In situ observation of Si faceted dendrite growth from low-degree-of-undercooling melts

We directly observed the transition of crystal growth behavior of Si in a low undercooling region. We succeeded in observing the initiation of faceted dendrite growth from a portion of parallel twins with increasing degrees of undercooling. The critical undercooling for growing a faceted dendrite was experimentally determined to be ΔT = 10 K. We also confirmed that parallel twins associated with faceted dendrite growth were formed between grain boundaries and not at grain boundaries during melt growth. The parallel-twin formation was explained in terms of a model of twin formation on the {1 1 1} facet plane at the growth interface.     

Crystallization behaviours around the glass transition temperature in an amorphous Fe–Nb–B alloy

Crystallization behaviours around the glass transition temperature in an amorphous Fe₇₀Nb₁₀B₂₀ alloy were investigated by means of transmission electron microscopy. Dense bcc-Fe nanocrystals initially appeared as the primary phase, followed by the dense formation of the (Fe,Nb)₂₃B₆ nanocrystalline phase. The bcc-Fe nanocrystals were formed even by annealing at a temperature that is 38 K lower than the glass transition temperature. A difference of the low temperature behaviours between the present conventional amorphous alloy and a bulk metallic glass was discussed.     

Solidification kinetics and phase formation of undercooled eutectic Ni–Nb melts

The non-equilibrium solidification behaviour of undercooled eutectic Ni₈₄Nb₁₆ and Ni_59.5Nb_40.5 melts has been analysed by in situ observation of recalescence events during electromagnetic levitation of undercooled melts. Levitated drops of controlled undercooling were quenched onto chill substrates and subjected to phase and microstructure analysis. For Ni₈₄Nb₁₆ a maximum melt undercooling of 276 K has been achieved. A transition from coupled eutectic to primary supersaturated α-Ni dendrite growth was revealed beyond a critical undercooling of 30 K. Beyond 110 K the primary Ni₃Nb phase occurred for substrate quenching. The undercooling of Ni_59.5Nb_40.5 melt was limited to 135 K. Bulk amorphous samples up to 2 mm thick have been prepared by quenching of undercooled Ni_59.5Nb_40.5 melts. On slow cooling the metastable phases decompose into an anomalous eutectic microstructure.     

Effect of denucleating glass composition on undercooling of Fe83Ga17 alloy melts

Adopting glass fluxing combined with superheating cycling method, the undercooling and its stability of Fe₈₃Ga₁₇ alloy melts were investigated using different kinds of denucleating glass: B₂O₃, 90% NaSiCa + 10% B₂O₃ (simplified as Na–Si–Ca–Al–B) and 70% Na–Si–Ca–Al–B + 30% Na₂B₇O₄. The results showed that different glass has different denucleating mechanism. The purification of B₂O₃ glass is only a physical process, by which the stable bulk undercooling cannot be obtained during superheating–cooling cycles. While taking Na–Si–Ca–Al–B glass as purifying agent, its denucleating mechanism is a comprehensively physicochemical process. But the stability of undercooling is still undesirable because of the separation between melt and glass during cooling process in superheating cycling. A stable bulk undercooling can be obtained by physicochemical denucleating process in the case of 70% Na–Si–Ca–Al–B + 30% Na₂B₇O₄ molten glass owing to its suitable viscosity.     

Structural and electronic properties of Si nanocrystals embedded in amorphous SiC matrix

Si-rich hydrogenated amorphous silicon carbide thin films were prepared by plasma-enhanced chemical vapor deposition technique. As-deposited films were subsequently annealed at 900 °C and 1000 °C to form Si nanocrystals embedded in amorphous SiC matrix. Raman spectra demonstrate the formation of Si nanocrystals with size around 7–9 nm. For the sample annealed at 1000 °C, the crystallinity can be reached to 70%. As increasing the annealing temperature, the dark conductivity is increased accompanying with the increase of crystallinity of the film. The dark conductivity reaches to 1.2 × 10^?6 S cm^?1 for the sample annealed at 1000 °C, which is 4 orders of magnitude higher than that of as-deposited film. It is found that the carrier transport process is dominated by the thermally activated transport process according to the temperature-dependent conductivity results.     

