Mössbauer probe spectroscopy studies of initial stage of Al-Fe mechanical alloying

The initial stage of mechanical alloying (MA) in a binary mixture of powdered Al and ⁵⁷Fe in an atomic ratio of 99: 1 has been studied using X-ray diffraction and ⁵⁷Fe Mössbauer probe spectroscopy. The proposed microscopic model of MA includes the formation of a nanostructured state (～15 nm) in FCC Al, the penetration of Fe atoms across Al grain boundaries, and the formation of isolated Fe atoms and Fe-Al clusters in boundary distorted zones of Al matrix interface similar to the local atomic environment in deformed Al₆Fe and Al₉Fe₂ phases. Based on the published data, it is assumed that with increasing Fe concentration in the initial mixture, and, correspondingly, increasing amount of clusters, distorted FCC interface regions are transformed into amorphous phase.     

Composition and non-equilibrium crystallization in partially devitrified co-rich soft magnetic nanocomposite alloys

The body-centered cubic (bcc) phase tends to preferentially nucleate during solidification of highly undercooled liquid droplets of binary alloy systems, including Fe–Co, Fe–Ni and Fe–Cr–Ni. We investigate a similar tendency during the partial devitrification of Co-rich amorphous precursors of composition (Co_1?xFe_x)₈₈Zr₇B₄Cu₁ by identifying the structure and composition of the nanocrystalline grains. The Co:Fe ratio of the bcc nanocrystals varies linearly with the Co:Fe ratio of the amorphous precursor, and can lie well within the single-phase face-centered cubic (fcc) region of the Fe–Co phase diagram at the crystallization temperature. Classical nucleation theory therefore suggests several potential explanations for the preferential nucleation of bcc phase from an amorphous precursor, including: (i) a reduced amorphous/bcc interface energy as compared to the close-packed phases; (ii) a lower strain of precipitation for bcc nuclei as compared to close-packed fcc and hexagonal close-packed nuclei; and (iii) stabilization of the bcc phase by dissolved glass-formers such as Zr and B.     

Design of cost-effective Fe-based amorphous coating alloys having high amorphous forming ability by thermodynamic calculation

In this study, new cost-effective Fe-based amorphous coating alloys having high amorphous forming ability were developed by varying the Fe content, while their microstructure, hardness, and corrosion resistance were also evaluated. Chemical compositions that have the lowest driving force of formation of crystalline phases such as Fe₃P, Fe₃C, and α-Fe were obtained from thermodynamically calculated phase diagrams of the representative Fe_xAl₂(P_10.83C_7.47B_1.7)_98-x alloy system at a crystallization temperature of 443 °C. Considering the intersections of driving force curves of Fe₃P and Fe₃C, Fe₃P and α-Fe, and Fe₃C and α-Fe, the Fe contents were found to be 77.8, 76.2, and 75.8 at.%, respectively. The microstructural analysis results of 1.5-mm-diameter suction-cast Fe-based alloys indicated that the Fe_76.5Al₂(P_10.83C_7.47B_1.7)_21.5 alloy had a fully amorphous microstructure, whereas crystalline phases were formed in other alloys. This alloy showed a better hardness and corrosion resistance than conventional thermal spray coating alloys, and its production cost could also be reduced by using less expensive alloying elements, which could provide a good way to practically apply this alloy to Fe-based amorphous thermal spray coatings.     

Aluminization of high purity iron and stainless steel by powder liquid coating

Powder liquid coating is investigated metallographically as an aluminization technique for high-purity iron [Acta Mater., in press] and stainless steel. In this process, Fe₂Al₅ forms initially during heat treatment, with c axis preferentially aligned with the sample normal. In Fe–18mass%Cr alloy, Cr exhibits almost the same concentration profile as Fe except for the temporary formation of a Cr₅Al₈ network in the early stage of heat treatment. Fe–25Cr–18Ni alloy forms a thinner aluminized layer compared to the other substrates, and contains an Al–Ni-rich layer and spherical precipitates (ordered B2). The diffusion of Al and Ni in the system (B2/bcc/fcc) is simulated using a new formulation of the diffusion equation for the ternary Fe–Al–Ni system taking the concentration-dependent interdiffusion coefficient into account. The bcc layer is found to be predominantly in a steady state due to the large interdiffusion coefficients, and characteristic uphill diffusion of Al in the B2 layer is attributed to the existence of Ni.     

