Metastable Bcc Research Articles

Polymorphic Transformation and Magnetic Properties of Rapidly Solidified Fe26.7Co26.7Ni26.7Si8.9B11.0 High-Entropy Alloys.

In this work, the microstructural evolution and magnetic performance of the melt-spun amorphous and amorphous-crystalline Fe26.7Co26.7Ni26.7Si8.9B11.0 high-entropy alloys (HEAs) during crystallization were investigated, respectively. Upon heating fully amorphous ribbons, a metastable BCC supersaturated solid solution together with a little Ni31Si12 crystals first precipitated and then the (Fe,Co)2B crystals formed until the full crystallization was achieved. With further increasing temperature after full crystallization, a polymorphic transformation from a metastable BCC phase to two types of FCC solid solutions occurred. For the amorphous-crystalline HEAs, the dominant crystallization products were the metastable FCC but not BCC crystals. During crystallization, the primary metastable FCC crystals first transform into the metastable BCC crystals and then the newly-generated BCC phase transforms into two types of FCC phases with further increasing temperature. This temperature dependence of the gradual polymorphic transformation results in the change of magnetic properties of the present high-entropy amorphous alloys.

Characterization of Ti-xNb (x = 25 – 35) Alloys in as Melt and Annealed States

The effect of Nb content on microstructure, mechanical properties and phase formation in as-melt and annealed binary Ti-Nb alloys were investigated using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analysis. The content of Nb varied in the range 25-35 mass % leading to significant changes in the microstructure. The annealed and furnace-cooled binary Ti-Nb samples exhibited HCP martensitic α` phase at a Nb content below 27.5 mass % and metastable BCC β phase at higher contents of Nb. The mechanical properties of alloys depended strongly on the Nb content and type of the dominating phase.

On Gibbs Energy for the Metastable bcc_A2 Phase with a Thermal Vacancy in Metals and Alloys.

An approach was proposed to obtain a reasonable thermodynamic description of a thermal vacancy in the metastable disordered body centered cubic (bcc_A2) phase, which had been consistently ignored in previous thermodynamic assessments. The present approach was first applied to obtain the thermodynamic descriptions for pure metastable bcc Ni and Zn, and then in the binary Ni-Zn system. The thermodynamic descriptions for both the metastable disordered bcc_A2 phase and the stable ordered bcc_B2 β phases in the Ni-Zn binary system were updated based on the corresponding experimental equilibria. With these updated thermodynamic descriptions, several drawbacks, including the multiple solutions for thermal vacancy concentrations and the artificial phase boundaries in previous assessments, can be avoided. Moreover, the calculated phase boundaries and invariant reactions related to the β phase agree well with the experimental data.

On the Formation of Spherical Metastable BCC Single Crystal Spatter Particles During Selective Laser Melting

On the Formation of Spherical Metastable BCC Single Crystal Spatter Particles During Selective Laser Melting

Hcp → ω phase transition mechanisms in shocked zirconium: A machine learning based atomic simulation study

There has been much controversy over the behavior of zirconium under shock strong enough to cause the pressure-induced hcp → ω phase transformation. Due to the short time- and length scales involved, direct measurements of the microstructure are extremely challenging. We have performed molecular dynamics simulations to investigate this issue, with Zr described by a machine-learned interatomic potential. Two different orientation relationships (ORs) between the hcp and ω phases are observed under shock driven conditions. Unlike the case with Ti that is in the same group, the ORs between the hcp and ω phases show less anisotropic phase transition sensitivity and in most cases follow the Silcock relationship with(0001)α||(12¯10)ω. Furthermore, we find that the α→ ω transformation in shocked Zr occurs via an intermediate metastable bcc structure during the loading process, whereas no such intermediate is found during the reverse ω→α transition when the shock releases.