Modeling dendrite growth in undercooled concentrated multi-component alloys

Most theoretical work on dendrite growth has focused on dilute binary alloys, while most industrial alloys are concentrated multi-component systems. By incorporating the local non-equilibrium effects both at the interface and in the bulk liquid, the thermodynamic database and diffusional interaction, a model was developed for dendrite growth in undercooled concentrated multi-component alloys. An experimental study of dendrite growth in undercooled Ni–18 at.% Cu–18 at.% Co melts was carried out and the measured interface velocities (V) were well predicted by the present model over the whole undercooling range (ΔT = 30–313 K). During dendrite growth the partition coefficients change non-monotonically due to interaction between the species and changes in the dendrite tip radius. Interaction between the species also leads to a lower interface velocity and larger ΔT and V as the ΔT–V relation plateaus. The previous definition of constitutional undercooling, i.e. the sum of the contributions of each solute, is not applicable to concentrated multi-component alloys. The controlling mechanisms during dendrite growth are discussed with respect to the results of the calculations.     

The origin of anomalous eutectic structures in undercooled Ag–Cu alloy

A melt encasement (fluxing) method was used to undercool Ag–Cu alloy at its eutectic composition. The recalescence of the undercooled alloy was filmed at a high frame rate. For undercoolings <60 K, a microstructure consisting of mixed anomalous and lamellar eutectic is observed. Analysis of eutectic spacing in the lamellar eutectic reveals little dependence upon the undercooling of the bulk melt and is consistent with growth at an undercooling of 1.5 K. Depending upon undercooling, the progress of the recalescence front may be either continuous or spasmodic, wherein periods of rapid growth are separated by significant interludes in which growth totally arrests. Analysis of spot brightness profiles reveals that, during continuous growth, the recalescence is characteristic of the advancement of a planar, space-filling front, while a double recalescence occurs during spasmodic growth, the first of which is characteristic of the propagation of a dendritic, or non-space-filling, front. It is concluded that, during spasmodic growth, the propagation of two-phase, or eutectic, dendrites is observed, which subsequently remelt to form the anomalous eutectic, while the lamellar eutectic grows during post-recalescence cooling.     

Criteria for tensile plasticity in Cu–Zr–Al bulk metallic glasses

(Cu_0.5Zr_0.5)_100?xAl_x (x = 5, 6, 8) bulk metallic glasses (BMGs) were deformed in tension. Besides ductility up to 0.5%, the material shows work-hardening behaviour. Both effects are attributed to the deformation-induced precipitation of B2 CuZr nanocrystals and the formation of twins in the nanocrystals larger than 20 nm. The precipitation of the nanocrystals alters the stress field in the matrix and is expected to retard shear band propagation, which in turn allows stresses in the nanocrystals to rise. This stress build-up is more severe in the larger grains and might be responsible for the subsequent twinning. Both deformation-induced nanocrystallization and twinning consume energy and avoid crack formation and with it premature failure.     

Nanocrystalline–amorphous transitions in Al–Mo thin films: Bulk and surface evolution

We investigate the bulk and surface features of the crystalline–amorphous transitions in binary Al–Mo alloy thin films as a function of Mo composition using transmission electron microscopy, X-ray diffraction and atomic force microscopy analysis, as well as thermodynamic modeling. Of the alloys tested, the minimum in the root mean square (rms) surface roughness and correlation length occurs at the Al–32 at.% Mo composition, which corresponds to the maximum volume fraction of the amorphous phase and the minimum volume fraction of the body centered cubic nanocrystallites. The rms surface roughness of the 32 at.% Mo films is on the order of a single nanometer, compared with nearly 80 nm for the 50 at.% Mo film. A structure–zone map is constructed to relate the surface morphology of the films to their bulk microstructure. A thermodynamic model developed by Miedema and coworkers was used to predict the general trends observed in the microstructural evolution as a function of film composition.     

The evolution of the microstructure in amorphous Al85Ce8Ni5Co2 alloy during heat treatment and severe plastic deformation: A comparative study

The effect of three different devitrification routes, i.e. isothermal heat treatment, high pressure torsion and ball milling of amorphous Al₈₅Ce₈Ni₅Co₂ alloy have been studied systematically. The phase selection during the different crystallization sequences is explained by different thermodynamic barriers and growth rates. Low temperature heat treatment results in the formation of α-Al and a metastable phase (φ). Although, the nucleation of Al₁₁Ce₃ phase is also likely in this temperature range, the lack of its growth is explained by kinetic considerations. Ball milling results in the formation of the stable phase mixture (α-Al and Al₁₁Ce₃). This process can be considered as a high temperature heat treatment at intermediate pressures (<2 GPa). High pressure torsion at the applied pressure of 6 GPa results in the primary formation of α-Al nanocrystals.     

