Fe Dendrites Research Articles

Failure Investigation of LiFePO4 Cells under Overcharge Conditions

The failure mechanism of LiFePO4 cells during overcharge conditions has been systematically studied using commercial A123 18650 cells at a 1C rate and different conditions – from 5% to 20% overcharge (SOC = 105% to 120%). SEM/EDX, high-energy synchrotron XRD (HESXRD), and cyclic voltammetry (CV) were used to characterize the morphology, structure, and electrode potentials of cell components both in situ and ex situ. The failure behaviors for A123 18650 cells experiencing different degrees of overcharges were found to be similar, and the 10% overcharge process was analyzed as the representative example. The Fe redox potentials in the 1.2 M LiPF6 EC/EMC electrolyte were measured during the overcharge/discharge process using CV, proving that Fe oxidation and reduction in the cell during the overcharge/discharge cycle is theoretically possible. A possible failure mechanism is proposed: during the overcharging process, metallic Fe oxidized first to Fe2+, then to Fe3+ cations; next, these Fe2+ and Fe3+ cations diffused to the anode side from the cathode side; and finally, these Fe3+ cations reduced first to Fe2+ cations, and then reduced further, back to metallic Fe. During overcharge/discharge cycling, Fe dendrites continued growing from both the anode and the cathode sides simultaneously, penetrating through the separator and forming an iron bridge between the anode and cathode. The iron bridge caused micro-shorting and eventually led to the failure of the cell. During the overcharge/discharge cycles, the continued cell temperature increase at the end of overcharge is evidence of the micro-shorting.

In situ martensitic phase reinforced Fe–Nb–Ni–Mn ultrafine composite with enhanced mechanical properties

Abstract An in situ formed martensitic dendrite reinforced Fe–11Nb–10Ni–5Mn ultrafine composite was developed by copper mold casting. This new class of ultrafine composite composed of micrometer-scale martensitic α′-Fe dendrites and ultrafine-scale alternating eutectics exhibits a high fracture strength of 1.6 GPa and a large compressive plasticity up to 10.5%. These excellent mechanical properties originate from the precipitation of the martensitic α′-Fe dendrites containing numerous nanometer-scale laths which are effective to generate multiple shear banding and restrict the rapid propagation of localized shear bands.

Effect of Ce Addition on the Microstructure and Properties of Cu-10Fe-3Ag<i> In Situ</i> Composite

Cu-10Fe-3Ag in situ composites containing (0–0.30%) Ce elements were prepared by cold drawing and intermediate heat treatments. Microstructure was observed, and mechanical properties and electrical conductivity were measured for alloys at various drawing strain ratio. Adding Ce element could reduce the size of primary Fe and Cu dendrites of Cu-10Fe-3Ag. Ultimate tensile strength increased but electrical conductivity decreased with the increase of drawing strain. Ce additions in Cu-10Fe-3Ag slightly increased the strength at low strain and effectively improved the conductivity at high strain. Both strain hardening rate and conductivity loss of Cu-10Fe-3Ag containing Ce were reduced at lower strain than Cu-10Fe-3Ag.

Effect of homogenization treatment on microstructure and properties for Cu–Fe–Ag in situ composites

Abstract The effects of homogenization treatment on the microstructure and properties of Cu–8Fe–0.1Ag and Cu–8Fe–0.5Ag (wt.%, similarly hereinafter) in situ composites were investigated. The composites were prepared by inductive melting, casting, homogenization treatment and subsequent cold work. Microstructures of the composites were observed and the tensile strength plus electrical conductivity was measured respectively. The results show that the prior homogenization treatment results in refinement of the primary Fe dendrites in Cu matrix and promotes the precipitation of the secondary Fe particles from the Cu matrix, which leads to increase in the strength and conductivity of the composites. During the following cold deformation, the homogenized alloys are readily elongated into fine and continuous filamentary composites compared with the as-cast, leading to high strength due to the increased interface density.

