Fe-Al System Research Articles

High spatial resolution PEELS characterization of FeAl nanograins prepared by mechanical alloying

We investigate the nanograin “chemical” structure in a nanostructured material of possible industrial application (Fe–Al system) prepared by conventional mechanical alloying via ball milling in argon atmosphere. We restrict ourselves to the structural and nanochemical behaviour of ball-milled nanocrystalline Fe–Al powders with atomic composition Fe 3Al, corresponding to a well-known intermetallic compound of the Fe–Al system. Scanning transmission electron microscopy (STEM) equipped with a parallel detection electron energy loss spectrometer (PEELS) has provided an insight on the “chemical” structure of both nanograins and their surface at a spatial resolution of better than 1 nm. The energy loss near edge structure of the Al L loss reveals that the Al coordination is similar to a B2 compound and the oxidation of the powder during processing may play a significant role in the stabilization of the intermetallic phases. Conventional transmission electron microscopy (TEM) was used for the structural characterization of the material after the ball milling; powder X-ray diffraction (XRD) aided the investigation.

The influence of cooling rate on the formation of an amorphous phase in Si-based multicomponent alloys and its thermal stability

By adding Ni, Cr, and Zr to the Si–Al–Fe system and using a higher surface velocity of the copper wheel of 63 m/s, the composition range of an amorphous single phase was extended to 60 at% Si. An amorphous Si 60Al 18Fe 12Ni 4Cr 3Zr 3 alloy was produced. The amorphous phase in the Si–Al–TM (TM = transition metals) alloys shows higher thermal stability than in the Al–Si–TM alloys. By the shift of differential scanning calorimetry (DSC) peaks with change of heating rate, the activation energy for the transformation was determined to be 343 kJ/mol for Si 55Al 20Fe 10Ni 5Cr 5Zr 5 alloy and 374 kJ/mol for Si 50Al 25Fe 10Ni 5Cr 5Zr 5 alloy.

Monte Carlo simulations of Mössbauer spectra in diffusion investigations

We study the diffusion process via a vacancy mechanism in an A-$B$ binary alloy with $B2$ order. The starting point of our Monte Carlo simulations were experiments done recently by M\ossbauer spectroscopy and nuclear resonant scattering on stoichiometric $B2$ ordered FeAl, which yielded a nonobvious jump model for the Fe atoms, namely a priority of effective jumps to third-nearest-neighbor sites over second-nearest-neighbor sites. In the simulations we chose ordering energies that ensured a phase diagram close to that of the real Fe-Al system. Allowing the vacancy to exchange with nearest-neighbor atoms exclusively, we obtained a perfect agreement between simulation and experiment for the case when an interaction between the vacancy and the atoms was included. We suggest that the remarkable features of Fe diffusion arise from highly correlated vacancy paths, which are due to an interaction of the vacancy with locally disordered areas in the $B2$ lattice created by the vacancy movement itself.

Mechanically activated synthesis studied by X-ray diffraction in the Fe–Al system

Abstract The nanocrystalline phase formation in the Fe–Al binary system induced by a new low temperature process, called ‘mechanically activated self-propagating high-temperature synthesis (MASHS)’, was investigated by X-ray diffraction (XRD) analysis. Some parameters of this alternative powder processing method which associates a short ball milling duration (leading to nanoscale 3-D polyinterfaces of the elemental components) and a self-propagating high-temperature synthesis (SHS) reaction, were studied. Their influence on the solid (liquid) state kinetic paths of phase formation was characterized. The XRD profile line analysis shows that it is possible to synthesize, for example, an FeAl intermetallic compound possessing a nanostructure (40 nm) using the MASHS process. Moreover, an infrared camera revealed that the mechanical activation step implemented before the SHS process decreases the ignition temperature of the exothermic reaction (100 K) between pure Fe and Al, whatever the composition.

A phenomenological treatment of diffusion in Al–Fe and Al–Ni alloys having B2-b.c.c. ordered structure

A model for diffusion in ordered phases with B2-b.c.c. structure is presented and generalized to multicomponent alloys. The model makes use of thermodynamic data to evaluate the thermodynamic factor as well as the degree of ordering at a given temperature. The activation energy for diffusion is expressed as a function of the degree of order. The model is suitable for implementation in software for simulation of diffusional phase transformations. The available experimental data for the Al–Ni and Al–Fe systems is discussed. Some inconsistencies in the data for the Al–Fe system are pointed out. A CALPHAD-like assessment procedure is applied successfully to a selected set of experimental information showing that the model is capable of representing this information in the Al–Fe and Al–Ni systems. The practical use of the model is demonstrated by the computer simulation of a diffusion experiment. It is shown that the results of the simulation agree well with the experimental observations.