Relating residual stress and microstructure to mechanical and giant magneto-impedance properties in cold-drawn Co-based amorphous microwires

The effect of cold-drawing on the tensile property and giant magneto-impedance (GMI) effect of melt-extracted Co-based amorphous microwires was evaluated through detailed analyses of the distribution of residual stress and microstructural evolution. The tensile ductility and tensile strength increased gradually with cross-sectional area reduction ratio (R) until 51%, and decreased with further deformation. The microwire with R = 51% exhibits the highest tensile ductility of 1.09% and tensile strength of 4320 MPa. Structural and thermodynamic analyses reveal that it is the mechanical deformation rather than thermal activation that induces the precipitation of nanocrystals and arrests the quick extension of shear bands leading to the enhanced ductility. Interestingly, the GMI effect also attains the maximum value of 160% at 10 MHz when R = 51% (30% larger than that of the as-cast wires), before decreasing with further cold-drawing. Such an identical evolution trend of both tensile and GMI properties can be ascribed to two underlying mechanisms: the generation of longitudinal and circumferential residual stresses and the growth of deformation-induced nanocrystals during cold-drawing. The role of residual stress is established herein not only as a trigger to accelerate the amorphous-to-nanocrystalline phase transformation but also as a decisive contributor to the mechanical and GMI performance. The unique simultaneous improvement of both mechanical and GMI properties of cold-drawn Co-based microwires opens up new possibilities for a variety of engineering applications, such as high-performance magnetic, stress and biological sensors.     

Stability of Al-rich glasses in the Al–La–Ni system

The stability of Al-rich glasses in the Al–La–Ni ternary system has been measured. Glasses with critical thicknesses ranging from 270 to 780 μm have been achieved. An in-situ two-phase amorphous region containing as much as ∼10 vol.% of nanocrystals was also observed with critical thicknesses ranging from 420 to 950 μm. Measurements of T_g, T_x and T_ℓ, as well as other empirical measures of thermal stability based on these quantities, confirm the unusual thermal stability of these glasses. These results suggest that bulk Al-based glasses with a maximum critical thickness exceeding 1 mm may be possible in alloys based on this system.     

Phase selection and microstructure formation in undercooled Co–61.8 at.% Si melts under various containerless processing conditions

The hypoeutectic composition Co–61.8 at.% Si was undercooled and solidified using electromagnetic levitation, electromagnetic levitation under a static magnetic field, electrostatic levitation and glass-fluxing. The samples generally showed two thermal events, either separated or continuous depending on undercooling. In situ monitoring of the two thermal events with a high-speed camera revealed a sudden decrease of dendritic growth velocities of primary phases at a critical undercooling of 88 K. Scanning electron microscopy studies of the solidified samples showed that the CoSi compound and the CoSi₂ compound nucleate as the primary phase for low and high undercoolings, respectively. The microstructure of the samples depends not only on undercooling, but also on the onset temperature or delay time of the second thermal event. Melt convection has no effect on the primary phase selection in undercooled melts, but it has a significant effect on the delay time and therefore on microstructure formation of the samples for high undercoolings.     

Zr–(Cu,Ag)–Al bulk metallic glasses

In this paper, we report the formation of a series Zr–(Cu,Ag)–Al bulk metallic glasses (BMGs) with diameters at least 20 mm and demonstrate the formation of about 25 g amorphous metallic ingots in a wide Zr–(Cu,Ag)–Al composition range using a conventional arc-melting machine. The origin of high glass-forming ability (GFA) of the Zr–(Cu,Ag)–Al alloy system has been investigated from the structural, thermodynamic and kinetic points of view. The high GFA of the Zr–(Cu,Ag)–Al system is attributed to denser local atomic packing and the smaller difference in Gibbs free energy between amorphous and crystalline phases. The thermal, mechanical and corrosion properties, as well as elastic constants for the newly developed Zr–(Cu,Ag)–Al BMGs, are also presented. These newly developed Ni-free Zr–(Cu,Ag)–Al BMGs exhibit excellent combined properties: strong GFA, high strength, high compressive plasticity, cheap and non-toxic raw materials and biocompatible property, as compared with other BMGs, leading to their potential industrial applications.     

