Fe Ni Cr Research Articles

Mössbauer study of metastable phase formation in vacuum deposited FeNiCr multilayers due to swift heavy ion irradiation

Conversion electron Mössbauer spectroscopy (CEMS) and X-ray diffraction (XRD) were used to study multilayers, consisting of a few atomic layers of iron, nickel and chromium with a composition 50%Fe–25%Ni–25%Cr, prepared by high vacuum deposition and subsequently irradiated by 246 MeV 86 Kr 8+ ions at room temperature. Metastable highly disordered microcrystalline ferromagnetic and paramagnetic phases have been detected in the Fe–Ni–Cr multilayers as a result of the swift heavy ion irradiation. These metastable phases never occur with thermally prepared alloys but are similar to those observed previously in the case of Fe–Ni–Cr alloys prepared by other non-equilibrium techniques as electrochemical deposition. The relative amount of these metastable phases increases with the irradiation fluence. It was shown that transformation of metastable phases into the stable ones occurs in high vacuum evaporated and ion beam mixed Fe–Ni–Cr films due to appropriate heat treatment in vacuum.

Characterization of the scale on oxidized Fe–Ni–Cr alloys using grazing emission X-ray fluorescence

Grazing emission X-ray fluorescence (GEXRF, or refracted X-ray fluorescence—RXF) has been used to characterize the oxide scale (predominately chromia) which formed on an alloy of 55Fe–25Cr–20Ni (wt%) after oxidation for 4 h at 750°C in O 2. Angle dependent X-ray emission spectra I( θ) were acquired for three elements, Cr, Fe and Ni. The measured data were fit by adjusting scale parameters in the calculated spectra such that I( θ) data were simultaneously fit for all three elements. It is shown that the calculated spectra are sensitive to scale thickness, to the Cr depletion zone that develops in the substrate at the scale–metal interface, and to the concentration of Fe and Ni atoms dissolved in the scale. The very demanding requirements imposed by simultaneously fitting all three measured I( θ) curves provide a satisfactory determination of these scale parameters. The GEXRF measurements showed that the scale was about 0.5 μm thick, with an underlying Cr depletion zone extending about 4 μm (midpoint) into the substrate. The average Cr concentration in this depletion zone was reduced from the preoxidized value of 25 to 22.2 wt%. The scale contained about 2.4 at% of Fe and less than 0.5 at% Ni. These measurements demonstrate that GEXRF can, in a single nondestructive measurement, provide key information needed to characterize a thermally grown chromia scale.

Mechanical properties of hot isostatically pressed nanograin iron and iron alloy powders

Hot isostatic pressing (hipping) was performed on a variety of iron and iron alloy powders produced by attrition ball milling. Microhardness and compression tests were used to determine the mechanical properties. Fracture morphology was studied by scanning electron microscopy. For each specific alloy, increasing the hipping temperature causes an increase in density up to a maximum. Hipping at still higher temperatures improves the bonding without any change in density. The minimum temperature required for obtaining the maximum density depends on the initial powder composition. While iron powder processed in argon (Fe [Ar]) reached a maximum density at 580°C, an identically processed iron carbon alloy (Fe–2C [N]) reached its maximum density above 850°C. Two types of compression strain–stress curves were obtained: up to a certain temperature, most powders showed an increase in compression yield stress with temperature. Hipping above this point causes a decrease in the compression yield stress, and an increase in elongation. This behaviour is entirely different than that of a stainless steel powder (Fe–18Cr–8Ni), which shows substantial work hardening for hipping temperatures ranging between 965 and 1050°C.

Structure of carbon nanotubes grown by microwave-plasma-enhanced chemical vapor deposition

Carbon nanotubes grown on a Ni substrate and an Fe–Ni–Cr alloy substrate by plasma-enhanced chemical vapor deposition were investigated by transmission electron microscope (TEM) and energy dispersive x-ray (EDX) analysis. TEM showed that the nanotubes on both substrates have a piled-cone structure with metal particles on top which determine the diameter of the nanotubes. Their diameter ranges from 60 to 80 nm. Moreover, EDX showed that the metal particles are composed of Ni when the nanotubes are grown on Ni substrate and of Fe and Ni in the case of the Fe–Ni–Cr alloy substrate.

