Metastable Bcc Research Articles

Calculation of ferromagnetic states in metastable bcc and hcp Ni by projector-augmented wave method

Based on a projector-augmented wave method, ab initio calculations reveal that hcp Ni is ferromagnetic with a magnetic moment of 0.59μB, which is slightly smaller than 0.60μB in the stable fcc Ni. For bcc Ni, it is found that a ferromagnetic is energetically favored when the lattice constant a>2.69Å, below which there may be a coexisting state of ferromagnetic and nonmagnetic behaviors. Interestingly, although bcc Ni has a smallest magnetic moment of 0.55μB, it has a largest average atomic volume among the three structures, differing sharply from the cases of Fe and Co.

Crystal Structure and Interaction Dependence of the Crystal-Melt Interfacial Free Energy

We examine via molecular simulation the dependence of the crystal-melt interfacial free energy gamma on molecular interaction and crystal structure (fcc vs bcc) for systems interacting with inverse-power repulsive potentials, u(r)=epsilon(sigma/r)(n), 6< or =n< or =100. Both the magnitude and anisotropy of gamma are found to increase as the range of the potential increases. Also we find that gamma(bcc)<gamma(fcc), consistent with recent observations that some fcc forming fluids nucleate via formation of metastable bcc nuclei. The anisotropy in gamma is also seen to be smaller in the bcc systems. By extrapolation, we also obtain an improved estimate of gamma for hard spheres.

Long-Lived Metastable bcc Phase during Ordering of Micelles

We report a metastable bcc phase that intervenes between a disordered micellar suspension and an fcc crystal in a block copolymer solution. Upon heating, the metastable bcc phase nucleates first, and then transforms over the course of hours to the stable fcc phase. At still higher temperatures, the fcc phase transforms to an equilibrium bcc phase. These results constitute an interesting experimental manifestation of Ostwald's step rule, and also support recent theory and simulation results whereby bcc nucleates more readily from a melt of spheres.

Structural modification of supersaturated BCC-Nd(Al) solid solution

The metastable Nd(Al) supersaturated solid solution with the BCC structure was synthesized by the melt-spinning method. The thermal stability of the supersaturated solid solution was studied by differential scanning calorimetry (DSC) and electrical-resistance measurements. The structural modification was determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). A constrained BCC lattice was found due to the smaller Al atoms entering the BCC-Nd lattice. Two evident transformation stages were found during the heating process. The lower activation energy and negative temperature coefficient of the resistivity (TCR) suggest that a continuous structural transformation occur, which results from the softening mode of the highly metastable BCC structure. The strongly distorted and ultra-fine microstructure was observed directly by TEM.

Magnetic study of Fe 65Ni 20Nb 6B 9 nanocomposite alloys

The alloy Fe 65Ni 20Nb 6B 9 was obtained from the elemental constituents in a high-energy planetary ball mill and subsequently thermally treated at 873 and 900 K in order to obtain the equilibrium phases. The as-prepared nanocrystalline alloy consists primarily of metastable BCC α-Fe(Ni) nanocrystals while the treated ones consist of a mixture of BCC (ferromagnetic) and FCC (paramagnetic at room temperature) phases. Hysteresis loops at 5 and 300 K present low remanence and coercivity. As-prepared sample exhibits the best soft magnetic properties. In all samples, the susceptibility curves suggest magnetic collective (long-range order) behavior with a maximum between 70 and 90 K. This feature is caused by nanometric-sized magnetic particles. The huge difference between blocking and irreversibility temperatures in the field cooling and zero-field cooling scans indicates a wide grain-size distribution.

Disordered atomic column state in Fe-Mo alloys

Changes in crystallographic features during the ($\mathrm{bcc}\ensuremath{\rightarrow}\mathrm{bcc}+C14$ structure) reaction have been investigated for Fe-Mo alloys containing around $10\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ Mo by transmission electron microscopy. The metastable bcc Fe-Mo alloys were annealed at the relatively low temperature of $1023\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in order to elucidate the details of the structural change in the initial stage. The annealing led to the appearance of precipitates in the bcc matrix, which did not have the $C14$ structure even after $1000\phantom{\rule{0.3em}{0ex}}\mathrm{h}$ of annealing. The structure of the precipitate was basically characterized by a disordered arrangement of distorted decagonal columns. The column was composed of distorted icosahedral clusters connected one-dimensionally. In addition, annealing was found to cause an increase in the number of $C14$ configurations for the local arrangement of these decagonal columns.