Mechanical alloying of the Mo-rich Mo-O-Fe ternary system

Using X-ray diffraction, M?ssbauer spectroscopy, and Auger spectrometry, it has been established that oxygen affects the sequence of solid-state reactions upon mechanical alloying of two types of powders mixtures consisting of an Mo/O composite and Fe with the elemental (atomic) ratios 74.4: 6.7: 18.9 (type 1) and 70.3: 11.7: 18 (type 2). For the samples of both types the process begins with the formation of a nanostructure in Mo and penetration of Fe atoms along oxygen-saturated grain boundaries. The feasibility of two types of reactions depending on the oxygen content has been considered. In the samples of the first type, a reaction Mo/O + Fe → bcc Mo + bcc Mo-O-Fe → bcc Mo_74.4O_6.7Fe_18.9 occurs. The final product is assumed to be a supersaturated solid solution in which O atoms are located in interstitial positions (lattice interstices) and Fe atoms in substitutional positions (lattice sites). For the samples of the second type, an appreciably different reaction has been suggested: Mo/O + Fe → bcc Mo + hcp Mo₆₃O₁₅Fe₂₂ → bcc Mo₈₁Fe₁₉ + Am Mo₃₁O₅₂Fe₁₇, where Am is an amorphous phase. The correlation (established by Butaygin and Povstugar (Dokl.-Chem. 398 (Part 2), 196–199 (2004)) between the rate of the consumption of the second component and the ratio of the yield strengths of the base (Mo) and second (Fe) elements has been confirmed.     

The correlation of microstructural development and thermal stability of mechanically alloyed multicomponent Fe-Co-Ni-Zr-B alloys

The microstructural evolution of multicomponent Fe_70-x-yCo_xNi_yZr₁₀B₂₀ (x = 0, 7, 21; y = 7, 14, 21, 28) alloys during mechanical alloying (MA) has been studied using XRD, SEM and TEM. Mixtures of elemental and pre-alloyed powders have been transformed initially into the single supersaturated bcc α-Fe solid solution phase for the alloys investigated. Subsequently, an amorphous phase has been obtained in Co-free alloys and Co-containing alloys with high Ni/Co ratios of 1 and 3. However, no amorphous phase was detected in another Co-containing alloy with a lower Ni/Co ratio (e.g. 0.33). The thermal stability of the as-milled powders has been investigated by a combination of DSC and the Pendulum magnetometer experiments. The DSC studies provide information on the thermodynamics and kinetics of crystallization of amorphous structure as a function of alloying contents. The Pendulum magnetometer studies reveal the phase transformation from nanocrystalline bcc α-Fe solid solution to amorphous structure during MA and the thermomagnetization behavior of the as-milled powder.     

Effect of alloying element segregation on the work of adhesion of metallic coating on metallic substrate: Application to zinc coatings on steel substrates

