Determination of impurity–point defect binding energies in alloys

Abstract

Modelling of radiation induced or thermal non-equilibrium segregation needs data on impurity–point defect binding energies. These data are generally unavailable although some attempts have been made to calculate them. In the present paper an approach to calculating impurity–interstitial (mixed dumbbell) binding energies is established on the basis of strain field arguments. Earlier work is slightly modified for more accurate calculation of oversized impurity–vacancy binding energies. The method is applied to predictions of various impurity–point defect binding energies in several transition metal matrixes. With the aid of these predictions, other experimental and theoretical results on impurity–point defect binding energies and radiation induced segregation are reasonably explained.

MST/3512     

Calorimetric and magnetic study of solid Fe–Cr–Co alloys

Abstract

An adiabatic calorimeter was used to measure the molar heat capacities and heats of transformation of iron alloys containing up to 14 at.-% Cr and 15 at.-% Co over the temperature range 700–1500 K. A Sucksmith magnetic balance was used to measure the magnetic properties of these alloys and of Fe–Co and Fe–Cr alloys previously investigated calorimetrically. In all but one of the ternary alloys the α → γ transformation occurred at a temperature below the Curie temperature (T_c), but the heats of transformation, corrected to the transformation temperature of pure iron (1184 K), correlated well with the difference between the extrapolated Curie temperature and 1184 K, as had been observed previously for alloys with clearly defined values of T_c. The fact that all observations fell on the same curve supported the proposal that a significant amount of magnetic order remains after the α → γ transformation for most iron alloys.

MST/198     

Solidification microstructures selection of Fe–Cr–Ni and Fe–Ni alloys

The transition of solidified phases in Fe–Cr–Ni and Fe–Ni alloys was investigated from low to high growth rate ranges using a Bridgman type furnace, laser resolidification and casting into a substrate from superheated or undercooled melt. The ferrite–austenite regular eutectic growth, which is difficult to find in typical production conditions of stainless steels, was confirmed under low growth rate conditions. The transition velocity between eutectic and ferrite cell growth had a good agreement predicted by the phase selection criterion. Which of either ferrite or austenite is easier to form in the high growth range was discussed from the point of nucleation and growth. Metastable austenite formation in stable primary ferrite composition was mainly a result of growth competition between ferrite and austenite. For a binary Fe–Ni system, a planar metastable austenite in the steady state, simultaneous growth such as eutectic and banded growth between ferrite and austenite in an initial transient region are confirmed.     

Electrochemical characteristics of nano and microcrystalline Fe–Cr alloys

The electrochemical behavior of nano and microcrystalline Fe–10Cr and Fe–20Cr alloys was determined using potentiodynamic polarization in 0.5 M H₂SO₄. Disks of the alloys were prepared by high-energy ball milling followed by compaction and sintering. In the current study, nanocrystalline Fe–Cr alloys reveal significantly different electrochemical characteristics, typified by lower anodic current densities and more negative passivation potentials, compared with their microcrystalline counterparts. In addition to the differences in grain boundary density, compositional characterization of corrosion films carried out by X-ray photoelectron spectroscopy indicates a higher Cr content in the film developed upon nanocrystalline Fe–Cr alloys. Mechanisms for observed enhancement in the corrosion performance of the nanocrystalline Fe–Cr alloys are discussed.     

Density and viscosity of ternary Cr–Fe–Ni liquid alloys

The density and viscosity of ternary Cr–Fe–Ni liquid alloys have been investigated over a wide temperature range. The density was measured using electromagnetic levitation as a container-less technique, while viscosity was measured by means of a high-temperature oscillating cup viscometer. Although, the concentration dependence of density shows the influence of the second order (binary) interaction parameter in excess volume, the influence of a third order (ternary) interaction parameter in excess volume can be neglected. The temperature dependences of the viscosities are well described by the Arrhenius law. The viscosity increases monotonically as Fe or Cr concentration increases. For constant temperature, the viscosity as a function of iron molar faction can be described by a thermodynamic model using the enthalpy of mixing as input parameter.     

