equations and computer simulations on a model of a quenched binary A-B alloy. The atoms are confined to the vertices of a simple cubic lattice, interact through attractive nearest neighbor interactions, and move by interchanges of nearest neighbor pairs (Kawasaki dynamics). We study in particular the time evolution of the number of clusters of A atoms of each size, at four different concentrations: ρA=0.035, 0.05, 0.075, and 0.1 atoms per lattice site. The temperature is 0.59 times the critical temperature. At this temperature the equilibrium concentration of A atoms in the B-rich phase is ρ A eq =0.0145 atoms/lattice site. The coefficients entering the Becker-Doring equations are obtained by extrapolation from previously published low-density calculations, leaving the time scale as the only adjustable parameter. We find good agreement at the three lower densities. At 10% density the agreement is, as might be expected, less satisfactory but still fairly good-indicating a quite wide range of utility for the Becker-Doring equations.

Clusters, metastability, and nucleation: Kinetics of first-order phase transitions

We describe and interpret computer simulations of the time evolution of a binary alloy on a cubic lattice, with nearest neighbor interactions favoring like pairs of atoms. Initially the atoms are arranged at random; the time evolution proceeds by random interchanges of nearest neighbor pairs, using probabilities compatible with the equilibrium Gibbs distribution at temperatureT. For temperatures 0.59Tc, 0.81 Tc, and 0.89T c, with densityρ of A atoms equal to that in the B-rich phase at coexistence, the density C1 of clusters ofl A atoms approximately satisfies the following empirical formulas: C1 ≈w(1 −ρ)3 andC 1, ≈ (1 −ρ)4Q1w1 (2 ⩽l ⩽ 10). Herew is a parameter and we defineQ l =∑ K e −βE(K) , where the sum goes over all translationally nonequivalentl-particle clusters andE(K) is the energy of formation of the clusterK. Forl > 10,Q 1 is not known exactly; so we use an extrapolation formulaQ l ≈Aw −l l −α exp(−bl σ), wherew s is the value ofw at coexistence. The same formula (withw > w s) also fits the observed values of C, (for small values ofl) at densities greater than the coexistence density (forT=0.59Tc): When the supersaturation is small, the simulations show apparently metastable states, a theoretical estimate of whose lifetime is compatible with the observations. For higher supersaturation the system is observed to undergo a slow process of segregation into two coexisting phases (andw therefore changes slowly with time). These results may be interpreted as a more quantitative formulation (and confirmation) of ideas used in standard nucleation theory. No evidence for a “spinodal” transition is found.