A thermodynamic model based on the ‘Macroscopic Atom’ approach is proposed to assess the effect of alloying element segregation on the adhesion of metallic coating on metallic substrate. The interfaces that occur in hot-dip galvanized steels are considered, which include: Zn/Fe, Zn/Fe₂Al₅, Zn/FeZn₁₃, FeZn₁₃/Fe₂Al₅, and Fe₂Al₅/Fe. The effect of the alloying element on the work of adhesion of these interfaces is investigated, which includes Mg, Al, Si, P, Ti, V, Cr, Mn, Fe, Ni, Zn, Nb, Mo, Sn and Bi. Among these elements, Bi, Sn and Mg are predicted to decrease the work of adhesion of the Zn/Fe interface, whereas P, Nb, Mo, V, Ti and Ni tend to enhance this adhesion. The effect of element M (M = Al, Si, Cr, Mn) is positive when it exists in the zinc coating or negative when it occurs in the iron substrate. Among these interfaces, the Fe₂Al₅/Fe interface with a value of 3.8 J m⁻² is the strongest, whereas the Zn/FeZn₁₃ interface with of a value of 1.7 J m⁻² is the weakest. Delamination of the coating upon deformation is predicted to occur along the FeZn₁₃/Fe₂Al₅ and Zn/Fe₂Al₅ interfaces. This agrees with microscopic observations of hot dip galvanized steel after tensile testing.     

Kinetic study of non-isothermal crystallization in Al80Fe10Ti5Ni5 metallic glass

This study investigated the crystallization behavior of a kinetically metastable Al₈₀Fe₁₀Ti₅Ni₅ amorphous phase. The Al₈₀Fe₁₀Ti₅Ni₅ amorphous phase was synthesized via the mechanical alloying of elemental powders of Al, Fe, Ti, and Ni. The microstructures and crystallization kinetics of the as-milled and annealed powders were characterized using X-ray diffraction, transition electron microscopy, and non-isothermal differential thermal analysis techniques. The results demonstrated that an Al₈₀Fe₁₀Ti₅Ni₅ amorphous phase was obtained after 40 h of ball milling. The produced amorphous phase exhibited one-stage crystallization on heating, i.e., the amorphous phase transforms into nanocrystalline Al₁₃(Fe,Ni)₄ (40 nm) and Al₃Ti (10 nm) intermetallic phases. The activation energy for the crystallization of the alloy evaluated from the Kissinger equation was approximately 538±5 kJ/mol using the peak temperature of the exothermic reaction. The Avrami exponent or reaction order n indicates that the nucleation rate decreases with time and the crystallization is governed by a three-dimensional diffusion-controlled growth. These results provide new opportunities for structure control through innovative alloy design and processing techniques.     

High Temperature Oxidation of Hot-Dip Aluminized T92 Steels

The T92 steel plate was hot-dip aluminized, and oxidized in order to characterize the high-temperature oxidation behavior of hot-dip aluminized T92 steel. The coating consisted of Al-rich topcoat with scattered Al₃Fe grains, Al₃Fe-rich upper alloy layer with scattered (Al, Al₅Fe₂, AlFe)-grains, and Al₅Fe₂–rich lower alloy layer with scattered (Al₅Fe₂, AlFe)-grains. Oxidation at 800 °C for 20 h formed (α-Al₂O₃ scale)/(AlFe layer)/(AlFe₃ layer)/(α-Fe(Al) layer), while oxidation at 900 °C for 20 h formed (α-Al₂O₃ scale plus some Fe₂O₃)/(AlFe layer)/(AlFe₃ layer)/(α-Fe(Al) layer) from the surface. During oxidation, outward migration of all substrate elements, inward diffusion of oxygen, and back and forth diffusion of Al occurred according to concentration gradients. Also, diffusion transformed and broadened AlFe and AlFe₃ layers dissolved with some oxygen and substrate alloying elements. Hot-dip aluminizing improved the high-temperature oxidation resistance of T92 steel through preferential oxidation of Al at the surface.     

Effects of crystallographic structure and Cr on the rate of void nucleation in BCC Fe: An atomistic simulation study

Atomistic Monte Carlo simulations based on modified embedded-atom method (MEAM) interatomic potentials have been carried out to clarify the differences in swelling rates between bcc and fcc Fe and between pure bcc Fe and bcc Fe−Cr alloys. Assuming that the transient regimes prior to the onset of steady-state swelling correspond to the void nucleation stage, the effect of crystallographic structure (bcc vs. fcc) or Cr alloying on the void nucleation rate under a given amount of supersaturated vacancies was examined. It was found that the void nucleation rate is much higher in fcc Fe than in bcc Fe. Randomly distributed Cr atoms slightly increase the void nucleation rate in bcc Fe, but microstructural evolutions such as the precipitation of Cr-rich phase have more decisive effects, serving as a vacancy sink. The reasons for the individual results are rationalized in terms of the binding energy of vacancy clusters and the size difference between Fe and Cr atoms.     

