A model for the initiation of hydrogen embrittlement cracking at notches in gaseous hydrogen environments

A model for the initiation of hydrogen embrittlement cracking in gaseous hydrogen environments is presented. The model is based on the stress-induced diffusion of hydrogen atoms to the regions of high triaxial stress ahead of a plastically strained notch. The influence of yield stress and notch geometry on the apparent threshold stress intensity for embrittlement are considered and derived analytically. The time dependence for crack initiation for apparent stress intensities above the threshold is derived from a simple diffusion model. The results of the model are in agreement with reported hydrogen embrittlement phenomena.     

Environmental hydrogen embrittlement of an α-β titanium alloy: Effect of microstructure

Environmental hydrogen embrittlement of a Ti-6 Al-4 V alloy has been studied as a function of test displacement rate and of variations inα- β microstructure. Embrittlement in low pres sure (∼1 atm) gaseous hydrogen was inversely dependent on test displacement rate and strongly dependent on microstructure. At a given displacement rate, microstructures having a continuous α-phase matrix were less severely embrittled than those having a continuous β-phase matrix. Further, brittle fracture occurred in the former microstructures by transgranular cleavage and in the latter microstructures by intergranular separation. These observations are consistent with previous studies made on slow strain-rate embrittlement of hydrogen-charged titanium alloys and are explained in terms of relative hydrogen transport rates within the α-phase and β-phase titanium.     

A new model for hydrogen-assisted cracking (hydrogen “embrittlement”)

A new model is presented for hydrogen-assisted cracking (HAC) which explains the observations of decreasing microscopic plasticity and changes of fracture modes with decreasing stress intensities at crack tips during stress-corrosion cracking and HAC of quenched-and- tempered steels. The model suggests that the presence of sufficiently concentrated hydrogen dissolved in the lattice just ahead of the crack tip aids whatever deformation processes the microstructure will allow. Intergranular, quasicleavage, or microvoid coalescence fracture modes operate depending upon the microstructure, the crack-tip stress intensity, and the concentration of hydrogen. The model unifies several theories but shows how the stress-sorption and lattice embrittlement models are unnecessary. The model shows that planar pressure effects are necessary at low stress intensities and are necessary only to augment the driving force from the applied loads. The basic hydrogen-steel interaction appears to be an easing of dislocation motion or generation, or both.     

Synthesis of ultrafine particles of intermetallic compounds by the vapor-phase magnesium reduction of chloride mixtures: Part I. Titanium aluminides

A new chemical synthesis process for the preparation of intermetallic compounds has been developed. It involves the vapor-phase reduction of mixtures of constituent metal chlorides by magnesium vapor to produce intermetallic compounds in the form of fine powder. The advantages of the process include (a) the use of inexpensive raw materials, (b) low reaction temperatures, and (c) products in the form of fine particles. Part I describes the synthesis of titanium aluminide particles by this method, whereas Part II presents the synthesis of nickel aluminides particles. Although nickel aluminides can also be prepared by the hydrogen reduction of nickel chloride and aluminum chloride vapor mixtures, titanium aluminides cannot be produced by hydrogen reduction because of unfavorable thermodynamics. The effect of AlCl₃/TiCl₃ partial pressure ratio on the formation of different titanium aluminides was studied. A two-phase mixture containing 80 mol pct of TiAl+20 mol pct of TiAl₃ formed at an AlCl₃/TiCl₃ ratio of 10. The amount of TiAl₃ was maximized to 72 mol pct at an AlCl₃/TiCl₃ ratio of 16. The maximum conversion of the limiting chloride TiCl₃ was 94 pct. The product particles were very fine in the size range of 0.2 to 0.3 μm.     

Synthesis of ultrafine particles of intermetallic compounds by the vapor-phase magnesium reduction of chloride mixtures: Part II. Nickel aluminides

The new chemical synthesis process developed in this laboratory for the preparation of the fine powders of intermetallic compounds by the vapor-phase reduction of mixtures of constituent metal chlorides by magnesium vapor, described in Part I for titanium aluminides, was applied to the synthesis of nickel aluminide particles. NiAl, NiAl₃, and Ni₂Al₃ were formed by reducing mixtures of NiCl₂ and AlCl₃ vapors. The effect of the partial pressure ratios AlCl₃/NiCl₂ and Mg/NiCl₂ on the formation of nickel aluminides was studied. The maximum content of NiAl obtained was 98 mol pct. At the AlCl₃/NiCl₂ ratio of 19, a two-phase mixture of 17 mol pct of NiAl+83 mol pct of NiAl₃ was produced. The product particles were very fine in the size range of 0.1 to 0.2 μm.     

The intergranular embrittlement of nickel by hydrogen: The effect of grain boundary segregation

The mechanical behavior of polycrystalline nickel specimens that were deformed in tension and cathodically charged with hydrogen simultaneously was investigated with particular emphasis on the fracture of such electrodes. This procedure leads to definite, if, however, weak serrated yielding and also markedly reduces the elongation at fracture compared to polycrystals unexposed to hydrogen. Moreover, in contrast to hydrogenated nickel monocrystals which neck down to give a chisel-edge fracture typical of ductile metals, hydrogenated polycrystal fractures are brittle and intergranular. The embrittlement of nickel by hydrogen is shown by means of Auger electron spectroscopy to be associated with the segregation of hydrogen recombination poisons to the grain boundaries. In essence, it is suggested that the entry of hydrogen into the nickel specimens occurs preferentially in the proximity of grain boundary intersections with the free surface, due to the presence therein of Sb and Sn which act as hydrogen recombination poisons and stimulate the absorption of hydrogen by the metal. The presence of such impurities in the grain boundaries suggests that a pressure mechanism is not involved in the intergranular cracking.     