Common mechanism for controlling polymorph selection during crystallization in supercooled metallic liquids

Despite the fundamental and technological importance, the physical scenario of polymorph selections during crystallization in metals remains poorly understood. Here, through the extensive molecular dynamics simulation, we study the crystallization pathway of face-centered cubic (fcc) metals aluminum and copper. We reveal two different crystallization pathways at ambient pressure: involving the metastable body-centered cubic (bcc) polymorph (Cu) or not involving the bcc polymorph (Al). Interestingly, by varying pressure on Al, we successfully control polymorph selection and observe crystallites containing the metastable bcc polymorph. These findings are in line with the phonon dispersion analyses, which indicate that the bcc structure of Cu at ambient pressure and of Al at high pressure remain stable whereas that of Al at low pressure is thermally unstable. More importantly, we demonstrate that it is the profile of the free energy surface related to the phase stability, rather than the strong cohesive interactions of metals, encodes the crystallization pathway in metals. Our findings not only shed important light on a common thermodynamic mechanism of polymorph selections, but also pave a new way for better controlling the crystallization pathway in industrial and metallurgical applications by regulating the stability of the intermediate metastable phases.

Free volume distribution and structural inhomogeneity in Ni50V50 amorphous alloy

The structure of Ni50V50 amorphous alloy was investigated by using neutron, X-ray diffraction and reverse Monte Carlo (RMC) modelling. The RMC configuration was decomposed into polyhedral holes whose faces are all triangles consisting of chemical bonds. Free volumes in the respective polyhedral holes were evaluated with reference to the packing efficiency of Ni-V metastable BCC phase. The tetrahedral holes, which account for about 70% of the whole space, are regarded as densely packed units because the average packing efficiency of them is approximately equal to that of the corresponding crystal phase. On the other hand, various types of polyhedral holes, which have a certain free volume, are observed and some of them are connected with each other. The densely packed coordination polyhedra consisting only of tetrahedral holes tend to be clustered.

Crystal nucleation and metastable bcc phase in charged colloids: A molecular dynamics study.

The dynamic process of homogenous nucleation in charged colloids is investigated by brute-force molecular dynamics simulation. To check if the liquid-solid transition will pass through metastable bcc, simulations are performed at the state points that definitely lie in the phase region of thermodynamically stable fcc. The simulation results confirm that, in all of these cases, the preordered precursors, acting as the seeds of nucleation, always have predominant bcc symmetry consistent with Ostwald's step rule and the Alexander-McTague mechanism. However, the polymorph selection is not straightforward because the crystal structures formed are not often determined by the symmetry of intermediate precursors but have different characters under different state points. The region of the state point where bcc crystal structures of large enough size are formed during crystallization is narrow, which gives a reasonable explanation as to why the metastable bcc phase in charged colloidal suspensions is rarely detected in macroscopic experiments.

Synthesis of TiCr2 intermetallic compound from mechanically activated starting powders via calcio-thermic co-reduction

Effect of mechanical activation of TiO2 and Cr2O3 oxides as starting materials was investigated for direct synthesis of TiCr2. Differential thermal analysis (DTA) indicated that increasing the ball milling time resulted in lower exothermic reaction temperatures between molten Ca–Cr2O3 and molten Ca–TiO2. A model-free Kissinger type method was applied to DTA data to evaluate the reaction kinetics. The results reveal that the activation energy of the exothermic reactions decreased with increasing the milling time. The structure, oxygen content, and average particle sizes of the obtained TiCr2 product were affected by the ball milling time of the starting materials. Increasing the milling time from 10 to 40 h decreased the average particle size and oxygen content of the obtained TiCr2 from 10 to 2 μm and from 1690 to 1290 ppm, respectively. The X-ray diffraction (XRD) results showed that TiCr2 compounds with metastable bcc phase can be produced using nano-sized starting materials, while only a slight amount of bcc phase can be obtained in the TiCr2 compounds, using micron-sized starting materials. The TiCr2 obtained by this method had a hydrogen absorption capability of 0.63 wt % and the kinetics of the hydrogen absorption increased for the 40 h milled sample.

