Iron Lattice Research Articles

Modeling of hydrogen-assisted cracking in iron crystal using a quasi-Newton method

A Quasi-Newton method was applied in the context of a molecular statics approach to simulate the phenomenon of hydrogen embrittlement of an iron lattice. The atomic system is treated as a truss-type structure. The interatomic forces between the hydrogen-iron and the iron-iron atoms are defined by Morse and modified Morse potential functions, respectively. Two-dimensional hexagonal and 3D bcc crystal structures were subjected to tensile numerical tests. It was shown that the Inverse Broyden's Algorithm-a quasi-Newton method-provides a computationally efficient technique for modeling of the hydrogen-assisted cracking in iron crystal. Simulation results demonstrate that atoms of hydrogen placed near the crack tip produce a strong deformation and crack propagation effect in iron lattice, leading to a decrease in the residual strength of numerically tested samples.

Influence of Al on strength and impact behaviour of hot rolled plain C–Mn steels

The influence of Al on the strength and impact properties of hot rolled plain C–Mn low S steels has been examined at two C levels, 0˙02 and 0˙1% and two N levels, 0˙001 and 0˙004–0˙005% (where percentage is to be taken as wt-%). It has been found that Al is an excellent element to add to hot rolled steels, an addition of 0˙2% to a plain 0˙1C–1˙4Mn steel with 0˙004–0˙005%N resulting in an impact transition temperature of −95°C at a strength level of ∼300 MPa. At this Al level, Al removes the N from solution but also importantly refines the grain boundary carbides and to a lesser degree the grain size resulting in a 40°C lowering of the impact transition temperature in a coarse grained steel (15–20 μm; d− 1/2, 7–8 mm−1/2). Strength is not affected by this Al addition. The decrease in strength due to the removal of nitrogen from solution as AlN is balanced by the solid solution hardening from the Al in solution in the iron lattice, (a 1%Al addition increasing the strength by ∼70 MPa), and the small grain refinement that occurs from AlN precipitation. Increasing the Al level to 1% in the 0˙1%C steels, results in the formation of martensite with consequent deterioration in impact behaviour. When the N level is reduced to very low levels, 0˙001%N, no grain refinement in the γ occurs in the 0˙02% and 0˙1%C steels at the 0˙2%Al level. The solid solution hardening effect of Al now dominates the properties with the impact transition temperature remaining constant due to Al refining the grain boundary carbides.

Approximation of Hydrogen Induced Delayed Fracture of Overlaid Cladding in Pressure Vessels Steel Structure

Distribution and electronic states of interstitial hydrogen atoms in the iron lattice are discussed in regard to their influence to the lattice bonding, and theory of hydrogen induced delayed fracture of steel based upon it is proposed. Characteristics of delayed fracture are accordingly well accounted for. Keyword: delayed fracture; cladding; hydrogen; steel; dislocation.

Glow Discharge Plasma Nitriding of AISI 304 Stainless Steel

Glow discharge plasma nitriding of AISI 304 austenitic stainlesssteel has been carried out for different processing time underoptimum discharge conditions established by spectroscopic analysis.The treated samples were analysed by X-ray diffraction (XRD) toexplore the changes induced in the crystallographic structure. TheXRD pattern confirmed the formation of an expanded austenite phase(γN) owing to incorporation of nitrogen as aninterstitial solid solution in the iron lattice. A Vickersmicrohardness tester was used to evaluate the surface hardness as afunction of indentation depth (μm). The results showed clearevidence of surface changes with substantial increase in surfacehardness.

Technique for measuring angular correlations and g-factors of excited states with large multi-detector arrays: An application to neutron rich nuclei produced by the spontaneous fission of 252Cf

Triple coincidences between prompt γ-rays emitted in the spontaneous fission of 252Cf were measured with Gammasphere. These data are used to measure the angular correlation of cascades of γ-rays from excited states of neutron rich fission fragments stopped in an unmagnetized iron foil. The hyperfine fields in the iron lattice cause attenuations of the angular correlations between γ-rays emitted from the excited states which have sufficiently long lifetimes. This attenuation is measured and used to calculate the g-factors of excited states in many neutron rich nuclei.