Influences of different prior heat treatments on the microstructural and mechanical properties of Cu–Fe filamentary composites

Cu–6 wt.% Fe and Cu–12 wt.% Fe filamentary composites were prepared by casting and cold drawing. And a different heat treatment of quenching and aging or homogenizing was introduced before cold drawing process, respectively. The microstructure was observed and the tensile strength measured for the composites at different drawing strains. The quenching and aging or homogenizing prior to drawing deformation refine the as-cast microstructure and result in the increase in interface density in the drawn microstructure. The drawn alloys with the homogenizing treatment show smaller filament spacing than those with the quenching and aging treatment because homogenizing results in smaller and more dispersive primary Fe dendrites before drawing deformation. The heat treatments can improve the strength of the composites by increasing precipitation strengthening and interface strengthening levels. With the reduction in filament spacing during drawing deformation, the strength of the alloys with smaller initial size of Fe dendrites increases more obviously.

The effectiveness of Equal Channel Angular Pressing and rod rolling for refining microstructures and obtaining high strength in a Cu–Fe composite

A directionally cast Cu–Fe composite has been deformation processed both by Equal Channel Angular Pressing (ECAP) followed by rod rolling and by rod rolling alone, and the microstructural evolution and resulting mechanical behaviour examined. Material processed by rod rolling alone shows higher strength and slightly better ductility. Matrix microstructure is characterised by elongated dislocation cells/subgrains, which are finer with higher boundary misorientations after rod rolling alone. The initial Fe dendrites transform slowly to thin ribbons with a narrower range of crystal orientations after rolling alone than after preliminary ECAP. Strengthening here is principally due to matrix microstructural refinement, with contributions due to dislocations in cell boundaries and within the cells and due to grain/subgrain boundaries. Fe in solid solution as well as in the form of precipitate particles leads to additional matrix hardening, with the coarse precipitates producing locally harder regions of finer microstructures due to deformation incompatibility with the matrix. The use of ECAP as an initial stage of processing has not led to the expected microstructural refinement and improved mechanical behaviour.

Microstructure and Properties of Cu-17Fe Alloy Aged Treatment in High Magnetic Field

Aging treatment of Cu-17Fe alloy was investigated by a vacuum heat treating furnace in high magnetic furnace. The microstructures were documented using scanning electron microscopy (SEM). The solid solubility of Fe in Cu matrix was analysed by energy spectrometer (EDS). The mechanical properties were measured with a Vickers hardness tester. The electrical conductivity was measured with a micro-ohmmeter. The results showed that high magnetic field can promote spheroidization of the Fe dendrites, the spheroidization intensifies and the solid solubility decreases with the increasing magnetic induction intensity of the high magnetic field exerted to the alloy, the solid solubility of Fe is minimum when the magnetic induction intensity is 10T at 500°C during precipitation. And Cu-17Fe alloy has a good strength/conductivity combination of 110Hv/63%IACS after aging treatment of 10T at 500°C for 1h.

Effect of Ag addition on the as-cast microstructure of Cu–8 wt.% Fe in situ composites

Ternary copper-based composites consisting of Cu, 8 wt.% Fe and 0.1–6 wt.% Ag were prepared by inductive melting and casting. The effect of Ag addition on the as-cast microstructure was investigated using optical microscopy (OM) and scanning electron microscopy (SEM). The results show that the primary Fe dendrites in the as-cast microstructure are refined significantly with the presence of Ag. Contact angle between Cu–Ag alloy and γ-Fe was measured using sessile drop technique to investigate the refinement mechanism of the primary Fe dendrites. In addition, the effect of Ag addition on the dissolution of Fe atoms in the Cu matrix at high temperature was investigated by means of energy dispersive X-ray spectroscopy (EDS). The results show that the presence of Ag inhibits the solubility of Fe in the Cu matrix at high temperature.

Effect of Ag micro-alloying on the microstructure and properties of Cu–14Fe in situ composite

This paper studied Ag micro-alloying in the deformation-processed Cu–14Fe in situ composite, by a comparison of Cu–14Fe and Cu–14Fe–0.06Ag. Each alloy was prepared by casting and processed into an in situ composite by hot and cold working. The microstructures were documented using light microscopy and scanning electron microscopy (SEM). The mechanical properties were measured with a tensile-testing machine. The electrical conductivity was measured with a micro-ohmmeter. For both alloys, the as-cast microstructure consisted of a Cu matrix and Fe dendrites; after hot and cold working the microstructure consisted of a Cu matrix containing Fe fibres elongated in the working direction. The as-cast Ag-containing alloy contained finer Fe dendrites. The Ag-containing in situ composite had thinner Fe fibres, higher tensile strength, higher ductility, and higher conductivity. The cold worked Cu–14Fe–0.06Ag in situ composite with cumulative cold deformation strain η=7.8 (where η=ln(A0/A) and A0 and A are the original and final cross-section areas, respectively), achieved a tensile strength of 930MPa and a conductivity of 56%IACS (International Annealed Copper Standard; 17.241nΩm is defined as 100%IACS). The Ag micro-alloyed in situ composite had a combination of properties comparable to that of a much more expensive alloy containing much more Ag. After 1h heat treatment at 300°C, the tensile strength was increased to 950MPa and the conductivity was increased to 56.4%IACS.