Incident angle effects on microstructure and magnetic properties of Fe–Nb thin films using IBAD

The effects of the incident angle on the microstructure and magnetic properties of Fe–Nb thin films grown by Ar ion beam assisted deposition have been investigated. A new metastable crystalline bcc phase and an amorphous phase were obtained at 45° ion incident angle, while a mixture of bcc Nb and bcc Fe was formed under the incident angles of the assisting ion beam of 30° and 60°. The difference caused by the variation of incident angle is discussed on the basis of energy transfer and competition between the effects of surface energy and ion beam channeling.

Hydrogen embrittlement of a metastable β-titanium alloy

The effect of hydrogen on the mechanical behavior of a metastable bcc β-titanium alloy (Beta-C) has been investigated. By keeping the microstructure constant as the hydrogen concentration was increased the direct effect of hydrogen on the mechanical properties was ascertained. Hydrogen embrittles Beta-C Ti in the aged condition when the hydrogen level exceeds 3.5 at.% H in the bulk. The embrittlement was accompanied by a change in fracture appearance that was shown to be the result of the operation of several hydrogen related fracture mechanisms.

Surface structure and topology in surface stabilized Co-nanoparticles with a thin Al2O3 amorphous layer

Heating an amorphous Co2+:AlO(OH)·αH2O gel under a reducing atmosphere of pure H2 gas at 700–850°C temperature results in surface stabilized Co-metal nanoparticles with a thin Al2O3 ceramic surface layer. At early temperatures during heating, the gel decomposes and disperses in a refined structure in divided Co2+ groups through a matrix of Al2O3 (amorphous). A reconstructive Co2++H2→Co+2H+ reaction operates in the divided groups to result in isolated Co-particles. Formation and existence of Al2O3 layer (in a limited thickness t≤R0, with R0∼4.28nm the critical Al2O3 dimension to grow as a stable crystallite) over growing Co-particles control the process in a high-energy metastable fcc or bcc Co allotrope structure. Average crystallite size thus hardly grows to be as big as 41nm. A large value of surface energy σ=0.790J/m2 in Al2O3, in comparison to 0.234J/m2 in fcc or 0.279J/m2 in hcp Co, inhibits a moderate diffusion of surface atoms and in turn controls the nucleation and growth in small Co-particles. Otherwise, a particle of pure Co metal grows rapidly in the hcp bulk structure. The results of the Al2O3 surface modified fcc and bcc Co are analyzed with X-ray photoelectron spectroscopy in correlation of X-ray diffraction and microstructure.

Nanoscale studies of segregation at coherent heterophase interfaces in α‐Fe based systems

AbstractHeterophase interfaces are characterized with respect to atomic‐scale local compositions in two α‐Fe‐based alloy systems by three‐dimensional atom probe microscopy. The proximity histogram method is utilized for determining the compositions of the precipitates, the matrix phase and the Gibbsian interfacial excess of solute. Suitable methods to measure segregation in multicomponent systems at heterophase interfaces independent of the position of the dividing interface are discussed. Very little Sb segregates to the heterophase interface between coherent plate‐shaped moybdenum nitride precipitates, which are &lt;1 nm thick, and the α‐Fe matrix: the relative interfacial excess of Sb is only 0.12 ± 0.1 nm−2. This is in sharp contrast to the interfacial excess of 7 ± 3 nm−2 for molybdenum nitride plates that have grown to 2–10 nm thick (or 20–30 nm in diameter for spheroidal precipitates) with a semicoherent interface with misfit dislocations, as reported earlier. Small coherent Cu‐rich precipitates with a metastable bcc structure in an α‐Fe matrix exhibit interfacial segregation of Ni and Mn at their interfaces with excesses of 1.47 ± 0.43 and 0.95 ± 0.31 nm−2, respectively. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Metastability of an immiscible Cu-Mo system calculated from first-principles and a derivedn-body potential

An ab initio calculation is performed to predict the structures, lattice constants, and cohesive energies of the metastable ${\mathrm{Cu}}_{75}{\mathrm{Mo}}_{25},$ ${\mathrm{Cu}}_{50}{\mathrm{Mo}}_{50},$ and ${\mathrm{Cu}}_{25}{\mathrm{Mo}}_{75}$ phases. With the aid of an ab initio calculation, an n-body Cu-Mo potential is derived and proven to be realistic in reproducing some intrinsic properties of the metastable Cu-Mo phases. Based on the Cu-Mo potential, a molecular dynamics simulation reveals that a crystal-to-amorphous transition takes place in a Cu-rich fcc solid solution when the solute concentration reaches/exceeds a critical value of 25 at. % Mo. Moreover, a molecular dynamics simulation also predicts the formation of metastable fcc and bcc Cu-Mo phases and determines the heats of formation of both crystalline and amorphous phases, thus constructing an energy diagram of the Cu-Mo system over the entire composition range. The calculation/simulation results are compared with the experimental observations, and the agreements between them are fairly good.