Point defects and their properties in FeAl and FeSi alloys

Positron annihilation as well as differential dilatometric measurements were performed to measure the vacancy concentration in FeAl and FeSi. The highest vacancy concentration is found at the stoichiometric B2 composition with more than 3.5% at the melting point in FeAl. Additionally, diffusion and isotope effect experiments were carried out in B2 FeAl to obtain information about the diffusion mechanism. The isotope effect decreases when the material transforms from D0 3 to B2. The effective formation volume of the defects in the B2 phases is larger (1.4 Ω) than one atomic volume Ω and is related to a defect involving more than one vacancy. The effective migration enthalpy of 0.5–1.8 eV as well as the effective formation enthalpy of 0.7–1.02 eV vary with the concentration of the alloys in an opposite manner. The results for the FeAl system suggest that different defect types may operate as diffusion vehicles in the different phases. The mobility of the defects dominates the thermomechanical behavior of FeAl alloys. The defect production during mechanical deformation of FeAl alloys is divided into two parts. The annealing out of mechanically produced defects occurs on the same time rate as that of thermal defects.

Substitution of Mg2+and Fe2+in the Trigonal-Bipyramidal-Coordinated Site inβ″-(Mg, Fe)Al(PO4)O

Solid solution compositions in the system FeAl(PO4)O–MgAl(PO4)O were synthesized hydrothermally at 735°C and 0.2 GPa and controlled oxygen fugacities. X-ray powder diffraction studies reveal that under these conditions a complete solid solution series is formed. It is characterized by the substitution of Mg2+and Fe2+in a fivefold-coordinated trigonal-bipyramidal site. The solid solution compositions are isotypic with the end members, having theβ″-MgAl(PO4)O structure, and crystallize in the monoclinic space groupP21/c.57Fe-Mössbauer spectroscopic measurements show that these phases are paramagnetic at room temperature. They are characterized by an isomer shift between 1.18 and 1.20 mm/s relative toα-iron and a compositionally dependent quadrupole splitting between 2.52 and 2.61mm/s for high-spin57Fe2+in the trigonal-bipyramidal-coordinatedMe2+site.

Magnetic properties of fcc (Co 95 Fe 5 ) 1-x Al x ribbons

Rapidly quenched (Co95Fe5)1-xAlx ribbons are investigated by X‐ray diffraction, magnetization, and Mossbauer effect measurements. A single fcc phase is obtained for all ribbons x ⩽ 10 at.%. The lattice constant increases linearly with x and is discussed in connection with magnetic moment. The influence of Al substitution on both magnetization and Fe‐atom hyperfine field (H) is studied. At 296 K, the magnetization decreases linearly while H drops nonlinearly as x increases. Al substitution leads to substantial differences in iron hyperfine fields in bcc and fcc systems. Fe moment is perturbed differently by Al substitution in fcc (Co95Fe5)1-xAlx and bcc Fe–Al systems.

THE MAGNETISM AND ITS PRESSURE DEPENDENCE OF Fe-Al SYSTEM

Using the tight-binding linear muffin-tin orbital(TB-LMTO) method, we investigated the magnetism and its pressure dependence of Fe-Al system in some bcc superstructures. The results indicate that the magnetic moment of Fe in the ferromagnetic state decreases with increasing concentration of Al in the system and the volume-compression. As the concentration of Al in the system is in excess of about 0.5 the magnetism of the system will transform from ferromagnetism to paramagnetism. On the other hand, there are no pressure-induced magnetic phase transitions in the bcc Fe-Al alloys; this is different from that in the fcc Fe(γ-Fe) and some fcc Fe-based alloys. The results are in good agree-ment with the experimental results.

Methodological aspects of calculations of the thermodynamic factor in interdiffusion

Self-diffusion can be studied in binary systems for which appropriate radioisotopes for both constituents of the alloy are available. With respect to the Fe-Al system suitable radioisotopes are available for the Fe component (e.g. 59Fe) but not for Al. In the framework of the Boltzmann-Matano method further insight into the diffusion behaviour of Fe-Al intermetallics can be gained from interdiffusion experiments. The interdiffusion coefficient is related via the modified Darken equation to the tracer diffusivities of the constituents. The thermodynamic factor Φ entering the Darken equation is proportional to the second derivative of the Gibbs free energy, G, of the alloy on the molar fraction of one of the components. These relations can be used to deduce the tracer diffusivity of Al provided that the tracer diffusivity of Fe, the interdiffusion coefficient and the thermodynamic factor are known. The mixing energy, which enters G, could be calculated on the basis of the thermodynamical data or by means of non-empirical calculations. We suggest for the latter ab initio calculations the use of modern non-empirical methods.