Thermodynamics and Equations of State of Iron to 350\u2009GPa and 6000\u2009K

The equations of state for solid (with bcc, fcc, and hcp structures) and liquid phases of Fe were defined via simultaneous optimization of the heat capacity, bulk moduli, thermal expansion, and volume at room and higher temperatures. The calculated triple points at the phase diagram have the following parameters: bcc–fcc–hcp is located at 7.3 GPa and 820 K, bcc–fcc–liquid at 5.2 GPa and 1998 K, and fcc–hcp–liquid at 106.5 GPa and 3787 K. At conditions near the fcc–hcp–liquid triple point, the Clapeyron slope of the fcc–liquid curve is dT/dP = 12.8 K/GPa while the slope of the hcp–liquid curve is higher (dT/dP = 13.7 K/GPa). Therefore, the hcp–liquid curve overlaps the metastable fcc–liquid curve at pressures of about 160 GPa. At high-pressure conditions, the metastable bcc–hcp curve is located inside the fcc-Fe or liquid stability field. The density, adiabatic bulk modulus and P-wave velocity of liquid Fe calculated up to 328.9 GPa at adiabatic temperature conditions started from 5882 K (outer/inner core boundary) were compared to the PREM seismological model. We determined the density deficit of hcp-Fe at the inner core boundary (T = 5882 K and P = 328.9 GPa) to be 4.4%.

Ion microprobe analyses of carbon in Fe–Ni metal in iron meteorites and mesosiderites

Carbon concentrations in kamacite, taenite, and plessite (kamacite-taenite intergrowths) were measured in 18 iron meteorites and 2 mesosiderites using the Cameca ims 1280 ion microprobe at the University of Hawai‘i with a 5–7μm beam and a detection limit of <1ppm. Our goal was to investigate the effects of carbon on the microstructure of iron meteorites during cooling and to evaluate how carbon partitions between metal phases and other carbon-bearing minerals (graphite, haxonite, cohenite) in various meteorite groups. Carbon concentrations range from ∼100 to ∼1000ppm in taenite and plessite in groups IAB, IIICD, and IIIAB, which contain graphite and/or carbides, but only 2–6ppm in groups IVA, IVB and the ungrouped iron, Tishomingo, which lack graphite and carbides. Carbon contents in kamacite range from ∼2 to ∼10ppm in most studied meteorites, including IIAB, but higher abundances were found in kamacite from IAB Pitts subgroup meteorites Pitts and Woodbine (12–15ppm). Our carbon abundances for kamacite are lower than most published ion probe data, indicating that earlier carbon measurements had contamination problems.Grains of taenite and fine-grained plessite in carbon-rich meteorites, which all have normal M-shaped nickel profiles due to slow cooling, have diverse carbon contents and zoning profiles. This is because taenite decomposed by diverse mechanisms over a range of temperatures, when nickel could only diffuse over sub-μm distances. Carbon diffusion through taenite to growing carbides was rapid at the upper end of this temperature range, but was very limited at the lower end of the temperature range. In mesosiderites, carbon increases from 12ppm in tetrataenite to 40–115ppm in cloudy taenite as nickel decreases from 50% to 35%. Low carbon levels in tetrataenite may reflect ordering of iron and nickel; higher carbon in cloudy taenite is attributed to metastable bcc phase, possibly martensite, with ∼300ppm carbon intergrown with tetrataenite. Pearlitic plessite, which only forms in carbon-rich irons, contains much less carbon than martensitic plessite: 10–20ppm and 300–500, respectively, in IAB irons. Pearlitic plessite consists of μm-scale intergrowths of low-nickel kamacite and tetrataenite, which formed during cooling from ∼450 to 300°C when haxonite was forming. Martensitic plessite decomposed to tetrataenite and metastable high-nickel kamacite at temperatures below 300°C, which depended on nickel content. Carbon accumulated in untransformed taenite when haxonite growth ceased, producing M-shaped carbon profiles.Bulk carbon concentrations inferred from our ion probe data are 3–4ppm in IVA, IVB, and Tishomingo, which has IVB-like depletions of moderately volatile siderophiles. Published bulk carbon contents of IVA and IVB irons are >10 times higher suggesting contamination problems. Our ion probe analyses and observations of carbide and graphite show that bulk carbon decreases with decreasing germanium and other moderately volatile elements from group IAB, through IIAB and IIIAB, to group IVA and IVB. These trends may have been inherited from fractionated chondritic precursors, or may have been produced by impacts that caused volatile loss, separation of mantle from core material, and relatively rapid cooling of irons poor in volatiles and carbon.