Structural and compositional changes during mechanical milling of the Fe–Crsystem

The structural and compositional changes during mechanical milling of Fe–Cr powder(10 wt% Cr) were followed by scanning electron microscopy (SEM), atomic forcemicroscopy (AFM), x-ray diffraction (XRD), Mössbauer spectroscopy and magnetizationmeasurement. All these techniques give complementary information about the alloyingprocess during milling. Different particles are cold welded and elongated to formcomposite granules in the initial stage of milling (5–10 h). The crystalline order isalso destroyed heavily and nanosize crystallites are formed during early milling.Chromium diffusion in the iron lattice starts somewhat late (after 20 h of milling) whencomposite granules flatten, reducing the distance between iron and chromium layers.Though compositional homogeneity is achieved up to nanometre scale in 65 hof milling, atomic diffusion of the species still continues up to 100 h of milling.

On the mechanism of formation of zinc pack coatings

The mechanism of the formation of zinc (Zn) pack coatings is studied with scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). DSC showed that the coatings formation takes place in three steps. The initial step (at 193.9 °C) is endothermic and involves the transformation of α-NH 4Cl to β-NH 4Cl and the NH 4Cl decomposition to NH 3 and HCl. During the second step (at 248.6 °C), which is exothermic, Zn 2+ salts are formed and especially ZnCl 2. Finally at 264.1 °C Zn is deposited by an endothermic reaction on the ferrous substrate through the decomposition of ZnCl 2. The as-cast Zn diffuses into the iron lattice forming the gamma (Γ-Fe 11Zn 40) and delta (δ-FeZn 10) phases. Al 2O 3 is not involved in the above-mentioned mechanism and acts only as filler.

Zerovalent Irons: Styles of Corrosion and Inorganic Control on Hydrogen Pressure Buildup

Apparent corrosion rates have been measured for several commercially available zerovalent irons by monitoring hydrogen evolution in closed cells. Sievert-type rate constants (ks) were determined to account for hydrogen entering the iron lattice. Thus corrected corrosion rates (Rcorr) are provided for all irons tested in this study. Because the rate of hydrogen entering the iron lattice increases with PH2(1/2), and the rate of hydrogen production from corrosion, far from equilibrium conditions, is independent of PH2, at some time under closed system conditions the two rates become equal and a steady-state PH2 is attained. A relation describing this condition has been derived: PH2SS = [Rcorr/ ks]2 For the granular irons considered in this study, PH2SS values vary from less than one to eight bars, in contrast to the calculated thermodynamic equilibrium PH2 values for anaerobic corrosion, which range from 138 to 631 bar depending on the assumed product of corrosion. Because groundwater flow at an iron permeable reactive barrier removes hydrogen gas in the dissolved state, PH2SS values will be less than calculated using the relation above. A method is presented to calculate PH2 values along the flow direction in a PRB, and thus the maximum PH2 value that can possibly develop, assuming no bacterial utilization of the produced hydrogen.

CLUSTER-SIMULATED DISLOCATION CORE: MOTION AND TRAP FOR IMPURITY ATOMS

The electronic structures of a cluster-simulated edge dislocation core in bcc iron were computed using the discrete variational method of density functional theory Energy calculations show that the iron lattice and the impurity atoms trapped in the core have resistance to the dislocation motion. The ubiquitous P -embrittlement of iron can be explained electronically.

Grain boundary impurities in iron

Impurities segregating to grain boundaries can be responsible for embrittlement or occasionally can strengthen a material. We have modelled the effects of B, C, S and P impurities at a (2 1 0) tilt boundary in Fe using first-principles quantum mechanical calculations allowing for full atomic relaxation. The grains can either be pushed apart or pulled together depending on the size of the impurity and the nature of local relaxations which are different for impurities at substitutional and interstitial sites. Calculations of local densities of states indicate that charge transfer is not significant in explaining changes in materials properties. Cohesive energy calculations indicate that interstitial sites are preferred to substitutional sites and that B leads to cohesive enhancement while P and S are strong embrittlers and C a weak embrittler. If carbon instead forms a carbide phase at the boundary, closely matching the bulk iron lattice, there will be cohesive enhancement.

XPS identification of bulk hole defects and itinerant Fe 3d electrons in natural troilite (FeS)

XPS measurements have been made of vacuum-fractured monoclinic pyrrhotite and troilite that highlight sulfur coordination (S 2p) and iron electronic state (Fe 2p) differences between these two minerals as a result of iron lattice site occupancy. Metal-like Fe states are observed in the Fe 2p spectrum of troilite due to increased occupancy of Fe lattice sites and increased Fe-Fe 3d orbital interaction within the constraints of the mineral structure. Additional electron population near the top of the troilite valence band is confirmed by XPS measurement. These results provide comprehensive experimental evidence for itinerent 3d electrons in the natural iron sulfides.

