Direct formation of β-FeSi2 in evacuated fused ampoule

A simple preparation method for β-FeSi₂ was proposed to activate the interfacial reaction between Fe powder and Si powder through a heat process in a sealed ampoule. The mechanism of the reaction process was investigated by using sputtered Fe films on Si substrates and sapphire substrates. It was found that Si vapor in the ampoule reacts with solid Fe and is saturated to form ε-FeSi. Once ε-FeSi is formed, it initiates the reaction with solid Si to form β-FeSi₂.     

Properties of Co-doped N-type FeSi2 processed by mechanical alloying

Iron-silicide was produced with a mechanical alloying process and consolidated through vacuum hot pressing. The as-milled powders were of metastable state and fully transformed into the ß-FeSi₂ phase through subsequent isothermal annealing. The as-consolidated iron silicides consisted of an untransformed mixture of α-Fe₂Si₅ and ?-FeSi phases and a partially transformed β-FeSi₂ phase was found in the low density compact. Isothermal annealing was carried out to induce transformation into a thermoelectric semiconducting β-FeSi₂ phase. The transformation behavior of the β-FeSi₂ was investigated utilizing DTA, SEM, and XRD analyses. Isothermal annealing at 830°C in vacuum led to a thermoelectric semiconducting β-FeSi₂ phase transformation, but some residual metallic α and ?-phases were unavoidable even after 96 hours of annealing. The iron silicide microstructures were investigated using SEM and TEM. The mechanical and thermoelectric properties of the β-FeSi₂ materials before and after isothermal annealing are characterized in this study.     

Phase analysis of the mechanically alloyed Fe−Si powder by differential dissolution technique

The binary Fe?Si elemental powders mixture (1∶2 in atomic proportion) has been milled for different milling times in an attrition mill. The phase characterization of mechanically alloyed powder was investigated using the chemical method of differential dissolution (DD) and the X-ray diffraction (XRD) method. In powder specimens milled for more than 15 hr, ∈-FeSi and unreacted Si were observed. The formation of a supersaturated solid solution of Si in ∈-FeSi induced by mechanical alloying (MA) was also verified. The lattice parameter of the ∈-FeSi of as-milled powders changed from 4.4876 Å to 4.4668 Å according to the increase of MA time. Based on the results of the DD analysis, unreacted Si could be classified as (1) crystalline Si, (2) Si supersaturated in ∈-FeSi, or (3) amorphous Si. Therefore formation of the β-FeSi₂ after annealing could be explained by the reaction between the ∈-FeSi and the Si classified into types (1) and (2). It seemed that the amorphous Si induced by MA did not react with the ∈-FeSi during annealing at 700°C.     

Phase stabilization in the Fe1−xMnxSi2 system induced by ball milling

Phase formation in the Mn doped iron disilicide system Fe_1−xMn_xSi₂ with 0.00 ≤ x ≤ 0.24 was studied using X-ray diffraction and Mössbauer spectroscopy. Samples were prepared at room temperature in Ar atmosphere by two different routes. The first one involved the simultaneous mill of the pure elements, and the second one the milling in two steps, first the premixture of metals and then the addition of Si. Both routes produced β-FeSi₂, ɛ-FeSi and α-FeSi₂ phases. In the first case, the segregation of MnSi and Si was also observed. The diffraction results and the obtained set of hyperfine parameters supported the coexistence of β-FeSi₂, α-FeSi₂ and ɛ-FeSi. The relative phase composition depended on the preparation route, being the obtained fraction of the ɛ-FeSi smaller when the second preparation route was followed. It seemed that the previous formation of a Fe_1−xMn_x alloy before to the silicide overcomes the chemical driving forces.     

Microstructure and thermoelectric properties of β-FeSi2 ceramics fabricated by hot-pressing and spark plasma sintering

The microstructure and thermoelectric properties of β-FeSi₂ ceramics by hot pressing (HP) and spark plasma sintering (SPS) are investigated. With increasing hot-pressing temperature, the density, electronic conductivity and thermal conductivity of the samples increase significantly, the thermoelectric figure of merit is improved slightly. The microstructure study indicates that the sizes of the β-FeSi₂ and ?-FeSi phases in the sample sintered by the SPS process are smaller than that by the HP process. The SPS sample shows excellent thermoelectric performance due to the low thermal conductivity.     