Electrochemical behavior of electrodeposited strongly disordered Fe–Ni–Cr alloys

Potentiodynamic and chronoamperometric polarization measurements, 57 Fe conversion electron Mössbauer spectroscopy, X-ray diffractometry and electron microprobe measurements were performed on amorphous and microcsrystalline Fe–25Ni–25Cr alloys coatings electrochemically deposited at varying times in comparison with a thermally prepared alloy of the same composition. Significant differences were found among the Mössbauer spectra of electrodeposited alloys reflecting differences in the short range ordering of the dominant ferromagnetic phases. The potentiodynamic polarization curves revealed a higher ability to spontaneous passivation for the more disordered deposit while anodic dissolution was characteristic for the more ordered electrodeposits and for the crystalline thermally prepared alloy. The onset of the passivation of amorphous alloy was enhanced either by the decreased anodic dissolution or the increased hydrogen evolution rate. At the early stage of passivation, consistent passivation characterization was found both by chronoamperometric and potentiodynamic curves and by polarization resistance measurements performed at open circuit potential.

THE PGE AND BASE-METAL ALLOYS IN THE PODIFORM CHROMITITES OF THE LUOBUSA OPHIOLITE, SOUTHERN TIBET

PGE and base-metal alloy minerals from podiform chromitites of the Luobusa ophiolite, southern Tibet include Os–Ir, Os–Ir–Ru, Pt–Fe, Ir–Ni–Fe, Fe–Ni–Cr, and Fe–Co with highly variable compositions. Small amounts of diamond, graphite, SiC, an unnamed chromium carbide, iron silicide (Fe–Si), and native Fe, Ni, Cr, Au, Cu and Si also are present. The alloys and native elements were recovered primarily from heavy-mineral separates of the chromitites, but some are included in, or attached to, magnesiochromite grains. They are mostly subhedral to anhedral and range in size from about 0.05 to 0.5 mm. Some grains have euhedral crystal forms and others have round shapes, suggestive of melt droplets. Fe–Ni-bearing, Ru-rich PGE alloys, Fe–Ni and Fe–Co alloys and native Fe and Ni in the Luobusa chromitites are interpreted as secondary minerals formed by alteration of PGE sulfides. The Pt–Fe, Os–Ir, and Ir–Ni–Fe alloys, Fe silicide and perhaps the native Si are considered to be xenocrysts from the mantle, transported to shallow depths by a rising plume and then captured by the melts from which the Luobusa chromitites crystallized.

The development of aligned precipitates during internal carbonitridation of Fe–Ni–Cr alloys

An austenitic Fe–25Cr–19Ni stainless steel alloy was carbonitrided at 1,000°C in an atmosphere with ac=1 and P(N2)=0.9atm to form two discrete precipitation zones. Local equilibrium between the precipitates and the austenite matrix carbon activity was achieved throughout the reaction zone. Small, globular Cr7C3 particles were formed immediately beneath the surface. High aspect ratio Cr23C6 lamellar plates were formed deeper in the precipitation zone and were found to have a cube–cube orientation relationship with the austenite matrix. The inward growth of these carbides was facilitated by the formation of an austenite/depleted austenite grain boundary at the precipitation front, which transformed the austenite to a more appropriate orientation and accelerated the segregation of chromium to the carbide tips.

The development of aligned precipitates during internal carbonitridation of Fe–Ni–Cr alloys

An austenitic Fe–25Cr–19Ni stainless steel alloy was carbonitrided at 1,000°C in an atmosphere with ac=1 and P(N2)=0.9atm to form two discrete precipitation zones. Local equilibrium between the precipitates and the austenite matrix carbon activity was achieved throughout the reaction zone. Small, globular Cr7C3 particles were formed immediately beneath the surface. High aspect ratio Cr23C6 lamellar plates were formed deeper in the precipitation zone and were found to have a cube–cube orientation relationship with the austenite matrix. The inward growth of these carbides was facilitated by the formation of an austenite/depleted austenite grain boundary at the precipitation front, which transformed the austenite to a more appropriate orientation and accelerated the segregation of chromium to the carbide tips.