Phase diagram of a highly diluted, disordered Ising system: The Al-rich, Al–Fe system

Disordered solid solutions containing a highly diluted magnetic element in which competitive exchange interactions are present can allow the observation of magnetic ordering (ferromagnetism, antiferromagnetism, and spin-glass behavior) at dilutor element contents different from those corresponding to ordered alloys. In this work we present a simulational study of the magnetic properties of the Al-rich, Al–Fe disordered alloy system, which meets the above mentioned requirements and is therefore a good candidate to show differences between the magnetic phase diagrams corresponding to the ordered and disordered states. Our simulations were carried out on the basis of a randomly site-diluted Ising model, using the Monte Carlo method as energy minimization tool. In our model, the spins of the Fe atoms are assumed to be coupled by a direct nearest-neighbor ferromagnetic exchange J (depending on the Al concentration), and by an Al-mediated antiferromagnetic superexchange interaction −αJ. Our results suggest the presence of ferromagnetic order for Al concentrations higher than those for which that order disappears in ordered alloys and the occurrence, for Fe atomic percentages between 10% and 25%, of a re-entrant spin glass phase whose freezing temperature decreases with the degree of dilution.

Growth Temperature Measurements and Solidification Microstructure Selection of Primary Al 3Ni and Eutectic in the αAl-Al 3Ni System

Competition between growth of contending primary phases and their eutectics is a key consideration in determining the solidification microstructure of a material that results from particular conditions. Application of the principle that solidification is led by the constituent which grows at the highest temperature under the prevailing conditions gives predictions that are in good accord with experimental fact. The necessary growth temperatures and their dependence on materials composition and solidification conditions have been measured, however, for rather few constituents in relatively few systems as yet. The present purpose is to extend to primary Al{sub 3}Ni and the Al-Al{sub 3}Ni eutectic in the Al-Ni system the measurements made earlier during Bridgman growth, for primary Al{sub 3}Fe(Al{sub 13}Fe{sub 4}) and for the Al-Al{sub 3}Fe and Al-Al{sub 6}Fe eutectics in the Al-Fe system. These measurements will be compared with predictions of dendrite and eutectic growth theory and related to the relevant boundary of the reported Al-Al{sub 3}Ni solidification microstructure selection diagram. Some preliminary results together with other findings were reported earlier in a cited reference.

Mechanically Activated SHS Reaction in the Fe-Al System: In Situ Time Resolved Diffraction Using Synchrotron Radiation

The Mechanical Activation Self propagating High temperature Synthesis (M.A.S.H.S.) processing is a new way to produce nanocrystalline iron aluminide intermetallic compounds. This process is maily the combination of two steps ; in the one hand, a mechanical activation where the Fe - Al powder mixture was milled during a short time at given energy and frequency of shocks and in the other hand, a Self propagating High temperature Synthesis (S.H.S.) reaction, for which the exothermicity of the Fe + Al reaction is used. This fast propagated MASHS reaction has been in -situ investigated using the Time Resolved X - Ray Diffraction (TRXRD) using a X - ray synchrotron beam and an infrared thermography camera, allowing the coupling of the materials structure and the temperature field. The effects of the initial mean compositions, of the milling conditions as well as of the compaction parameters on the MASHS reaction are reported.

Sign of the interaction parameter in disordered Fe-Al alloys

In the framework of the coherent potential approximation the authors show that the interaction parameter for disordered Fe-Al alloy strongly depends on the concentration. The calculations were provided within linear muffin-tin orbital formalism in the atomic sphere approximation. Calculation data for B2 phase for different concentrations were used to extract the interaction parameter, V(0), for disordered solid solutions and were compared with the parameter treated from the X-ray scattering data. The concentration dependence of V(0) leads to the failure of the application of the regular solid solution model to the study of the phase relations in the Fe-Al system. Parameter V(k{sub s}), which is responsible for ordering tendency, also depends on concentration, and it is in accordance with the experimental data.

Mossbauer spectroscopic study of heat-treated and control-cooled Fe3Al alloys

The Fe–Al system forms a model object of transition metal-sp element system. Fe–Al alloys containing 21–31 at% Al are known to exhibit several attractive physical properties which render them candidate materials for structural and magnetic applications. These magnetic and physical properties can be varied by altering the composition and the processing routes. Keeping this in mind, Mossbauer spectra of five Fe–24 at% Al and Fe–25 at% Al alloy samples processed via different routes have been studied. The analysis of the Mossbauer parameters, distribution of hyperfine magnetic fields, P(H) and H-values were used to identify various Fe-atom configurations (nearest neighbour or next nearest neighbour) associated with the phases formed by different processing routes. The results obtained from the present study indicate that the average hyperfine field 〈H〉 depends on the average rate of cooling followed during the precipitation of these phases.