Microstructure and properties of Cu–Fe microcomposites with prior homogenizing treatments

Cu–6 wt.% Fe and Cu–12 wt.% Fe microcomposites were prepared by cold drawing. A homogenizing treatment at 950 °C for 3 h followed by furnace cooling was carried out before drawing. The microstructure was observed and the tensile strength and electrical resistivity were determined. The original cast or homogenized microstructure develops into the filamentary composite structure in drawing process. The homogenizing treatment promotes the precipitation of secondary Fe particles in the Cu matrix and disperses primary Fe dendrites. Fe precipitates and dispersive Fe dendrites in the homogenized microstructure result in finer and more uniform composite filaments in the drawn microstructure. The tensile strength and electrical resistivity increase with the increase in drawing strain. The homogenizing treatment increases the strength, especially for the microcomposites at high draw ratios and decreases the resistivity, especially for Cu–6 wt.% Fe. The improvements in strength and conductivity as a result of the prior homogenizing treatment can be attributed to the increased interface obstacle and reduced solute scattering in the filamentary structure.

Microstructure and strength of Cu–Fe–Ag in situ composites

Three Cu–Fe–Ag in situ composites, i.e. Cu–14 wt.%Fe–1 wt.%Ag, Cu–14 wt.%Fe–3 wt.%Ag and Cu–11 wt.%Fe–6 wt.%Ag were prepared by cast and drawn process. Microstructure evolution and mechanical properties of the composites were investigated. The results show that the presence of Ag can strengthen the composite and the fracture surfaces of Cu–Fe–Ag composites exhibit shear characteristics at lower draw ratio. The ultimate strength of the above three Cu–Fe–Ag composites reached 1119 MPa, 1578 MPa and 1357 MPa in sequence at draw ratio η = 6.1, however that of the same processed Cu–12 wt.%Fe is only 978 MPa. The higher strength of Cu–Fe–Ag results from the strengthening effect of Ag: (1) refining the primary Fe dendrites and reducing the spacing between the filaments; (2) solution and precipitation strengthening the copper matrix and (3) strengthening the composite in the form of aligned Ag filaments if the amount Ag is enough for the formation of Cu–Ag eutectic under certain solidification conditions.

Strength of deformation processed Cu–Fe–Ag in situ composites

Three Cu–Fe–Ag in situ composites, i.e. Cu–14 wt.%Fe–1 wt.%Ag, Cu–14 wt.%Fe–3 wt.%Ag and Cu–11 wt.%Fe–6 wt.%Ag, were prepared by cast and drawn process. Strength of the composites was investigated in detail. The results show that the presence of Ag can refine the primary Fe dendrites, leading to much higher strength of Cu–Fe–Ag composites, which reached 1578 and 1357 MPa for Cu–14Fe–3Ag and Cu–11Fe–6Ag respectively at η = 6.1, however that of the same processed Cu–12 wt.% Fe is only 978 MPa. Strength of Cu–Fe–Ag can be expressed as σ = 1319 × λ − 1 / 2 + 57 × C Fe–M 1 / 2 + Δ σ Ag, where λ is filament spacing, C Fe–M is the Fe content in the matrix and Δ σ Ag is strengthening effect of Ag in the matrix. As Ag content is lower, Δ σ Ag = 100 × C Ag 1 / 2 and as it is near or beyond 6 wt.%, Δ σ Ag should be determined by specific experiment due to a second-stage work hardening of the matrix. The calculated strength of Cu–Fe–Ag composites fits the experimental results well.