Hydrogen accommodation in Zr second phase particles: Implications for H pick-up and hydriding of Zircaloy-2 and Zircaloy-4

Ab-initio computer simulations have been used to predict the energies associated with the accommodation of H atoms at interstitial sites in α, β-Zr and Zr–M intermetallics formed with common alloying additions (M = Cr, Fe, Ni). Intermetallics that relate to the Zr₂(Ni,Fe) second phase particles (SPPs) found in Zircaloy-2 exhibit favourable solution enthalpies for H. The intermetallic phases that relate to the Zr(Cr,Fe)₂ SPPs, found predominantly in Zircaloy-4, do not offer favourable sites for interstitial H. It is proposed that Zr(Cr,Fe)₂ particles may act as bridges for the migration of H through the oxide layer, whilst the Zr₂(Ni,Fe)-type particles will trap the migrating H until these are dissolved or fully oxidised.     

Dynamic equilibrium of MoSi2 polymorphs during mechanical milling

The phase transformation of α-MoSi₂ into β-MoSi₂ induced by mechanical milling (MM) was studied. Planetary ball milling was performed on an α-MoSi₂ powder under six different milling conditions. The X-ray diffraction results show that the dynamic equilibrium between α-MoSi₂ and β-MoSi₂ is reached at low milling intensities, while a single phase of β-MoSi₂ is formed when milled at high milling intensities. The single-phase β-MoSi₂ formation is found to be due to Fe impurity. The mechanism of the phase transformation of MoSi₂ is discussed, and the phase fractions under the dynamic equilibrium are explained by the redistribution of the close-packed layers via synchroshear processes.     

Solid-state reactions upon mechanical alloying of an Fe32Al68 binary mixture

The sequence of solid-state reactions that occur upon mechanical alloying of powder mixtures of Al and Fe taken in an atomic ratio of 68: 32 has been studied by the methods of X-ray diffraction analysis, M?ssbauer spectrometry, and Auger spectrometry. Upon the formation of a nanocrystalline state (<10 nm), there takes place a mutual penetration of Al atoms into Fe and Fe atoms into Al particles. The rate of consumption of the fcc Al is substantially higher than that of the bcc Fe. The process of the mechanical alloying (MA) was found to be two-stage. At the first stage, up to 2 at % Fe is dissolved in the fcc Al, and an amorphous Fe₂₅Al₇₅ phase is formed in the interfaces, whose amount reaches 70 at % at the finish of the initial stage. In the interfaces of the ??-Fe phase, a disordered bcc phase of composition Fe₆₆Al₃₄ is formed, which contains up to 12 at % Al segregates. At the second stage, the amorphous phase crystallizes into an orthorhombic intermetallic compound Fe₂Al₅. The residual ??-Fe, bcc Fe₆₆Al₃₄, and segregated Al form a bcc phase of composition Fe₃₅Al₆₅.     

The mechanically activated combustion reaction in the Fe–Si system: in situ time-resolved synchrotron investigations

Mechanical high-energy ball milling of Fe+2Si elemental powder mixtures was used to activate self sustaining combustion reaction in the case of iron disilicide synthesis. The reaction path as well as the influence of the microstructural parameters on phase transformation have been investigated in detail. Time-resolved X-ray diffraction (TRXRD) using the fast recording kinetics offered by the synchrotron radiation was coupled to an infrared camera in order to study the internal structure of the combustion wave. The crystallite size and the amount of mechanically induced phases play an important role during the combustion; the reaction path and the end product composition mainly depend on the degree of mechanical activation (i.e. shock power and ball milling duration). β-FeSi₂ is formed during a slow diffusion process in the post-combustion zone. The polyinterfaces created at a nanometric scale during the mechanical activation stage are responsible for this peculiar behaviour.     

Influence of high pressure and manganese addition on Fe-rich phases and mechanical properties of hypereutectic Al−Si alloy with rheo-squeeze casting

The influence of high pressure and manganese addition on Fe-rich phases (FRPs) and mechanical properties of Al?14Si?2Fe alloy with rheo-squeeze casting (RSC) was investigated. The semi-solid alloy melt was treated by ultrasonic vibration (UV) firstly, and then formed by squeeze casting (SC). Results show that the FRPs in as-cast SC alloys are composed of coarse β-Al₅(Fe, Mn)Si, δ-Al₄(Fe, Mn)Si₂ and bone-shaped α-Al₁₅(Fe, Mn)₃Si₂ phases when the pressure is 0 MPa. With RSC process, the FRPs are first refined by UV, and then the solidification under pressure further causes the grains to become smaller. The peritectic transformation occurs during the formation of α phase. For the alloy with the same composition, the ultimate tensile strength (UTS) of RSC sample is higher than that of the SC sample. With the same forming process, the UTS of Al?14Si?2Fe?0.8Mn alloy is higher than that of Al?14Si?2Fe?0.4Mn alloy.     