Polymorph selection and nucleation pathway in the crystallization of Hertzian spheres.

The crystallization process of Hertzian spheres is studied by means of molecular dynamics simulations in an NPT ensemble where the total number of particles N, the pressure P, and the temperature T are kept constant. It has been observed that the bond orientational ordering rather than the translational ordering (density) plays a primary role. The crystal polymorphs are determined by the state points. Under the conditions of small supercooling, the system is likely to be nucleated into crystals that have a preference for the metastable bcc structure, which can be regarded as a manifestation of the Alexander-McTague mechanism. In contrast, small nuclei are found to have a preference for fcc symmetry under conditions of a high degree of supercooling. Prestructured precursors that act as seeds and wet on the nuclei during nucleation always have a high degree of bcc-like ordering, despite different state points. The results above may provide a clue to the understanding of the crystallization process in core-softened particles.

Microstructural dependence of magnetic and magnetostrictive properties in Fe–19 at% Ga

A combination study of magnetic and magnetostrictive properties in directionally cast (DC) (un-annealed) and differently heat-treated Fe–19 at% Ga samples was carried out at room temperature. Slow cooling leads to an increase in the occupation of [2 0 0] easy magnetic axis. However, structural ordering of Ga atoms into a metastable D03 phase decreases the saturation magnetostriction (λs) and saturation magnetization (Ms), while increases coercivity (Hc). Water quenching suppresses the formation of stable fcc ordered phase (L12) and largely preserves disordered bcc single-phase (A2) structure down to room temperature, leading to enhanced magnetostriction and magnetization. Slow cooling promotes the ordering of metastable bcc ordered phase (D03). Magnetic force microscopy (MFM) study exhibits that the water-quenched (WQ) sample consists of a well-aligned stripe domain structure, while irregular maze-like domain structure is observed in furnace-cooled (FC) sample. The results confirm that in addition to an inhibitory effect of D03 ordering on magnetic domain wall motions, irregular magnetic domains also contribute to decrease in magnetic and magnetostrictive properties of FC sample.

Electronic structure of metastable bcc Cu–Cr alloy thin films: Comparison of electron energy-loss spectroscopy and first-principles calculations

Metastable Cu–Cr alloy thin films with nominal thickness of 300nm and composition of Cu67Cr33 (at%) are obtained by co-evaporation using molecular beam epitaxy. The microstructure, chemical phase separation and electronic structure are investigated by transmission electron microscopy (TEM). The thin film adopts the body-centered cubic crystal structure and consists of columnar grains with ~50nm diameter. Aberration-corrected scanning TEM in combination with energy dispersive X-ray spectroscopy confirms compositional fluctuations within the grains. Cu- and Cr-rich domains with composition of Cu85Cr15 (at%) and Cu42Cr58 (at%) and domain size of 1–5nm are observed. The alignment of the interface between the Cu- and Cr-rich domains shows a preference for {110}-type habit plane. The electronic structure of the Cu–Cr thin films is investigated by electron energy loss spectroscopy (EELS) and is contrasted to an fcc-Cu reference sample. The experimental EEL spectra are compared to spectra computed by density functional theory. The main differences between bcc-and fcc-Cu are related to differences in van Hove singularities in the electron density of states. In Cu–Cr solid solutions with bcc crystal structure a single peak after the L3-edge, corresponding to a van Hove singularity at the N-point of the first Brillouin zone is observed. Spectra computed for pure bcc-Cu and random Cu–Cr solid solutions with 10at% Cr confirm the experimental observations. The calculated spectrum for a perfect Cu50Cr50 (at%) random structure shows a shift in the van Hove singularity towards higher energy by developing a Cu–Cr d-band that lies between the delocalized d-bands of Cu and Cr.