Heterometallic two-dimensional polymer containing cyanide-bridged praseodymium(III) and transition metals

A heterometallic cyanide-bridged 3d–4f heterometallic coordination polymer [Pr(DMF)4(H2O)3.5Fe(CN)6]·H2O (DMF = N,N′-dimethylformamide) has been synthesized and characterized by X-ray diffraction analysis. It crystallizes in the monoclinic space group P2(1)/n with cell parameters: a = 17.669(5) A3, b = 8.927(3) A3, c = 19.967(6) A3, β = 95.807(5)°, V = 3133.2(16) A3, Z = 4. Multiple hydrogen bonds extend the dinuclear units into a novel 2-D topological architecture of an irregular triangular lattice of iron and praseodymium ions. The magnetic properties have been investigated.

The magnetic moment of Ln 3Fe 5O 12 garnets

The calculation of the magnetic moment of Ln 3Fe 5O 12 garnet is made in the framework of Dirac's theory. The coupling between the light and heavy rare earths and the iron lattice moments is the same but with a total contribution of the latter of 2.80 μ B for europium and light rare-earth garnets and 5.00 μ B for the others.

A note on Dr. J.L. Snoek

Abstract J.L. Snoek is the eponym of the renowned relaxation effect: the Snoek peak, which is associated with the redistribution of carbon atoms in the bcc iron lattice under the application of the oscillatory stress. A brief review of the Snoek effect and a sketch of Snoek’s life are given; the latter is based on two Dutch articles.

Susceptibility of Steels Exploited in Water-Steam Environments to Hydrogen-Induced Cracking

We perform the detailed investigation of 10Kh2M and 12Kh1MF ferrite-pearlite steels operating in water-steam environments in different boilers and power plants in Poland and Ukraine for up to 240,000 h with an aim of evaluation of their susceptibility to hydrogen-induced cracking (blistering). For both types of steel, we establish similar regularities of the influence of operating conditions on the properties of materials. An abrupt deterioration of the mechanical properties of materials was observed after 110,000 h of operation. It was accompanied by an increase in the concentration of residual hydrogen and pronounced changes in the parameters of the crystal lattice of iron as well as in the volume fraction and morphology of the pearlite phase. The results of hydrogen-permeation tests were used to choose the parameters characterizing the susceptibility of steels to blistering. For both types of steel, we determine the correlation between their susceptibility to blistering and changes in the microstructure in the process of operation.

Microstructure and magnetostriction of Fe 1− xTb x alloys prepared by solid-state synthesis

Fe 1− x Tb x was mechanically alloyed over a range of compositions 0.1< x<0.9. Based on X-ray diffraction measurements, a complete diagram of the resulting phases was drawn as a function of alloying time and composition. Depending on the composition, the microstructure of the as-milled powder is nanocrystalline or amorphous, with solubility ranges differing from those observed in the thermodynamically equilibrium structures. Terbium is soluble in the iron lattice with 0< x<0.33. A single-phase amorphous structure was isolated with x=0.33, and a single-phase Laves Fe 2Tb structure with 0.4< x<0.5. At higher terbium concentrations, a mixture of Fe 2Tb and terbium was obtained. The magnetic properties of the mechanically alloyed binary Fe 2Tb were studied in more detail. The mechanically alloyed binary Fe 2Tb powders were cold compacted and annealed. The highest magnetostriction was measured after annealing the samples at 500°C, resulting in a crystal size of 7–8 nm. Without an external load, the saturation magnetostriction λ s for the nanocrystalline Fe 2Tb was measured to be 1225×10 −6. This is 30% higher than that of the zone-melted coarse-grained Terfenol-D with the composition of Fe 2Tb 0.3Dy 0.7. When a constant magnetic field was applied perpendicular to the primary driving magnetic field, the saturation magnetostriction λ s of the nanocrystalline Fe 2Tb sample was measured to be 1582×10 −6. This value is 80% higher than the magnetostriction in unstressed zone-melted Terfenol-D.