Thermodynamic Description of the Al-Mo and Al-Fe-Mo Systems

The Al-Mo and Al-Fe-Mo systems were critically assessed using the CALPHAD technique. The solution phases (liquid, fcc and bcc) were described by a substitutional solution model. The non-stoichiometric compound AlMo₃ was described by a two-sublattice model (Al,Mo)(Al,Mo)₃ in the Al-Mo binary system and (Al,Fe,Mo)(Al,Fe,Mo)₃ in the Al-Fe-Mo ternary system. Other compounds Al₆₃Mo₃₇, Al₈Mo₃, Al₃Mo, Al₄Mo, Al₁₇Mo₄, Al₂₂Mo₅, Al₁₂Mo and Al₅Mo in the Al-Mo system were treated as stoichiometric compounds in the binary system and as line compounds Al _m (Fe,Mo) _n in the Al-Fe-Mo ternary system. The compounds μ and Fe₂Mo in the Fe-Mo system were treated as (Al,Fe)₇Fe₂(Fe,Mo)₄ and (Fe,Mo)₂(Al,Mo) in the Al-Fe-Mo system, respectively. Compounds Al₅Fe₄, Al₂Fe, Al₅Fe₂ and Al₁₃Fe₄ in the Al-Fe system were treated as (Al,Fe,Mo), Al₂(Fe,Mo), (Al,Fe)₅(Al,Fe,Mo)₂ and (Fe,Mo)_0.235Al_0.6275(Al,Va)_0.1375 in the Al-Fe-Mo system, respectively. Ternary compounds τ₁ and τ₂ were treated as Al₈(Al,Fe)Mo₃ and (Al,Fe,Mo)(Va)₃, respectively. A set of self-consistent thermodynamic parameters of the Al-Fe-Mo system was obtained.     

Bulk amorphous Al85Fe15 alloy and Al85Fe15-B composites with amorphous or nanocrystalline-matrix produced by consolidation of mechanically alloyed powders

An Al80Fe14B6 powder mixture was subjected to mechanical alloying. Presence of an amorphous structure in the milling product was revealed by XRD investigations. The calorimetric study showed that the amorphous phase crystallised above 370 °C. The milled Al₈₀Fe₁₄B₆ powder was consolidated under a pressure of 7.7 GPa in different conditions: at 350 °C and at 1000 °C. Besides, the mechanically alloyed amorphous Al₈₅Fe₁₅ powder was consolidated at 360 °C. The amorphous structure was retained after consolidation applied at 350 °C and 360 °C. Compaction at 1000 °C caused crystallisation of the amorphous phase and appearance of metastable nanocrystalline phases. Structural investigations revealed that both bulk Al₈₀Fe₁₄B₆ samples are composites with boron particles embedded in amorphous or nanocrystalline matrix. The hardness of the nanocrystalline-matrix composite and of the amorphous-matrix one is equal to 707 HV1 and 641 HV1 respectively, whereas that of bulk amorphous Al₈₅Fe₁₅ alloy is 504 HV1. The specific yield strength of amorphous-matrix and nanocrystalline-matrix composites, estimated using the Tabor relationship, is 625 and 650 kNm/kg respectively, while that of amorphous Al₈₅Fe₁₅ alloy is 492 kNm/kg. We also suppose that application of high pressure affected crystallisation of amorphous phase, influencing the phase composition of the products of this process.     