Thermal expansions and mechanical properties of electrodeposited Fe–Ni alloys in the Invar composition range

Electrodeposited Invar Fe–Ni alloys with 36 to 40 mass% Ni were prepared from plating baths containing saccharin as a stress reducer and containing various Fe2+ concentrations. The Invar Fe–Ni alloys contained of small amount of S (∼0.02 mass%). The coefficients of thermal expansion (CTEs) of the as-deposited Invar Fe–Ni alloys were approximately 9 to 11ppm/°C and were larger than those of pyrometallurgically produced Invar alloys. When the alloys were heat-treated at 400 to 500°C, their CTEs drastically decreased to approximately 5ppm/°C. Furthermore, upon heat treatment at 600°C, the CTEs reached approximately 2 to 4ppm/°C depending on alloy composition; these CTEs are comparable with those of pyrometallurgically produced alloys. The as-deposited Invar Fe–Ni alloys were mainly composed of metastable bcc phases, resulting in larger CTEs. When the alloys were annealed at 400°C or above, the equilibrium fcc phases became the predominant phases, accompanied by a drastic decrease of the CTEs. The bcc-to-fcc transformation led to a decrease of the CTEs and to thermal contractions. Upon the heat treatment, an S (sulfide) at bcc grain boundaries segregated not as a thin film but as a granular sulfide form at primary bcc grain boundaries in the electrodeposited Invar alloys. Upon heat treatment at 500°C or above, bcc grain eliminated accompanied by fcc grain growth and the granular sulfide agglutinated further. Consequently, the agglutinating granular sulfide was entrapped in the matrix grains or grain-boundary triple-points of transformed fcc grains. In addition, it is considered that these two-type morphologies of the sulfide, as a thin film at grain boundaries or as a precipitate, will be determined by grain growth form during an annealing. Ductile behavior of the electrodeposited Invar Fe–Ni alloys was confirmed, irrespective of whether the alloys were heat-treated. Upon heat treatment at 400 to 500°C, the Invar Fe–Ni alloys exhibited high strength with good ductility, consistent with their low CTE. After the heat treatment, no severe embrittlement of the electrodeposited Invar alloys was observed despite the codeposition of S because the S existed as a granular sulfide, thereby preventing grain-boundary embrittlement.

Metastable phases in Zr-Excel alloy and their stability under heavy ion (Kr2+) irradiation

Zr-Excel alloy (Zr-3.5Sn-0.8Nb-0.8Mo, wt.%) has been proposed as a candidate material of pressure tubes in the CANDU-SCWR design. It is a dual-phase alloy containing primary hcp α-Zr and metastable bcc β-Zr. Metastable hexagonal ω-Zr phase could form in β-Zr as a result of aging during the processing of the tube. A synchrotron X-ray study was employed to study the lattice properties of the metastable phases in as-received Zr-Excel pressure tube material. In situ heavy ion (1 MeV Kr2+) irradiations were carried out at 200 °C and 450 °C to emulate the stability of the metastable phase under a reactor environment. Quantitative Chemi-STEM EDS analysis was conducted on both un-irradiated and irradiated samples to investigate alloying element redistribution induced by heavy ion irradiation. It was found that no decomposition of β-Zr was observed under irradiation at both 200 °C and 450 °C. However, ω-Zr particles experienced shape changes and shrinkage associated with enrichment of Fe at the β/ω interface during 200 °C irradiation but not at 450 °C. There is a noticeable increase in the level of Fe in the α matrix after irradiation at both 200 °C and 450 °C. The concentrations of Nb, Mo and Fe are increased in the ω phase but decreased in the β phase at 200 °C. The stability of metastable phases under heavy ion irradiation associated with elemental redistribution is discussed.

Polymorph selection in the crystallization of hard-core Yukawa system

Colloid-colloid interactions in charge-stabilized dispersions can to some extent be represented by the hard-core Yukawa model. The crystallization process and polymorph selection of hard-core Yukawa model are studied by means of smart Monte Carlo simulations in the region of face-centered-cubic (fcc) phase. The contact value of hard-core Yukawa potential and the volume fraction of the colloids are fixed, while the Debye screening length can be varied. In the early stage of the crystallization, the precursors with relatively ordered liquid structure have been observed. Although the crystal structure of thermodynamically stable phase is fcc, the system crystallizes into a mixture of fcc and hexagonal close-packed (hcp) structures under small Debye screening length since the colloidal particles act as effective hard spheres. In the intermediate range of Debye screening length, the system crystallizes into a mixture of fcc, hcp, and body-centered-cubic (bcc). The existence of metastable hcp and bcc structures can be interpreted as a manifestation of the Ostwald's step rule. Until the Debye screening length is large enough, the crystal structure obtained is almost a complete fcc suggesting the system eventually reaches to a thermodynamically stable state.