Structural characterization of Fe(1 1 0) islands grown on α-Al 2O 3(0 0 0 1)

The morphology, size distribution, epitaxial relationships and lattice distortion in nanometer sized Fe(1 1 0) oriented islands grown on Al 2O 3(0 0 0 1) are studied and discussed as a function of the amount of deposited Fe. Combined scanning electron microscopy and transmission electron microscopy (TEM) as well as atomic force microscopy measurements show the dependence of these parameters with island size. For the smallest island sizes, an iron lattice distortion with respect to bulk values is found at their edges. Electron energy loss spectroscopy experiments demonstrate the absence of oxidation of the Fe islands either due to the high temperature deposition on Al 2O 3 or to the polishing procedure prior to TEM observations. From the analysis of the relative intensity for the L 2 and L 3 Fe peaks, a high spin ground state for the Fe atoms is deduced, probably correlated with the highly distorted regions at the edges of the islands.

Boron solubility in Fe–Cr–B cast irons

Boron solubility in the as-cast and solution treated martensite of Fe–Cr–B cast irons, containing approximately 1.35 wt.% of boron, 12 wt.% of chromium, as well as other alloying elements, has been investigated using conventional microanalysis. The significant microstructural variations after tempering at 750 °C for 0.5–4 h, compared with the original as-cast and solution treated microstructures, indicated that the matrix consisted of boron and carbon supersaturated solid solutions. The boron solubility detected by electron microprobe was between 0.185–0.515 wt.% for the as-cast martensite and 0.015–0.0589 wt.% for the solution treated martensite, much higher than the accepted value of 0.005 wt.% in pure iron. These remarkable increases are thought to be associated with some metallic alloying element addition, such as chromium, vanadium and molybdenum, which have atomic diameters larger than iron, and expand the iron lattice to sufficiently allow boron atoms to occupy the interstitial sites in iron lattice.

An ab initio model of electron transport in hematite (α-Fe2O3) basal planes

Transport of conduction electrons through basal planes of the hematite lattice was modeled as a valence alternation of iron cations using ab initio molecular orbital calculations and electron transfer theory. A cluster approach was successfully implemented to compute electron-transfer rate-controlling quantities such as the reorganization energy and electronic coupling matrix element. Localization of a conduction electron at an iron lattice site is accompanied by large iron–oxygen bond length increases that give rise to a large internal component of the reorganization energy (1.03 eV). The internal reorganization energy calculated directly is shown to differ from Nelsen’s four-point method due to the short-range covalent bridge interaction between the Fe–Fe electron transfer pair in the hematite structure. The external reorganization energy arising from modification of the lattice polarization surrounding the localization site is predicted to contribute significantly to the total reorganization energy. The interaction between the reactants and products electronic states near the crossing-point configuration is 0.20 eV and is consistent with an adiabatic electron-transfer mechanism. Electron transfer is predicted to possess a small positive activation energy (0.11 eV) that is in excellent agreement with values deduced from conductivity measurements. Measured electron mobility can be explained in terms of nearest-neighbor electron hops without significant contribution from iron atoms further away. Comparison of the predicted maximum polaron binding energy with the predicted half bandwidth indicates compliance with the small-polaron condition. Therefore the localized electron treatment is appropriate to describe electron transport in this system.

The Influence of Carbon Diffusion on the Character of the .GAMMA.-.ALPHA. Phase Transformation in Steel

During the austenite-ferrite phase transformation the excess amount of carbon that can not be dissolved in the growing ferrite phase accumulates in the austenite phase ahead of the moving interface. This local carbon enrichment along the interface in the austenite decreases the driving force for transformation. Two processes therefore determine the actual interface velocity: the transformation rate of the iron lattice from fcc to bcc and the diffusion of the carbon along its gradient into the austenite grain. If the diffusion of the carbon is the rate-determining process, the transformation occurs in the diffusion-controlled mode. If the rate of transformation is determined by the lattice transformation, this is called the interface-controlled or mean-field mode. When the transformation rate is not determined by a single process, this is referred to as the mixed mode of transformation.The mixed-mode character can be identified by the carbon concentration in the austenite at the α-γ interface, xCγ, γα. If xCγ, γα is close to the concentration given by the local α-γ equilibrium, the character of the transformation is predominantly diffusion controlled. On the other hand, if diffusion is relatively fast, xCγ, γα is close to the average concentration in the austenite, and the transformation has a interface-controlled character. Due to the build-up of concentration gradients, the transformation character can gradually change during the transformation. In this paper, a two-dimensional study of a mixed-mode transformation is performed for Fe-C alloys of different carbon contents, and the character of the transformation is quantitatively determined.