Sequential growths of AlN and GaN layers on as-polished 6H–SiC(0001) substrates

Microstructures of surfaces and defects generated during initial and subsequent growths via metalorganic vapor-phase epitaxy of AlN(0001) films on 6H–SiC(0001) substrates and GaN(0001) films on AlN/SiC(0001) substrates have been investigated using atomic force microscopy and cross-sectional and plan-view transmission electron microscopy. Scratches present on the SiC surfaces did not appear to bias the nucleation of AlN. The lateral growth rate of AlN was greater than the vertical growth rate, leading to almost planar layers at 15 and 100 nm thicknesses. Partially coalesced islands were observed after nominally ～15 nm of growth. Increasing the thickness to 100 nm resulted in complete island coalescence, formation of undulating films from the polishing scratches in the SiC substrate, a surface microstructure containing steps, terraces and small pits, and a reduced dislocation density relative to the 15 nm layers. The AlN/SiC interfaces contained steps and complex dislocation networks. GaN islands nucleated and grew on the AlN films. Complete coalescence of these islands occurred at thicknesses less than 100 nm. Dislocation density in the GaN films was reduced by increasing the thickness of either the AlN and or the GaN. Arguments are developed to account for these observations.     

Metastable phase formation in Ti–Al–Nb undercooled melts

Metastable phase formation processes in ternary Ti–Al–Nb alloys were studied by containerless electromagnetic levitation for melt undercooling up to 300 K below the liquidus temperature. Dendrite growth velocities of 15–25 m s⁻¹ for highly undercooled Ti–Al–Nb melts were consistent with primary β-phase formation, which is promoted by Nb addition. From double-recalescence events in Ti₄₀Al₅₀Nb₁₀ and Ti₄₅Al₅₀Nb₅ melts beyond a critical undercooling a subsequent β to α phase transformation in the semi-solid state was inferred. A second recalescence near 1300 °C, which was attributed to an α to γ solid state transformation, was observed in the pyrometer trace for the Ti₄₅Al₅₀Nb₅ and Ti₄₀Al₅₀Nb₁₀ alloys. The γ phase formation was suppressed in favour of a homogeneous α₂ phase in undercooled Ti₄₅Al₄₅Nb₁₀ samples quenched onto a chill substrate, whereas in Ti₄₀Al₅₀Nb₁₀ high undercooling enabled a direct γ phase solidification.     

The characterization of structure,thermal stability and magnetic properties of Fe–Co–B–Si–Nb bulk amorphous and nanocrystalline alloys

Recently bulk amorphous alloys have attracted great attention due to their excellent magnetic properties. The glass-forming ability of bulk amorphous alloys depends on the temperature difference (ΔT_x) between glass transition temperature (T_g) and crystallization temperature (T_x). The increase of ΔT_x causes a decrease of the critical cooling rate (V_c) and growth of the maximum casting thickness of bulk amorphous alloys. The aim of the present paper is to characterize the structure, the thermal stability and magnetic properties of Fe₃₆Co₃₆B₁₉Si₅Nb₄ bulk amorphous alloys using XRD, Mössbauer spectroscopy, DSC and VSM methods. Additionally the magnetic permeability μ_i (at force H ≈ 0.5 A/m and frequency f ≈ 1 kHz) and the intensity of disaccommodation of magnetic permeability Δμ/μ(t₁) (Δμ = μ(t₁ = 30 s) ? μ(t₂ = 1800 s)), have been measured, where μ is the initial magnetic permeability measured at time t after demagnetisation, the Curie temperature T_C and coercive force H_c of rods are also determined with the use of a magnetic balance and coercivemeter, respectively.Fe–Co–B–Si–Nb bulk amorphous alloys were produced by pressure die casting with the maximum diameters of 1 mm, 2 mm and 3 mm.The glass transition temperature (T_g) of studied amorphous alloys increases from 807 K for a rod with a diameter of 1 mm to 811 K concerning a sample with a diameter of 3 mm. The crystallization temperature (T_x) has the value of 838 K and 839 K for rods with the diameters of 1 mm and 3 mm, respectively. The supercooled liquid region (ΔT_x = T_x ? T_g) has the value of about 30 K. These values are presumed to be the origin for the achievement of a good glass-forming ability of the Fe–Co–B–Si–Nb bulk amorphous alloy. The investigated amorphous alloys in the form of rods have good soft magnetic properties (e.g. M_s = 1.18–1.24 T). The changes of crystallization temperatures and magnetic properties as a function of the diameter of the rods (time of solidification) have been stated.     