Detailed study of the crystallization behaviour of the metallic glass Fe 75Si 9B 16

The crystallization behaviour of the metallic glass Fe 75Si 9B 16 was studied by means of transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and magnetic measurements. A detailed interpretation of the results led to some interesting conclusions. The material is crystallised in three distinguished steps. During the first step, iron crystallises in two different forms, i.e. pure iron spherulites and Fe(Si) dendrites. High resolution TEM reveals some short-ranged order iron cluster which explains the two morphologies. The primary crystallization follows a homogeneous nucleation and diffusion-controlled three-dimensional growth. The other two steps (i.e. the eutectic crystallization of bcc Fe(Si) and bct Fe 3B plus the separation of Fe 3B into bcc Fe(Si) and bct Fe 2B and the independent crystallization of the bct Fe 2B from the amorphous matrix), as one peak in the DSC measurements, follow a homogenous nucleation and interface reaction controlled three-dimensional growth. Finally, a satisfactory coincidence of the DSC and magnetic curves was achieved, something which has to be emphasised, as it is being described for the first time.

Containerless Solidification and Net Shaping by Splat Quenching of Undercooled Nd<SUB>2</SUB>Fe<SUB>14</SUB>B Melts

High-speed optical temperature measurement and digital imaging elucidated the solidification behavior of undercooled Nd2Fe14B melt through containerless processing by an electromagnetic levitation method. The Fe phase solidified primarily from the melt. Subsequently, the remaining melt was undercooled below the peritectic temperature and the Nd2Fe14B phase surrounded the primary Fe dendrites, yielding the recalescence. The clear interface of the thermal field propagated and covered the entire sample. Detailed microstructural observation showed that the Nd2Fe14B phase surrounding the different Fe dendrites mutually came into contact with the several points. This suggested that many sites for nucleation of the Nd2Fe14B phase are not necessary for the successive growth of the Nd2Fe14B phase that was maintained by the spread of the Nd2Fe14B phase to the different primary Fe dendrites. This resulted in the macroscopic interface of the thermal field during recalescence. Moreover, the undercooled melt was dropped from the levitation coil and quenched by a pair of copper chill plates with moulds, the shape of which is a hemisphere cap, in order to obtain a small bulk sample for industrial purpose. The spherical sample with the diameter of 5 mm was successively obtained without decreasing the cooling rate. This result suggests the possibility of the net shaping of a small magnet from the melt.

The temperature-dependent growth of Fe submonolayer film on Au(001) studied by barrier height imaging using scanning tunneling microscopy

The growth of Fe submonolayer films on Au(001) at various substrate temperatures (300–500 K) was investigated by scanning tunneling microscopy (STM). STM topographic and barrier height images show that the growth mode and morphology of the films markedly depend on growth temperature. Fe islands with a monolayer height nucleate and grow on flat terraces of Au(001) at 300 K. However, Fe islands with double-layer height and small pinholes were observed on terraces at 353 and 393 K, and step edges were decorated by Fe dendrites. A clear element-specific contrast between Fe islands and Au substrate was observed below 400 K. At elevated growth temperature, above 400 K, a segregated Au layer was observed. The mechanism of the surface segregation of Au is also discussed in terms of the surface diffusion through the pinholes.

Effect of TiC additions to the microstructure and magnetic properties of Nd9.5Fe84.5B6 melt-spun ribbons

Rapidly solidified samples of Nd9.5Fe84.5B6 with and without 3 at. % TiC were prepared by melt spinning and melt extraction and then annealed in vacuum (3×10−6 Torr) at temperatures from 600 to 750 °C. For alloys melt spun under similar conditions, the overquenched state was achieved at wheel speeds >10 m/s for the TiC added alloy while >20 m/s was necessary without TiC. The overquenched samples contained a smaller fraction of α-Fe in smaller grains than the undercooled samples where Fe dendrites formed near the free surface during solidification. These Fe dendrites were not removed by annealing. In addition, large orientated 2-14-1 grains nucleated on the Fe dendrites. This combination is detrimental to the magnetic properties. The addition of TiC results in improved control of the microstructure over a larger fraction of the ribbon volume enhancing the magnetic properties.