Defect types and defect properties in FeAl alloys

Abstract The thermomechanical behavior of high temperature intermetallic alloys is related to point defects and their concentration. Positron annihilation is a sensitive tool to obtain various information about defect properties. In the FeAl system the formation enthalpy of defects decreases from 1.6 to 2.0 eV in pure α -Fe to 1.37 eV (Fe–7at.%Al) and 0.91eV in the D0 3 structure. Additionally, the formation volume of the defects increases from 0.88 Ω in Fe–7at.%Al to 1.42 Ω in Fe–40at.%Al (Ω=mean atomic volume). The change of the formation volume indicates different types of defects. In quenching experiments the migration enthalpy is measured using the fast Doppler broadening technique and changes from 0.5 to 1.2 eV. Furthermore, the knowledge of the positron lifetimes gives an additional hint about the main defects and their positions in the lattice. The results suggest that thermal defects in the B2 phase are mainly triple defects at low temperature and double vacancies at higher temperatures. In the A2 as well as in the D0 3 structure single vacancies are supposed to be the predominant defect types.

Metastable phases of Al–Fe system by mechanical alloying

The preparation and characterization of metastable phases of the Al–Fe alloy system by mechanical alloying are reported. In Al-rich (up to 10 at.% Fe) alloys, the supersaturated f.c.c. solid solution of Fe in Al (up to 1 at.% Fe) is formed. Almost complete amorphization is confirmed in the composition range 17–33 at.% Fe. The metastable disordered b.c.c. solid solution of about 10 nm in grain size has also been formed by ball-milling for over 180 h in Fe-rich (above 50 at.% Fe) alloys. Examination of lattice parameter and magnetization have shown that the composition range and degree of disorder are comparable to those formed by crushing and sputter deposition.

Microstructural Features of Iron Aluminides Formed by the Reaction between Solid Iron and Liquid Aluminium / Gefügeuntersuchungen an bei der Reaktion zwischen Eisen und Aluminiumschmelze gebildeten Eisenaluminiden

A model experiment was carried out to study the interaction between solid iron and liquid aluminium which occurs during the reaction synthesis of Fe-Al intermetallics. Couples made from iron and aluminium were reacted in the temperature range 973K to 1373K for different time intervals (3 h to 0.5 h). The microstructural features of the product formed were characterized in order to understand the phenomenon occurring and its expected consequences on the reaction product. The reaction product consisted of a multilayered structure and a number of intermetallics belonging to Fe-Al system were found to form. The morphological features of the reaction product have been explained on the basis of internal stresses developed inside the intermetallic product and thermal currents set up inside liquid aluminium due to the exothermic nature of the reaction between iron and aluminium in the given temperature range.

Defect analysis with positron annihilation ? Applications to Fe aluminides

Measurements of positron annihilation at various temperatures at constant pressure give information about the formation of thermal defects and their enthalpy in metals and alloys. In the FeAl system the formation enthalpy decreases from 1.6-2.0eV in pure α-Fe to 1.37eV (Fe7at%nAl) and 0.91eV in the D0 3 structure. Additionally, the formation volume of the defects can be determined from measurements varying the pressure at constant temperature. The formation volume increases from 0.88Ω in Fe7at%Al to 1.42Ω in Fe40at%Al(Ω = mean atomic volume). The change of the formation volume indicates different types of defects. With the further knowledge of the positron lifetimes in these materials we suggest triple defects as the main defects in this system. In quenching experiments the migration enthalpy is measured using the fast Doppler broadening technique. In Fe 3 Al we observe a migration enthalpy of 0.5 eV. which is influenced by an apparing change of order in a temperature range above the DO 3 B 2 transition as determined from the phase diagram by Massalski. Thickness reduction of Fe6.3at%Si single crystals changes positron lifetimes with increasing deformation, particularly by rolling less than 5%.

Pressure dependence of positron trapping rate, lifetime and defect formation in metals and alloys

In order to determine the pressure dependence of positron annihilation parameters like bulk lifetime and positron trapping rate, positron lifetime and Doppler broadening experiments have been carried out. In aluminium a slight decrease in the bulk lifetime of 3 ps at 530 MPa has been found. Vacancies have been quenched in Fe25at%Al and Fe40at%Al to study the effect of high hydrostatic pressure on the positron trapping rate of vacancies. We like to emphasize that there is no significant pressure dependence of the trapping rate up to 570 MPa shown by these alloys. Measurements of positron annihilation at various temperatures at constant pressure give information about the formation of thermal defects and their enthalpy in metals and alloys. Additionally, the formation volume of the defects can be determined from measurements varying the pressure at constant temperature. The defect formation volumeV v f has been measured in pure Al and in intermetallic phases of the FeAl system at different temperatures. In the ordered B2 structure a effective defect formation volume of 1.42 Ω was observed, in A2 of 0.88 Ω, in DO3of 0.96 Ω and in pure Al of 0.67 Ω, respectively (Ω=mean atomic volume). While the latter values reflects single vacancy defects, the large formation volume in B2 points to the prevalence of double vacancies and/or, more likely at low temperatures, triple defects.