Density and thermal expansion of Cr–Fe, Fe–Ni, and Cr–Ni binary liquid alloys

Densities and their temperature coefficients of liquid Cr–Fe, Fe–Ni, and Cr–Ni binary alloys have been measured containerless using the technique of electromagnetic levitation. Data have been obtained in a wide temperature range including the supercooled region. The density measurements indicate that these binary systems have a small and positive excess volume, whereas the excess free energies are negative. The temperature coefficients of these alloys can be estimated from those of the pure components. Hence, possible contributions from the temperature dependence of the excess volume can be ignored to calculate the temperature coefficient of density.     

Initial oxidation of Fe–Cr alloys: in situ STM and ex situ SEM observation

Abstract

In order to understand the initial oxidation of Fe–Cr alloys a single crystal of Fe–15Cr (100) was oxidized at 440°C under controlled oxygen partial pressure in a UHV system and the surface morphology was observed using in situ STM (basic pressure 1×10^?10 mbar); in addition, polycrystalline Fe&15Cr was oxidized at 400°C in an IR-furnace in atmospheric air and the morphology was observed using ex situ SEM. The chemistry of the surface oxide layers was studied by XPS.

Preparation of the single crystal in the UHV system did not lead to segregation of Cr to the surface during heating. In situ STM investigation showed that oxidation of Fe–Cr commenced by nucleation of Cr oxide on the surface, due to selective oxidation of Cr. When the Cr at the surface and at the interface was completely consumed by nucleation of Cr oxide, Fe oxidized and covered the initial Cr oxide nuclei, resulting in an Fe oxide layer on the surface. Ex situ experiments showed that initial oxidation of the mechanically prepared polycrystalline alloy depended on the defect distribution in the surface. It started with formation of whisker-type Fe oxides along defects and proceeded with spherical-type nucleation and growth of Fe oxide. In both experiments, the final product on the surface was Fe₂O₃.     

Wear behavior–hardness–microstructure relation of Fe–Cr–C and Fe–Cr–C–B based hardfacing alloys

The aim of this study is the investigation of the effect of ferroboron and the amount of powder mixture (ferroboron + ferrochromium) on wear resistance of Iron (Fe)–Chromium (Cr)–Carbon (C) based hardfacing alloys. Powder mixture, consisting of ferrochromium (FeCr) and ferroboron (FeB), was added to massive wire during welding process. Hardfaced layers were obtained by three different powder mixtures and two different powder/massive wire proportions. Hardfacing was applied to AISI 1020 steel substrate by open arc welding. Hardness test, Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD) analysis, dry sand/rubber wheel abrasion test were executed. Test results showed that increasing ferroboron content and increasing powder mixture amount enhanced the wear resistance.     

Toward the correlation of indentation hardness in micro- and nano-scale: understanding of indentation edge behaviors in Fe–Cr alloys

Journal of Materials Science - Nanoindentation hardness tests are used to measure indentation hardness at the micro- and nanoscales and further to predict Vickers hardness on larger scales. Hence,...     

The role of cementite in the metal dusting of Fe–Cr and Fe–Ni–Cr Alloys

Abstract

Model Fe–25 w/o (weight percent) Cr and Fe–25 Cr–Ni alloys containing 2.5, 5, 10 and 25w/o nickel were exposed to a CO–26H₂–6H₂O (vol. pct) mixture at 680°C under thermal cycling conditions. The supersaturated carbon activity was calculated to be 2.9 (referred to graphite) and M₃C was predicted to form on Fe–25Cr and Fe–25 Cr–2.5 Ni, but not on higher nickel content alloys. Metal dusting occurred on all alloys, accompanied by internal carburisation. Transmission electron microscopy of the dusting deposit showed that much of the carbon consisted of hollow graphite nanotubes. Small, metal-rich particles were found at the carbon filament tips. These were identified as single crystal Fe₃C in the case of Fe–25 Cr, and M₃C, containing low levels of nickel, in the case of Fe–25 Cr–2.5 Ni and Fe–25 Cr–5 Ni. In contrast, the particles found at the filament tips on the higher nickel, two phase, alloys were both M₃C and austenitic Fe–Ni. Strong orientation relationships were determined between the graphite and cementite particles, however, no consistent and clear crystallographic relationship was deduced between the graphite and austenite particles. It is concluded that carbon deposition from the gas is catalysed by both Fe₃C and austenite. Subsequent carbon nanotube growth reflects the orientation relationship between Fe₃C and the graphite.     