Heterogeneous nucleation on potent spherical substrates during solidification

For a spherical-cap nucleus to become a “transformation nucleus”, the linear dimension (d) of the flat substrate must exceed the critical nucleus size (2r¹). This Turnbull criterion (d ⩾ 2r¹) defines a minimum undercooling for grain formation on, and effective inoculation with, flat nucleating substrates. However, for nucleation on potent substrates the spherical-cap model is no longer tenable. The free growth model has in general considered the growth of a two-dimensional nucleus on a potent flat substrate. Inspired by the particle-core structures observed in magnesium alloys after inoculation with nearly spherical zirconium particles, a model has been proposed, on the basis of an adsorption and surface diffusion mechanism, for heterogeneous nucleation and grain formation on potent spherical substrates of d ⩾ 2r¹. The critical undercooling required is found to be approximately the same as that defined by Turnbull’s patch nucleation theory. The model shows excellent agreement with experiments compared from different perspectives.     

Fe- and Co-based bulk glassy alloys with ultrahigh strength of over 4000 MPa

Since the first synthesis of Fe-based bulk glassy alloys in Fe–(Al,Ga)–(P,C,B,Si) system in 1995, a number of Fe- and Co-based bulk glassy alloys have been developed up to date because their alloys are expected to exhibit high mechanical strength and good soft magnetic properties. The maximum diameter of Fe- and Co-based bulk glassy alloys exhibiting high fracture strength of over 4000 MPa is 5 mm for Fe–Co–B–Si–Nb system and 3 mm for Co–Fe–Ta–B–Mo–Si system. The addition of a small amount of Nb or Ta is essential for the increase in their glass-forming ability through the formation of network-like atomic configurations. The primary crystalline phase from supercooled liquid is a metastable complex FCC Fe₂₃B₆ or Co₂₃B₆ phase and hence the change to the local atomic configurations leading to the precipitation of the metastable Fe₂₃B₆ or Co₂₃B₆-type phase is thought to play an important role in the achievement of high glass-forming ability. The highest fracture strength reached as high as 4250 MPa for Fe–Co–B–Si–Nb alloy and 5545 MPa for Co–Fe–Ta–B–Mo alloy. The fracture strength has a good linear relation with Young's modulus, glass transition temperature or liquidus temperature. It is, therefore, concluded that the origin for the ultrahigh strength is attributed to the strong bonding nature among the constituent elements. Considering that Fe–Si–B amorphous alloy wires developed for several years between 1979 and 1983 have been used as high strength materials for the last two decades, the newly developed high-strength Fe- and Co-based bulk glassy alloys are expected to be used as a new type of ultrahigh strength material by utilizing the advantage points of much higher strength and three-dimensional material form.     

Growth of nanocomposite films: From dynamic roughening to dynamic smoothening

This paper reports several new findings on the breakdown of dynamic roughening in thin film growth. With increasing energy flux of concurrent ion impingement during pulsed DC sputtering, a transition from dynamic roughening to dynamic smoothening is observed in the growth behavior of TiC/a-C nanocomposite films. The nanocomposite films show a negative growth exponent and ultra-smoothness (RMS roughness ～0.2 nm at a film thickness of 1.5 μm). Based on high-resolution cross-sectional transmission electron microscopy observations we conclude that during growth an amorphous front layer of 2 nm covers the nanocomposite film and suppresses the influence of nanocrystallites on the roughness evolution of the nanocomposite films. We were able to predict the evolution of surface roughness based on a linear equation of surface growth which contains two diffusivity parameters that control the atomic mobility along the growing outer surface. The model is in good agreement with atomic force microscopy measurements of roughness evolution.