Mechanical properties of FeSi (ε), FeSi2 (ζα) and Mg2Si

FeSi, α-FeSi₂ (high-temperature modification) and Mg₂Si are the major phases in ferro-silicon-magnesium (FeSiMg) based foundry alloys used for the production of ductile cast iron. Small quantities of these phases were produced by induction melting for a more detailed study of the behaviour of each phase. The chemical composition of each phase was verified by electron microprobe analyses (EPMA). Investigation of the microstructure showed the existence of a small amount of second phases in the produced samples. Compression tests were performed to determine the elastic modulus of each compound. The fracture toughness of FeSi was established from three-point bending tests and was compared with values calculated from Vickers indentations. For α-FeSi₂ and Mg₂Si, the fracture toughness was estimated from hardness indentation tests. Palmqvist type cracks developed from the corners of the indentation marks in all three phases.     

Thermal expansion of semiconducting silicides β-FeSi2 and Mg2Si

The thermal expansion of β-FeSi₂ and Mg₂Si was investigated at high temperatures (ranging from 300 to 1173 K for β-FeSi₂ and from 293 to 873 K for Mg₂Si) using powder X-ray diffraction. The linear thermal expansion coefficients α_L for the three lattice parameters of β-FeSi₂ range from 10.6(2) to 11.8(4) × 10⁻⁶ K⁻¹, which indicates small anisotropy, which is in contrast to the large anisotropy reported previously. The volumetric thermal expansion coefficient α_V for β-FeSi₂ is relatively large among the transition-metal disilicides. α_L for Mg₂Si can be expressed by the linear expression of T: α_L = 11(1) × 10⁻⁶ + 6.9(2) × 10⁻⁹T K⁻¹. α_V for Mg₂Si is larger than that of the transition-metal disilicides, including β-FeSi₂. Based on a comparison of α_L among Mg₂Si, several metals and silicides, the candidates for electrode materials are discussed. In particular, temperature dependence and value of α_L for Ni is close to those for Mg₂Si, which suggests that Ni is a good candidate electrode material with respect to thermal expansion.     

First-principles calculations of phonon and thermodynamic properties of Fe-Si compounds

The phonon approach and the Debye model are combined to predict the vibrational thermodynamic contribution for the following Fe-Si compounds: Fe₃Si, Fe₂Si, Fe₅Si₃, FeSi, β-FeSi₂ and α-FeSi₂. Both the ultrasoft pseudopotential (USPP) and the projector augmented wave (PAW) methods are employed to describe the electron-ion interactions. The generalized gradient approximation including PW91 and PBE is employed to describe the exchange-correlation functional. Lattice parameters, bulk modulus, phonon dispersions, and finite temperature thermodynamic properties are calculated and compared with available experimental data, and good agreement is observed. The thermodynamic data obtained in the present work provide better understanding of the stability of binary Fe-Si compounds and can be used for further thermodynamic modeling of this system.     

Thermoelectric properties of mechanically alloyed iron disilicides consolidated by various processes

Mn doped p-type iron disilicide powders have been produced by a mechanical alloying process. As-milled powders were of metastable state and mostly transformed to β-FeSi₂ phase by subsequent isothermal annealing. As-milled powders were consolidated by various processes such as sintering of the cold compact in vacuum, vacuum hot pressing (VHP), and spray drying/atmospheric plasma thermal spraying. Phase transitions during the processes were investigated using XRD, EDS, and SEM. As-consolidated specimens consisted of a mixture of α-Fe₂Si₅ and ε-FeSi phases, which were gradually transformed into a thermoelectric semiconducting α-FeSi₂ phase by subsequent isothermal annealing in the vicinity of 845°C in vacuum. However, some residual α and ε phases remained even after prolonged annealing. Thermoelectric properties were measured as a function of temperature and correlated with phase transformation. They showed optimum values in the vacuum hot pressed specimen due to a higher fraction of β phase and/or higher density.     

Precipitation during homogenization cooling in AlMgSi alloys

Precipitation during the industrial cool down takes place predominantly above 300 °C in the EN AW-6082 and 6005 alloys. The phase precipitation throughout cooling is equilibrium β phase. A considerable capacity is retained after the cool down for further precipitation during a subsequent heating cycle. The β-Mg₂Si is once again the predominant phase that forms during a scan heating cycle employed in exactly the same manner with the industrial billet preheating operation. The precipitation in the 6060 alloy, on the other hand, occurs predominantly below 300 °C with additionally β′-Mg₂Si particles formed below 200 °C.     