A study of the factors contributing to the metal dusting of Fe–Cr–Ni alloys in highly carburising atmospheres

Abstract

Metal dusting is a recurring problem within the petrochemical industry that has serious consequences for plant efficiency and reliability, as well as for operational safety. The underlying factors governing its occurrence are not fully understood and therefore studies under closely monitored conditions in atmospheres that simulate those encountered in service are essential.

For this preliminary study, candidate reformer and pyrolysis tube materials have been selected and exposed at 560°C for periods of up to 200 hours in a highly reducing atmosphere. The intention has been to focus on some of the main variables that are thought to possibly contribute to metal dusting and evaluate their contribution through a series of controlled experiments. In addition to alloy composition, the principal parameters that have been addressed are surface condition and system pressure.

In the series of experiments described in this paper, machined and ground as well as polished specimens have been exposed to an H₂–24.4%CO–2.4%H₂O gas mixture at pressures of nominally 1.5, 2 and 5 bar (absolute). As a consequence of the character of this gas (i.e. carbon activity, ac, >>1), extensive coke deposition occurs on the metallic specimens. Thus, a thermal cycle has been introduced into the test procedure, generally at 25 hour intervals, so that the samples can be inspected and coke removed.

Cross-sectional optical and scanning electron microscope examinations of the morphology of the coke deposit and the nature and extent of attack have provided a deeper understanding of the importance of alloy composition and the contributions made by surface treatment and system pressure.     

Role of nickel in the oxidation of Fe–Cr–Ni alloys in air–water vapour atmospheres

Abstract

A series of experimental austenitic alloys has been produced in which the nickel content ranges from 14 to 43%, with constant levels of 20%Cr, 1%Mn and 0.5%Si. A combination of isothermal, discontinuous and cyclic oxidation testing has been used to elucidate the performance in dry air and in air with 10%, 45% or 62% water vapour at 700°C and 1000°C. Evaluation was by means of thermogravimetry, surface analysis with glow discharge optical emission spectroscopy and scanning electron microscopy.

Nickel is shown to have several roles: it accelerates the kinetics of chromia formation yet suppresses chromia spallation at 700°C. At 1000°C, it strongly decreases the breakaway oxidation and spalling associated with iron oxide formation. This effect is particularly marked in environments containing water vapour, where the material loss may be decreased 10-fold by an increase in the nickel content. Results correlate to thermodynamic and kinetic data which show nickel to increase the chromium activity and diffusivity in the alloy.     

Bimodal grain size distribution: an effective approach for improving the mechanical and corrosion properties of Fe–Cr–Ni alloys

The effect of nanocrystalline grain size and bimodal distribution of nano- and microcrystalline grain sizes on the oxidation resistance and mechanical properties of Fe-based alloys has been investigated. Nanocrystalline and bimodal Fe–10Cr–5Ni–2Zr alloy pellets, prepared by mechanical alloying route, have been compared with conventional microcrystalline stainless steel alloys having 10 and 20 wt% Cr. Zr addition has been shown to improve the grain size stability at high temperatures. A significant improvement in the ductility of bimodal alloys with respect to nanocrystalline alloys was seen presumably due to the presence of the microcrystalline grains in the matrix. The high temperature oxidation of nanocrystalline and bimodal alloys at 550 °C shows superior oxidation resistance over microcrystalline alloy of similar composition (Fe–10Cr–5Ni) and comparable to that of microcrystalline alloy having twice as much Cr (Fe–20Cr–5Ni). Secondary Ion Mass Spectroscopy depth profiling confirms the hypothesis that nanostructure facilitates the enrichment of Cr at the oxide metal interface resulting in the formation of a passive oxide layer.     