Precipitate morphologies and growth kinetics in the internal carburisation and nitridation of Fe–Ni–Cr alloys

Model Fe–Ni–Cr alloys were carburised in gas of unit carbon activity, nitrided at p N 2 =0.9 atm, sequentially carburised and nitrided, and simultaneously carburised and nitrided at 1000°C. Internal precipitation kinetics were always parabolic. Carburisation rapidly produced numerous small carbides, and nitridation led to the slow growth of large, sparsely distributed and strongly directional precipitates. In both cases, alloy penetration rates were controlled by volume diffusion of oxidant. The low stability of Cr 2N led to variation in precipitate volume fraction with depth, the distribution being controlled by the diffusion path, not nucleation. Carburisation of nitrided alloys converted prior nitrides to carbides whilst retaining the parent precipitate morphology. The process was diffusion controlled and slow, reflecting the high carbon activity at the nitride–carbide interface. Carbonitriding produced large volume fractions of carbide with, in some cases, nitride-like morphologies, but no obvious nitride phase. Rapid rates reflected fast diffusion along the aligned precipitate-matrix interfaces.

Time–dependent nature and origin of displacive transformation

We have investigated athermal and isothermal martensitic transformations (typical displacive transformations) in Fe–Ni and Fe–Ni–Cr alloys under pulsed and static magnetic fields and hydrostatic pressures in order to understand the time-dependent nature of martensitic transformation, that is, the kinetics of martensitic transformation. Also, we have calculated electronic structures of B2 and ζ′2 phases in AuCd by FLAPW and/or LAPW methods in order to understand the origin of B2–ζ′2 transformation. The following results were obtained. (i) The two transformation processes are closely related to each other, that is, the athermal process changes to the isothermal process under a hydrostatic pressure and the isothermal process changes to the athermal one under a magnetic field. (ii) These findings of (i) can be explained by the phenomenological theory, which gives a unified explanation for the two transformation processes previously proposed by our group. (iii) The calculation of the generalized susceptibility, x(q), for the B2 phase of AuCd shows that there exists a nesting vector of near 1/3<110>2Π/a as in the B2 phase of TiNi calculated previously. The density of states at the Fermi energy of the ζ′2 phase is lower than that of the B2 phase, which is similar to the case of B2–R transformation in TiNi previously calculated.

Oxidation behaviour of nanocrystalline Fe–Ni–Cr–Al alloy coatings

Nanocrystalline Fe–Ni–Cr–Al alloy coatings with ∼4 wt-%Al were produced using the unbalanced magnetron sputter deposition technique with a composite 310S stainless steel target embedded with aluminium plugs. The oxidation behaviour of the coatings was studied, during which complete external α-Al2O3 scales were formed. During isothermal oxidation tests at 950, 1000, and 1050°C, the oxidation kinetics followed an essentially parabolic rate law, and the oxidation constants were measured to be 2·06 × 10-3, 4·23 × 10-3, and 1·14 × 10-2 mg2 cm-4 h-1 respectively. During a cyclic oxidation test at 1000°C the α-Al2O3 scale showed good scale spallation resistance. The surface hardness of the coatings was measured with a Knoop indentor before and after oxidation. After oxidation, the coating surface hardness was still significantly higher than that of the uncoated specimen, demonstrating the potential this coating has in the improvement of high temperature erosion resistance.

Evolution of magnetic and mechanical properties in stainless steel films implanted with Kr ions

Mössbauer spectroscopy and X-ray diffraction techniques were used to analyse the magnetic and mechanical properties of nano-crystalline Fe–Ni–Cr films in the as-sputtered state and after ion implantation with a dose of 10 17 Kr 2+/cm 2. Before implantation, the films are found to be magnetic (hyperfine field around 29.7 T) and the body-centred cubic structure is in a high compressive stress state (−3.2 GPa). Measurements done after implantation treatments show that the hyperfine field decreases from 29.7 down to 29.4 T while residual stresses increase from −3.2 up to 1.4 GPa (the compressive state is decreasing). No phase transformation is observed.

Microstructural examination of Ni-ion irradiated Fe–Ni–Cr alloys followed to micro-zone deformation

Examination of deformation microstructures induced by micro-indentation technique by means of transmission electron microscopy (TEM) following an MV range metallic ion irradiation was performed in order to study the influence of irradiation-produced microstructural defects on plastic deformation behavior of materials. In this work, specimens of an Fe–Cr–Ni austenitic model ternary alloy before and after irradiation with 4 MeV Ni ions at 723 K were indentation-tested at loads as low as 9.8 mN using an instrumented micro-indentation testing system. The dislocation microstructures of the specimens' regions beneath the indents were successfully examined with a TEM following a cross-sectional thin film processing with a focused ion beam (FIB) processing system. This paper describes the experimental procedure developed in this study and the results from the first series of the experiments.