Mechanism of Exfoliation (Layer Corrosion) of AI-5%Zn-1%Mg

Abstract

The mechanism of exfoliation in AlZn5Mg1 alloy has been studied by making a survey of the phasesb occurring in the alloy in the naturally aged and artificially aged conditions, preparing the phases in pure state and investigating their electrochemical properties. It has beenfound that in the naturally aged alloy, the attack is confined to lamellar zones in the structure giving rise to the exfoliation. Finely dispersed α-Al(Fe, Me)Si phase particles arranged in streaks along the extrusion or rolling direction act as cathodes and the anodic areas consist of narrow zones of the Zn- and Mg-rich matrix, next to the particles. Between the streaks of α-Al(Fe,Me)Si particles, layers of matrix are left unattacked. Since the main factor determining the amount of α-Al(Fe,Me)Si phase is the Fe content, an increase in Fe content will decrease resistance to exfoliation. In the artificially aged condition, the alloy is not prone to exfoliation, but shows a type of general attack. α-Al(Fe,Me)Si particles again constitute the cathodes of the corrosion cells, but the anodic phase is the M-phase (MgZn₂), which is evenly distributed.

Since the zone next to a weld bead is essentially in the solution treated condition, it will become resistant to exfoliation on post-weld artificial ageing. At some distance from the weld bead, however, there will be an ‘over-aged’ zone, where neither the hardness nor the resistance to exfoliation will be very much increased on artificial ageing. This is due to the formation of especially wide precipitate-free zones around α-Al(Fe,Me)Si and E-phase particles and around grain boundaries on ‘over-ageing’. Since the precipitate-free zones are due to vacancy depletion, these zones are supersaturated with respect to Zn and Mg and thus prone to corrosion. The attack will be confined to the matrix, especially along streaks of α-Al(Fe,Me)Si and E-phase particles.     

Evolution of second phases and mechanical properties of 7075 Al alloy processed by solution heat treatment

The effects of solution treatment on the evolution of the second phases and mechanical properties of 7075 Al alloy were studied with scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), differential scanning calorimetry (DSC), hardness and tensile tests. The results show that Mg(Zn, Cu, Al)₂ phases gradually dissolve into the matrix, yet the size and morphology of Al₇Cu₂Fe phase exhibit no change with the increase of the solution treatment temperature and time due to its high melting point. When the solution treatment temperature and time continue to increase, the formation of coarse black Mg₂Si particles occurs. Compared to the as-cast alloy, the microhardness, tensile strength, and elongation of the sample under solution heat treatment at 460 °C for 5 h are increased by 55.1%, 40.9% and 109.1%, respectively. This is because the eutectic Mg(Zn, Cu, Al)₂ phases almost completely dissolve and basically no coarse black Mg₂Si particles are formed.     

Formation and characterisation of a chromate conversion coating on AA6060 aluminium

AA6060-T6, an AlMg0.5Si0.4 model alloy and α-Al(Fe,Mn)Si phase electrodes have been subjected to chromate treatment in a commercial chromate-fluoride based solution. The coated surfaces were subsequently examined by use of field emission SEM, TEM, AES and electrochemical measurements in 0.1 M NaCl solution in order to study the effect of substrate microstructure on coating formation and properties. Non-uniform growth of the chromate conversion coating (CCC) on AA6060-T6 resulted in a porous morphology, with cracks extending down to the base metal. Poor coverage was particularly observed at the grain boundaries. The thickness of the CCC after completed treatment was about 150-200 nm, while significantly thinner films were formed on the α-Al(Fe,Mn)Si particles. AlMg0.5Si0.4 in the artificially aged (T6) condition exhibited a coating morphology similar to AA6060-T6, while CCC formation on homogenised AlMg0.5Si0.4 was characterised by growth of localised oxide particles and filaments, resulting in poor coverage. These observations indicated that precipitation of Mg₂Si particles due to heat treatment promoted nucleation of the CCC. Chromate treatment caused a significant reduction of cathodic activity on AA6060 during subsequent polarisation in chloride solution. This was attributed to rapid formation of a thin chromium oxide film on the α-Al(Fe,Mn)Si particles during the chromate treatment, resulting in significant cathodic passivation of the phase. Inhibition of the oxygen reduction reaction at cathodic intermetallic particles is suggested as an important factor contributing to the high performance of chromate pre-treatments on aluminium.