Morphology and crystallographic phase of V–C particles formed in Fe–Cr–Ni–V–C alloys

Abstract

The morphology and crystallographic phase of V–C carbide particles formed in cast Fe–Cr–Ni–V–C alloys were investigated by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy (TEM). The combination of results obtained with these techniques revealed that cuboidal, cruciform and spherical carbide particles were formed, depending on the alloy composition, all having the cubic-VC_1?x structure (Fm-3m). Detailed TEM observations suggested that small carbide particles were initially cubic in shape and became spherical with increasing particle size. All cuboidal and spherical carbides were single crystallites with no grain boundary at any particle sizes, even after growing to 6 μm in diameter.     

Positron annihilation study on deformation-induced Au precipitation in Fe–Au and Fe–Au–B–N alloys

We have investigated the potential self-healing of deformation-induced defects by Au precipitation during isothermal aging at 550 °C in Fe–Au and Fe–Au–B–N alloys using positron annihilation spectroscopy. Two different samples with 0 and 24 % pre-strain were used to study the influence of dislocations on the Au precipitation. Dislocations introduced prior to the aging process play an essential role in the formation of Au precipitates. The Coincidence Doppler broadening (CDB) technique shows that Au precipitation in the matrix occurs in the pre-strained samples only. TEM observations confirm the heterogeneous nature of the Au precipitation which occurs exclusively on dislocations and grain boundaries. The evolution of S and W parameters derived from the CDB indicates a three-stage precipitation process for the pre-strained samples. Both the hardness tests and the positron annihilation spectroscopy indicate that the addition of boron and nitrogen to the Fe–Au alloy causes a deceleration of the Au precipitation in the pre-strained samples, but does not alter the defect-induced mechanism of the Au precipitation. The defect-induced Au precipitation provides a promising site-specific autonomous repair mechanism to extend the lifetime of Fe-based alloys for high temperature structural applications.     

Oxidation and abrasive wear of Fe–Si and Fe–Al intermetallic alloys

In the present work, intermetallic alloys Fe–Si and Fe–Al (Fe₃Si–C–Cr and Fe₃Al-C), produced by induction melting, were evaluated regarding their oxidation and abrasive resistance. The tests performed were quasi-isothermal oxidation, cyclic oxidation, and dry sand/rubber wheel abrasion. As reference, the ASTM A297-HH grade stainless steel was tested in the same conditions. In the oxidation tests, the Fe–Al based alloy presented the lowest oxidation rate, and the Fe–Si based alloy achieved the best results in the abrasion test, showing better performance than the HH type stainless steel.     

Effect of silicon content on the microstructure and properties of Fe–Cr–C hardfacing alloys

In this study, the surface of St52 steel was alloyed with preplaced powders 55Fe39Cr6C, 49Fe39Cr6C6Si, and 45Fe39Cr6C10Si using a tungsten-inert gas as the heat source. Following surface alloying, conventional characterization techniques, such as optical microscopy, scanning electron microscopy, and X-ray diffraction were employed to study the microstructure of the alloyed surface. Microhardness measurements were performed across the alloyed zone. Room-temperature dry sliding wear tests were used to compare the coatings in terms of their tribological behavior. It was found that the as-deposited coatings contained higher volume fractions of carbides (Cr₇C₃). The presence of 6%Si in the preplaced powders caused an increase in microhardness and wear resistance.     

Effect of chromium content on electrical resistivity and tensile properties of Ti–Fe–Cr alloys

Abstract

The effects of substituting chromium for iron and the use of low cost ferrochromium alloys in the production of β-Ti–Fe–Cr alloys have been studied with respect to phase constitution, stability, and mechanical properties, in solution treated and quenched states using resistivity, hardness measurement, X-ray diffraction, and tensile testing. Resistivity at room and liquid nitrogen temperatures, and hardness decreased while the ratio of resistivity at liquid nitrogen temperature to that at room temperature increased with increases in chromium content. Alloys of Ti–Fe–Cr, with almost the same electron per atom value, with higher chromium content have smaller volume fractions of athermal omega than alloys with higher iron content. There is less solution hardening in the former alloys than in the latter alloys. Tensile strength decreased with increases in chromium content, while elongation and reduction in area significantly increased. The balance between tensile strength and ductility (elongation and reduction in area) improved in the alloys with added chromium as a substitute for iron. Therefore, no negative influences of ferrochromium alloying on mechanical properties was observed in this study.