Mass-spectrometric determination of the thermodynamic mixing behavior of liquid ternary Fe–Ni–Cr alloys

Thermodynamic investigations on liquid ternary Fe–Ni–Cr alloys have been performed by means of the computer-aided Knudsen cell mass spectrometry. Ternary thermodynamically adapted power (TAP) series are used for the algebraic representation of the thermodynamic excess properties. At 1950 K, the molar excess Gibbs energy G E is negative in the predominant part of Gibbs triangle (minimum value: −2980 J/mol at x Ni=0.55, x Cr=0.45), and positive only near the liquid binary Cr–Fe alloys (maximum value: 940 J/mol at x Cr=0.375, x Fe=0.625). The molar heat of mixing H E is exothermic for nearly all Fe–Ni–Cr melts (minimum value: −4770 J/mol at x Fe=0.38, x Ni=0.62). Only the Cr-richest ternary melts near the Ni–Cr binary alloys are slight endothermic (maximum value: 80 J/mol at x Ni=0.125, x Cr=0.87). The molar excess entropy S E is slight negative for ternary Fe–Ni–Cr melts near the two binary boundary systems Fe–Ni and Cr–Fe with a minimum S E value of −1.27 J/mol K at x Cr=0.40 and x Fe=0.60, and slight positive at the side of binary Ni–Cr melts (maximum S E value: 1.35 J/mol K at x Ni=0.50, x Cr=0.50). The Fe-activities of liquid ternary Fe–Ni–Cr alloys as determined, in this work, show slight negative deviations from the ideal behavior in the range of composition below the section of constant mole fraction x Cr/ x Ni=1, and slight positive deviations from Raoult's law above this section line. The Ni-activities of all ternary Fe–Ni–Cr melts show negative deviation from the ideal behavior. The Cr-activities show slight positive deviations from Raoult's law in the range of composition on the left-hand side of the section of constant mole fraction x Ni/ x Fe=1, and slight negative deviations from the ideal behavior on the right-hand side of this section line. The results of this work can be used successfully for phase-diagram calculations.

Computation of phase equilibria in the Fe–Ni–Cr system based upon mass spectrometric investigations

Phase diagram calculations require only the differences between the chemical potentials of the involved phases. This makes it possible to use even artificial thermodynamic parameter sets which then have to be corrected by means of additional terms. It is demonstrated that mass spectrometric investigations yield a simpler, but more effective algebraic description of the thermodynamic ternary mixing behavior than the results of previous phase diagram assessments: mass spectrometric measurements in the liquid phase, and in the restricted field of fcc-phase in the system Fe–Ni–Cr together with the algebraic descriptions of the three binary boundary systems Fe–Ni, Ni–Cr, and Cr–Fe as reported in previous works make it possible to compute ternary phase equilibria in the system Fe–Ni–Cr, showing better agreement with the experimental phase equilibria data than the assessments as reported by Kundrat and Elliot, Hillert and Qiu, and Lee. Additional advantage of the mass spectrometric determined molar Gibbs energy is that the description of the magnetic contributions Z α,mag k ( T, x α ) has been merely reduced to the expressions of the pure species.

On the role of recovery in creep of precipitation-strengthened polycrystalline nickel-base alloys

An attempt is made to shed some new light on the role of recovery in creep of precipitation-strengthened polycrystalline nickel-base creep-resistant alloys, comparing some basic creep data for a Ni–16Cr–10W–4Mo–Ti–Al alloy with similar data for a low carbon 18Cr–12Ni stainless steel and a 15M 2B–Fe–18Cr–12Ni in situ composite. The comparison suggests that the minimum creep strain rate of precipitation-stregthened polycrystalline nickel-base creep-resistant alloys is controlled rather by recovery (driven by internal stress affected by the presence of γ′-particles), than by dislocation glide/climb (driven by effective stress) past these particles. An urgent need to study in detail recovery in creep of precipitation-strengthened polycrystalline nickel-base alloys is emphasized.