Extension of the primary solidification region of Nd2Fe14B by levitation of undercooled melts

Undercooled Nd–Fe–B melts with compositions near the Nd2Fe14B phase (Φ phase) were studied by the electromagnetic levitation technique. In situ measurements of the solidification kinetics were accomplished and melt drops with controlled undercooling were quenched onto chill substrates in order to reveal the solidification sequence and microstructure formation processes. Crystal growth velocities of the Φ phase of the order of 1 mm s−1 were estimated from in situ measurements. Consistent with the nucleation theory, which predicts primary γ-Fe phase crystallization up to 650 K undercooling for the stoichiometric alloy, γ-Fe nucleation could not be avoided during undercooling but the quenching of the melt drops led to dissolution of the γ-Fe nuclei during Φ-phase formation. The γ-Fe phase growth was suppressed in a seam adjacent to the chill surface, whereas Fe dendrites occurred in the remaining parts of the quenched samples. Reduced solute rejection on growth is believed to be responsible for the preferred metastable Φ-phase formation from the undercooled melt on chill substrates. The investigation gives some clues to understand different microstructures and magnetic properties of powder precursors for permanent magnets obtained from different rapid solidification routes.

Microstructure of Ni2B Laser-Induced Surface-Alloyed α-Fe

ABSTRACTSubstrate surfaces of α-Fe were laser alloyed with a CW-CO2laser using Ni2B powder. Single laser scans and laser scans with a 50% overlap between successive passes were carried out to obtain hard surface coatings on the α-Fe substrate. High hardness values in the range of 520 - 600 Hv1 were achieved by laser boriding of α-Fe, compared to a substrate hardness of ∼ 110 Hv1. The improvement in hardness is caused by an off-eutectic microstructure, which consists of Fe dendrites and Fe3B and Fe lamellae. Both phases, Fe3B and Fe, contain Ni in solid solution. For samples treated with a single laser scan the Fe phase solidifies in the bcc structure. In contrast, a 50 % overlap between successive laser passes causes the iron to crystallize in the fcc γ-phase. The difference in crystallographic structure is caused by the amount of Ni dissolved in Fe during laser alloying: only ∼ 9 at.% Ni is contained in the α-Fe phase after a single laser scan, while up to ∼ 40 at.% of Ni is dissolved in the γ-Fe phase after a 50 % overlap between each laser scan due to the nearly complete melting of all injected Ni2B particles. However, the Fe3B phase solidifies during laser alloying independent of the amount of Ni in the orthorhombic Fe3C structure, which is metastable for Fe contents exceeding 45 at.%.

Surface morphology of iron dendrites in the Yanzhuang H6 meteorite

In the Yanzhuang H6 meteorite are small (10μm to 7mm) Fe-Ni dendrites in pockets and veins. We have characterized the surface morphology of several of the dendrites by using scanning electron microscopy (with X-ray microanalysis). The dendrite surface has a network of growth ledges. At the edge of the apexes of the dendrites, there are four concentric “rings” which are consistent with a 〈111〉 dendrite growth direction to produce four ring patterns parallel to the “100” planes. The Fe dendrites are partially coated with a material containing growth ledges which is rich in Fe and S, and is presumed to be FeS (troilite).

Effect of silicon alloying additions on growth temperature and primary spacing of Al 3Fe in Al-8wt%Fe alloy

Alloys of Al-8.4Fe-1.7Si, Al-8.5Fe-3.4Si and Al-8.5Fe-5.6Si (wt%) designated A, B and C, respectively, were prepared from high purity (99.99%) aluminum, Japanese electrolytic iron (99.9%) and superpure silicon (99.99%). Melting was carried out in a recrystallized alumina crucible by using a Radyne induction furnace and was followed by chill casting under flowing argon into steel molds of cavity dimension 15 mm thick, 50 mm wide and 150 mm high. Rods 3 mm in diameter were fabricated directly from the ingots. Lengths of the rods, which were contained in 3 mm bore tubular alumina crucibles, were melted in a Bridgman growth facility. After maintaining the melt at 100K above the liquidus temperatures liquidus: 1,118, 1,108 and 1,092 K for 1.7, 3.4 and 5.6 wt%Si, respectively, for about 10 minutes, crucibles containing the melt were withdrawn at a speed of 0.34 mm/s into a water bath. The following conclusions can be drawn from analysis of the specimens. Addition of silicon to Al-8wt%Fe alloy results in an increase in growth undercooling and primary spacing of Al{sub 3}Fe dendrites Bridgman grown at 0.34 mm/s and 10K/mm. This increase in growth undercooling, relative to predicted local liquidus temperatures which have been corrected for observed macrosegregation ofmore » Fe, is in good accord with the predictions of the Kurz-Giovanola-Trivedi model for needle-like dendrite growth. The silicon content of the Al{sub 3}Fe dendrites obtained is consistent with previously reported measurements for a range of cast Al-Fe-Si alloys.« less