Comparison among Mechanical Properties of Investment-Cast c.p. Ti, Ti-6Al-7Nb and Ti-15Mo-1Bi Alloys

The present study compares castability and mechanical performance among alloys of c.p. Ti, Ti-6Al-7Nb and a newly-developed Ti-15Mo-1Bi in regard to their as-cast state. Experimental results indicate that average cast length of Ti-6Al-7Nb appears a little lower than c.p. Ti and Ti-15Mo-1Bi, although the differences are not significant. C.p. Ti has an hcp a phase of lath-type morphology with serrated, irregular grain boundaries. Ti-6Al-7Nb is dominated by hexagonal α' phase with acicular martensitic morphology. In particular, Ti-15Mo-1Bi is comprised of equiaxed, metastable bcc β phase. The hardened layer thicknesses of three materials are similar. Ti-15Mo-1Bi has the highest surface microvickers hardness, while c.p. Ti has the lowest. The bulk hardness of c.p. Ti is much lower than the other two alloys which display similar hardness. Notably, Ti-15Mo-1Bi and Ti-6Al-7Nb have similar bending strength which is far higher than c.p. Ti. The bending modulus of Ti-15Mo-1Bi is significantly lower than c.p. Ti and Ti-6Al-7Nb. Ti-15Mo-1Bi also shows a much larger elastic recovery angle than two other materials. Tensile test indicates that Ti-15Mo-1Bi has a similar UTS but much higher YS and elongation along with much lower tensile modulus compared to Ti-6Al-7Nb.

Metastable phase formation in undercooled Fe–Co melts

Abstract Soft magnetic Fe–Co alloys display the primary fcc phase solidification for >18 at.% Co in conventional near-equilibrium solidification processes. Undercooled Fe–Co melt drops within the composition range of 70–50 at.% Fe have been investigated with the electromagnetic levitation technique. The solidification kinetics was measured in situ using a high-resolving Si photodiode. The melt drops were undercooled up to 263 K below the liquidus temperature and subsequently quenched onto a chill substrate in order to characterise the solidification sequence and the microstructure. The transition from stable fcc phase to primary metastable bcc phase has been observed after reaching a critical undercooling level which increases with rising Co content. The growth velocity drops obviously after transition to metastable bcc phase formation. Moreover, parabolic flight experiments were performed in order to study the phase selection under reduced gravity conditions. The investigation of the solidification events yielded a much smaller critical undercooling and an increased life time of the metastable bcc phase.

Magnetic properties of Co and Co-Ag alloys in equilibrium/nonequilibrium structures studied byab initiocalculations

Ab initio calculations predict that the magnetic moments of the metastable fcc and bcc Co are 1.63 and $1.69{\ensuremath{\mu}}_{B},$ respectively, which are greater than the $1.57{\ensuremath{\mu}}_{B}$ value of the equilibrium hcp Co. For the Co-Ag system, ab initio calculations reveal that the ${\mathrm{CoAg}}_{3},$ ${\mathrm{Co}}_{3}\mathrm{Ag},$ and CoAg alloys are energetically favored to be in ${D0}_{19},$ ${D0}_{19},$ and $B2$ structures, respectively, and all prefer to be ferromagnetic rather than paramagnetic. Moreover, the magnetic moments of Co atom are enhanced to 1.62 and $1.76{\ensuremath{\mu}}_{B}$ in ${D0}_{19}$ ${\mathrm{Co}}_{3}\mathrm{Ag}$ and $B2$ CoAg alloys, respectively, while reduced to $1.43{\ensuremath{\mu}}_{B}$ in the ${D0}_{19}$ ${\mathrm{CoAg}}_{3}$ alloy. The results seem to suggest that an expansion of average atomic volume would enhance the magnetic moment of Co, which, however, would be reduced by nonmagnetic element alloying. Interestingly, the predictions agree reasonably well with those obtained from experimental as well as theoretical studies.

Hydrogen solubility of BCC titanium alloys

Hydrogen absorption–desorption properties of Ti–M–V–B (M=Cr, Mn) metastable BCC alloys have been examined at hydrogen pressures of 0.01–7 MPa and a temperature of 303 K using an automatic Sieverts apparatus. Ti 37Mn 25V 38B x alloys were prepared by arc melting and Ti 31Cr 30V 39B x ( x=0, 6) alloys were prepared by a suction casting method. The crystal structure and lattice parameters of the alloys were examined by an X-ray diffraction analysis (XRD) in addition to pressure–composition isotherms. The Ti 37Mn 25V 38 alloy and the Ti 31Cr 30V 39B x ( x=0, 6) alloys were BCC single phase and had large hydrogen storage capacities.