Welding of nickel base superalloy single crystals

The welding and weldability characteristics of single crystal nickel base superalloy PWA 1480 were investigated over a range of welding conditions and orientations using electron beam and laser welding processes. Using differential thermal analysis, the freezing range for this alloy was established and the formation of the γ/γ′ eutectic during the last stages of solidification was identified. Both electron beam and laser welds showed extensive fusion zone cracking, and the cracks were identified as solidification cracks, i.e. hot cracks. Crack free welds could be made over a very narrow range of welding conditions with preheat. Most of the microstructural features observed in the weld are similar to those previously observed in Fe–Ni–Cr alloy single crystal welds. The welds contain misoriented stray grains which playa critical role in the promotion of hot cracks in the welds. The origin of these grains is explained in terms of constitutional supercooling and the growth conditions. Detailed atom probe field ion microscopy analysis was carried out to determine the γ/γ′ compositions and the segregation at the γ/γ′ interface. The results revealed no solutal segregation at this interface, and the composition of the γ/γ′ was close to that of the equilibrium composition.

Atomistic simulation of dislocation core structure and dynamics in Fe–Ni–Cr–N austenite

Atomistic computer simulations based on the use of the conjugate gradient and molecular dynamics methods were employed to determine the core structure and dynamics of the a/2 edge and screw dislocations in Fe-Ni-Cr and Fe-Ni-Cr-N austenites. The embedded-atom method was used to quantify the interactions between iron, nickel, chromium and nitrogen atoms. In Fe-Ni-Cr austenite, both the edge and screw dislocations dissociate along one of the {1 1 1} planes, forming stacking fault ribbons. The ribbon widths were found to be comparable to their values calculated using the continuum theory. The analysis of dislocation dynamics showed that the phonon drag interferes more with the motion of screw dislocations, reducing their mobility in comparison with the mobility of edge dislocations. In Fe-Ni-Cr-N austenite, the structure of the dislocation core of the a/2 edge dislocation does not seem to be significantly affected by the presence of nitrogen. In sharp contrast, the core structure of the dissociated a/2 screw dislocation undergoes a major change, resulting in spreading of the core on to two or more non-parallel planes. As a result, mobility of the screw dislocations is substantially lower than that of the edge dislocations. This finding is consistent with the experimental observations that the dislocations are predominantly of the screw character in Fe-Ni-Cr-N austenite.

Effect of minor elements on irradiation assisted stress corrosion cracking of model austenitic stainless steels

A low impurity Fe 18Cr 12Ni (HP) and its heats doped with Si and C (HP + Si and HP + C) were irradiated to 6.7 × 10 24 n/m 2 ( E ≫ 1 MeV) at 513 K. The slow strain rate tensile (SSRT) tests were carried out at a constant strain rate of 1.7 × 10 −7 s −1 in high purity, 573 K water. Scanning electron microscopy on the fracture surface revealed that HP and HP + Si failed mainly by the intergranular stress corrosion cracking (SCC), while the major failure mode in HP + C was the transgranular SCC. All alloys exhibited radiation hardening. HP + Si exhibiting the smallest hardening showed uniform elongation of 17%, while HP and HP + C did not. Transmission electron microscopy was also carried out. Frank loops and unidentified small clusters were formed in HP and HP + C, while only small clusters were observed in HP + Si.

High temperature creep behaviour of a 15M 2B [sbnd]Fe [sbnd]18Cr [sbnd]12Ni composite

The creep behaviour of a 15M 2B Fe 18Cr 12Ni in situ composite was investigated at temperatures ranging from 873 to 1173 K and applied stresses in the range2.01 × 10 −4 G–7.78 × 10 −3 G (where G is the shear modulus). The apparent activation energy of creep, Q c, was found to increase slightly with increasing applied stress. By neglecting this stress dependence of Q c and using its weighted average value of Q c av, a slight temperature dependence of the stress sensitivity parameter of the minimum creep rate, m, was avoided. This parameter was found to depend on the applied stress σ, increasing from a value close to 3.5 atσ=20MPa to a value close to 10 atσ=400MPa. The creep in the composite is most probably lattice diffusion controlled, the difference between the value of Q c av=359kJ mol −1 and that of the activation enthalpy of lattice diffusion,Δ H¯ L = 290kJ mol −1, being accounted for by the temperature dependence of the shear modulus. The creep strength of the composite is nearly the same as that of the matrix solid solution (low carbon 18Cr 12Ni steel). Possible reasons for the negligible strengthening effect of M 2B particulates are discussed. The creep-rate-controlling dislocation mechanism is most probably the same in the composite and the 18Cr 12Ni steel, namely recovery of the dislocation structure dependent on lattice diffusion and associated with subgrain or cell structure formation. It is shown that the stress sensitivity parameter m increasing with increasing stress and reaching a value as high as 10 cannot be explained in terms of the measured internal stress as a back stress.