The effect of melt-spinning processing parameters on crystal structure and magnetic properties in Fe–Mn–Ga alloys

We have succeeded to fabricate body-centered cubic (bcc) single phase of Fe–Mn–Ga alloys using melt-spinning technique. Heusler type L2₁ structure of Fe₂MnGa alloy are predicted to have half-metallic properties, however bulk Fe₂MnGa alloys crystallize into face-centered cubic (fcc) lattice with small admixture of bcc phase. By changing either ejection temperature or rotation speed of melt-spinning processing parameters, fcc or bcc lattice can be obtained from same precursor ingot. For stoichiometric Fe₂MnGa as-spun alloy, super-lattice diffraction peaks indicative of L2₁ structure are observed from XRD measurements. The as-spun bcc alloys transform into ferromagnetic hexagonal lattice by thermal annealing.     

Size effect on the Fe nanocrystalline phase transformation

A thermodynamics for the phase transformation from γ(fcc) to α(bcc) in nanocrystalline (NC) Fe is considered. Gibbs free energies of the interfaces in NC γ- and α-Fe particles were calculated, respectively, by means of a quasiharmonic Debye approximation, yielding a larger increase in the total Gibbs free energy of α-Fe than that of γ-Fe. This is attributed to the difference in their interfacial energies. As a result, the fcc NC Fe can be thermodynamically stable at room temperature when the grain size is sufficiently small. Taking into account the thermodynamic equilibrium condition, the critical grain size for the γ-Fe phase to exist in stable form at 300 K was quantitatively calculated for different excess volumes ΔV, a parameter describing the state of interface based on a dilated crystal model. The assumptions made in the present model and the factors influencing the critical grain size are discussed.     

Aluminum Alloying Effects on Lattice Types, Microstructures, and Mechanical Behavior of High-Entropy Alloys Systems

The crystal lattice type is one of the dominant factors for controlling the mechanical behavior of high-entropy alloys (HEAs). For example, the yield strength at room temperature varies from 300 MPa for the face-centered-cubic (fcc) structured alloys, such as the CoCrCuFeNiTi _x system, to about 3,000 MPa for the body-centered-cubic (bcc) structured alloys, such as the AlCoCrFeNiTi _x system. The values of Vickers hardness range from 100 to 900, depending on lattice types and microstructures. As in conventional alloys with one or two principal elements, the addition of minor alloying elements to HEAs can further alter their mechanical properties, such as strength, plasticity, hardness, etc. Excessive alloying may even result in the change of lattice types of HEAs. In this report, we first review alloying effects on lattice types and properties of HEAs in five Al-containing HEA systems: Al _x CoCrCuFeNi, Al _x CoCrFeNi, Al _x CrFe_1.5MnNi_0.5, Al _x CoCrFeNiTi, and Al _x CrCuFeNi₂. It is found that Al acts as a strong bcc stabilizer, and its addition enhances the strength of the alloy at the cost of reduced ductility. The origins of such effects are then qualitatively discussed from the viewpoints of lattice-strain energies and electronic bonds. Quantification of the interaction between Al and 3d transition metals in fcc, bcc, and intermetallic compounds is illustrated in the thermodynamic modeling using the CALculation of PHAse Diagram method.     

Application of classical nucleation theory to phase selection and composition of nucleated nanocrystals during crystallization of Co-rich (Co,Fe)-based amorphous precursors

Classical steady-state nucleation theory is applied to Co-rich Fe,Co-based alloys to provide a rationale for experimental observations during the nanocrystallization of Co-rich (Co,Fe)₈₉Zr₇B₄ and (Co,Fe)₈₈Zr₇B₄Cu₁ amorphous precursors. The amorphous precursor free energy is estimated using density functional theory. This simple theory suggests: (i) strain or interface energy effects could explain a tendency for a body-centered cubic (bcc) phase to form during crystallization. Dissolved glass formers (Zr,B) in crystalline phases may also contribute; (ii) similar face-centered cubic (fcc) and hexagonal close-packed (hcp) free energies could explain the presence of some hcp phase after crystallization even though fcc is stable at the crystallization temperature; (iii) nanocrystal compositions vary monotonically with the Co:Fe ratio of the amorphous precursor even when multiple phases are nucleating because nucleation is not dictated by the common tangency condition governing bulk phase equilibria; and (iv) Fe-enrichment of the bcc phase can be attributed to a relatively small free energy difference between the amorphous and bcc phases for high Co-containing alloys.     