Molecular dynamics simulations of stress-induced phase transformations and grain nucleation at crack tips in Fe

The molecular dynamics simulation technique is used to study a stress-induced new grain formation mechanism at the crack tip of a nanocrystalline α-iron sample at temperatures ranging from 100 to 600 K. The stress-induced formation of new bcc grains, created inside existing grains, is found to occur through a metastable bcc to fcc phase transformation at the crack tip of the sample. A Nishiyama–Wassermann orientation relationship is found between the original bcc grain and the fcc phase and a Kurdjumov–Sachs orientation relationship is found between the new bcc grain created and the fcc transition phase. The new grain nucleation is observed to increase with increasing temperature and stacking faults associated with the fcc phase are observed at the higher temperatures.

Ion beam induced formation of metastable alloy phases in Cu–Mo system during ion beam assisted deposition

The Cu–Mo alloy film with positive heat of mixing was prepared by ion beam assisted deposition (IBAD) technique. When the energy of Ar + ions was 2 keV during IBAD, metastable bcc Cu was formed in the Cu 20Mo 80 film. A Mo-based bcc supersaturated solid solution was obtained within the composition range of 0–50% Cu fraction. An fcc supersaturated solid solution was also formed around the composition of 15 at.% Mo. The formation mechanisms of the metastable alloy phases were discussed in terms of the process far from equilibrium.

Structure development in amorphous Al–La alloys

Melt-spun Al alloys containing 2.7–10 at.% La were studied by means of x-raydiffraction, transmission electron microscopy, electrical resistivity, thermal andmicrohardness measurements. Fully amorphous structures were only obtained forthin AlLa7.7 and AlLa10 ribbons and exhibit concentration fluctuations. Phaseseparation into two metastable bcc structures is observed; they appearsuccessively and are finally replaced by the equilibrium phases α(fcc)Al and (orthorhombic)Al11La3. The transformation kinetics is analysed interms of a general Johnson–Mehl–Avrami–Kolmogorov model and of local kineticparameters. The Avrami exponent monitors changes in the crystallizationbehaviour; nucleation of the stable phases occurs after partial dissolution of themetastable phases. The low values of the activation energy reflect the highinstability of these glasses.

Diffuse interface analysis of crystal nucleation in hard-sphere liquid

We show that the increase of the interface free energy with deviation from equilibrium seen in recent Monte Carlo simulations [S. Auer and D. Frenkel, Nature (London) 413, 711 (2001)] can be recovered if the molecular scale diffuseness of the crystal–liquid interface is considered. We compare two models, Gránásy’s phenomenological diffuse interface theory, and a density functional theory that relies on the type of Ginzburg–Landau expansion for fcc nucleation, that Shih et al. introduced for bcc crystal. It is shown that, in the range of Monte Carlo simulations, the nucleation rate of the stable fcc phase is by several orders of magnitude higher than for the metastable bcc phase, seen to nucleate first in other fcc systems. The nucleation barrier that the diffuse interface theories predict for small deviations from equilibrium is in far better agreement with the simulations than the classical droplet model. The behavior expected at high densities is model dependent. Gránásy’s phenomenological diffuse interface theory indicates a spinodal point close to glass transition, while a nonsingular behavior is predicted by the density functional theory with constant Ginzburg–Landau coefficients. Remarkably, a minimum of the nucleation barrier, similar to the one seen in polydisperse systems, occurs if the known density dependence of the Ginzburg–Landau coefficients is considered.

Phase evolution in laser-deposited titanium-chromium alloys

Ti-Cr alloys have been laser deposited from powder feedstock consisting of a blend of elemental powders using the laser-engineered net-shaping (LENS) process. The microstructure of the as-deposited Ti-Cr alloys primarily consists of a metastable bcc matrix of β-Ti(Cr) with precipitates along the grain boundaries. The grain-boundary precipitates have been identified to be of three types, essentially pure hcp Ti, an alloyed hcp phase designated α-Ti(Cr), and the C14 TiCr2 Laves phase. Initial stages of decomposition, visible within the β matrix, suggest a spinodal clustering process resulting in a fine dispersion of second phases. Diffraction studies have revealed the presence of fine precipitates of α within the β matrix. The evidence for the precipitation of the metastable ω phase within the β matrix is not strong. The phase evolution in the LENS-deposited Ti-Cr alloy has been discussed in the context of rapid solidification and the enthalpy of mixing of the elemental powders.