Ageing characteristics of the metastable gamma phase in U–9 wt.% Mo alloy: experimental observations and thermodynamic validation

Ageing characteristics of the metastable bcc γ-phase in U–9 wt.% Mo alloy, a candidate for high uranium density nuclear fuel for research and test reactors, have been investigated in this study. Analyses of the aged microstructures, employing X-ray diffraction and various microscopy techniques, revealed the decomposition mechanism of the metastable γ-phase to the stable α-U and γ′ (U2Mo) phases. A discontinuous precipitation reaction, leading to the generation of partially transformed cellular colonies with lamellae comprising of either the α-phase or the γ′-phase in γ-phase matrix, was found to be operative. The in situ transformation of γ interlamellar regions to the γ′-phase was noticed occasionally within the γ + α lamellar colonies. Thermodynamic analysis of the U–Mo system, using free energy–composition diagrams, could associate the observed attributes of the decomposition of γ-phase to the extent of Mo segregation and the chemical driving force required for the nucleation of α- and γ′-phases in the γ-matrix.

Metastable bcc phase formation in 3d ferromagnetic transition metal thin films sputter-deposited on GaAs(100) substrates

Co100−xFex and Ni100−yFey (at. %, x = 0–30, y = 0–60) films of 10 nm thickness are prepared on GaAs(100) substrates at room temperature by using a radio-frequency magnetron sputtering system. The detailed growth behavior is investigated by in-situ reflection high-energy electron diffraction. (100)-oriented Co and Ni single-crystals with metastable bcc structure are formed in the early stage of film growth, where the metastable structure is stabilized through hetero-epitaxial growth. With increasing the thickness up to 2 nm, the Co and the Ni films start to transform into more stable hcp and fcc structures through atomic displacements parallel to bcc{110} slide planes, respectively. The stability of bcc phase is improved by adding a small volume of Fe atoms into a Co film. The critical thickness of bcc phase formation is thicker than 10 nm for Co100−xFex films with x ≥ 10. On the contrary, the stability of bcc phase for Ni-Fe system is less than that for Co-Fe system. The critical thicknesses for Ni100−yFey films with y = 20, 40, and 60 are 1, 3, and 5 nm, respectively. The Co100−xFex single-crystal films with metastable bcc structure formed on GaAs(100) substrates show in-plane uniaxial magnetic anisotropies with the easy direction along GaAs[011], similar to the case of Fe film epitaxially grown on GaAs(100) substrate. A Co100−xFex film with higher Fe content shows a higher saturation magnetization and a lower coercivity.

Interfacial mixing of nickel vanadium multilayers induced by cold rolling

During the cold rolling of multilayers of Ni and V with an average composition Ni70V30, Ni30V70 and Ni57V43, deformation induces phase transformation and an interfacial mixing. After repeated cold rolling at room temperature, X-ray diffraction (XRD) and scanning transmission electron microscope (STEM) results demonstrate that between pure Ni and V layers a metastable fcc solid solution phase forms in Ni70V30, a metastable bcc solid solution phase forms in Ni30V70 and metastable fcc and bcc solid solution phases form in Ni57V43 with suppression of nucleation of intermetallic phases. At an intermediate stage of the reaction when both pure Ni and V still remain, a smoothly varying composition profile was determined from the electron energy loss spectrum (EELS) across the multilayers. The composition dependent effective interdiffusion coefficient that was calculated based on the obtained profile was found to increase monotonically from 1.2×10−15cm2/s to 1.6×10−13cm2/s with increase of Ni mole fraction. Compared to the stored energy due to dislocation and interfaces, the excess chemical free energy from the interfacial mixing is the largest portion of total stored energy from deformation, which represents a form of mechanochemical transduction.