Role of C: Zr stoichiometry in the formation of a nanocrystalline structure and magnetic properties in Fe87-x Zr13Cx films

X-ray diffraction analysis was used to study the structure of as-sputtered and annealed Fe-13 at. % Zr-C films, which were produced by reactive magnetron sputtering and characterized by stoichiometric and nonstoichiometric (with respect to ZrC) at. % C to at. % Zr ratios. A special packet of programs was used to resolve wide reflections observed in X-ray diffraction patterns of the films. The as-sputtered films of all compositions were found to be amorphous in terms of X-ray diffraction. The thermal stability of the amorphous phase increases as the C: Zr ratio in the films departs from the stoichiometric ratio (1: 1) characteristic of the monocarbide ZrC. Annealing leads to the formation of a mixed (amorphous + nanocrystalline) structure. Depending on the carbon content and annealing temperature, the phase composition of the films is represented by different combinations of phases, such as bcc α-Fe (the basis phase), fcc ZrC, monoclinic Fe₂C, monoclinic Fe_2.5C, orthorhombic Fe₃C, and Fe₂₃Zr₆. After annealing at 550°C, the best magnetic properties are characteristic of the films having the stoichiometric composition with respect to ZrC (at. % C: at. % Zr ～ 1).     

Nanocrystalline Al–Fe intermetallics – light weight alloys with high hardness

Nanocrystalline Al–Fe alloys containing 60–85 at.% Al were produced by consolidation of mechanically alloyed nanocrystalline or amorphous (Al85Fe15 composition) powders at 1000 °C under a pressure of 7.7 GPa. The hardness of the alloys varied between 5.8 and 9.5 GPa, depending on the Al content. The specific strength, calculated using an approximation of the yield strength according to the Tabor relation, was between 544 and 714 kNm/kg. Based on the results obtained, we infer that application of high pressure affected crystallisation of amorphous Al₈₅Fe₁₅ alloy, influencing the phase composition of the crystallisation product, and phase changes in nanocrystalline Al₈₀Fe₂₀ alloy, inhibiting them.     

Structure and phase transformations in Fe–Ni–Mn alloys nanostructured by mechanical alloying

Ternary Fe₈₆Ni_xMn_14−x alloys, where x = 0, 2, 4, 6, 8, 10, 12, 14, 16 at.%, were prepared by the mechanical alloying (MA) of elemental powders in a high-energy planetary ball mill. X-ray diffraction analysis and Mössbauer spectroscopy were used to investigate the structure and phase composition of samples. Thermo-magnetic measurements were used to study the phase transformation temperatures. The MA results in the formation of bcc α-Fe and fcc γ-Fe based solid solutions, the hcp phase was not observed after MA. As-milled alloys were annealed with further cooling to ambient or liquid nitrogen temperatures. A significant decrease in martensitic points for the MA alloys was observed that was attributed to the nanocrystalline structure formation.     

Phase Equilibria of the Al–Zn–Bi–Fe Quaternary System at 600 °C

Isothermal sections of the Al–Zn–Bi–Fe quaternary system at 600 °C with fixed Al content of 75 at.% and fixed Zn content of 50 at.% were determined by scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy and x-ray powder diffraction analysis. One four-phase region and four four-phase regions were identified in the 75 at.% Al and 50 at.% Zn sections, respectively. The Bi-poor L₁ phase and Bi-rich L₂ phase are in equilibrium with (Al), FeAl₃, Fe₂Al₅, α-Fe, and δ, respectively. No new quaternary phase was found in these two sections. Bi is almost insoluble in all Fe–Al and Fe–Zn compounds.