Autocatalytic model for the reduction of nickel chloride with hydrogen gas

The mechanism of hydrogen reduction of nickel chloride consists of formation of nuclei of nickel followed by the growth of these nuclei. The incubation period observed at the start of the reduction¹ is usually associated with nuclei formation. The growth of the nuclei corresponds to the acceleratory period.     

Thermodynamics of B2 intermetallic compounds with triple defects: a Bragg-Williams model for (Ni,Co)Al

Applying the Bragg-Williams approximation, a pair interaction model has been developed for describing the composition and temperature dependence of thermodynamic properties of ternary intermetallic B2 phases. Taking the activities and the integral enthalpies of formation as input data, values for the enthalpy and the entropy of bonds between atoms have been obtained. An analytical expression has been derived for the dependence of the concentration of point defects (vacancies and antistructure atoms) on composition and temperature. The model has been applied successfully to the available experimental data for the Ni-Al, Co-Al, and Ni-Co-Al systems. It has been shown that the values calculated accordingly for the effective enthalpy (and entropy) of formation of vacancies agree well with those from experimental observations and theoretical ab initio calculations.     

A critical-strain criterion for hydrogen embrittlement of cold-drawn, ultrafine pearlitic steel

A stress-modified, critical-strain model of fracture-initiation toughness has been adapted to the case of hydrogen-affected pearlite shear cracking, which is a critical event in transverse fracture of cold-drawn, pearlitic steel wire. This shear cracking occurs via a process of cementite lamellae failure, followed by microvoid nucleation, growth, and linkage to create shear bands that form across pearlite colonies. The key model feature is that the intrinsic resistance to shear-band cracking at a transverse notch or crack is related to the effective fracture strain at the notch root. This fracture strain decreases with the logarithm of the diffusible hydrogen concentration (C _H). Good agreement with experimental transverse fracture-initiation-toughness values was obtained when the sole adjustable parameter of the model, the critical microstructural dimension (l*), was set to the mean dimension of shearable pearlite colonies within this steel. The effect of hydrogen was incorporated through the relationship between local effective plastic strain (ɛ _eff ^f ) and C _H, obtained from sharply and bluntly notched tensile specimens analyzed by finite-element analysis (FEA) to define stress and strain fields. No transition in the transverse fracture-initiation morphology was observed with increasing constraint or hydrogen concentration. Instead, shear cracking from transverse notches and precracks was enabled at lower global applied stresses when C _H increased. This shear-cracking process is assisted by absorbed and trapped hydrogen, which is rationalized to either reduce the cohesive strength of the Fe/Fe₃C interface, localize slip in ferrite lamellae so as to more readily enable shearing of Fe₃C by dislocation pileups, or assist subsequent void growth and link-up. The role of hydrogen at these sites is consistent with the detected hydrogen trapping. Large hydrogen-trap coverages at carbides can be demonstrated using trap-binding-energy analysis when hydrogen-assisted shear cracking is observed at low applied strains.     

The low-temperature embrittlement of niobium and vanadium by both dissolved and precipitated hydrogen

Like other exothermic occluders of hydrogen, niobium (columbium) and vanadium may be embrittled by the presence of a precipitated hydride. However, alloys containing hydrogen concentrations well below the solubility limits at room temperature show very interesting features when tested in tension at subambient temperatures. Niobium containing 25 ppm H, from which little hydride precipitates above 77 K, shows a ductility minimum on cooling which is strain-rate-dependent and may be attributed to diffusion of hydrogen to microcrack nucleation sites. When the hydrogen content is increased a further reduction in ductility oc-curs at lower temperatures due to the martensitic precipitation of niobium hydride. The behavior displayed by V-H alloys is broadly similar, except that here the influence of hydro-gen diffusion overshadows the embrittling effect of the hydride precipitate. This means that the embrittlement associated with hydride precipitation is less easily distinguished from that involving hydrogen in solution in vanadium than in niobium. P. McINTYRE, formerly Research Student, Department of Metal-lurgy, The University of Newcastle upon Tyne     

Fuzzy logic model of lime enhanced hydrogen reduction of cuprous sulphide

Traditional methods for metal extraction from sulphide minerals cause environmental pollutions. In recent decades, direct hydrogen reduction in the presence of lime has attracted an increasing attention among metal-sulphide separation methods. This study aimed to represent an alternative technique for the modelling lime enhanced direct hydrogen reduction process with a fuzzy logic model. The focus of this paper is to find a well defined relationship among the vital variables including bulk temperature, ratio of molar quantities of the solid reactants, pellet diameter, porosity, sulphide particle size, weight loss of the pellet, sulphur fixation in the pellet, and reaction completion time using a fuzzy logic model scheme. Finally, the results predicted by the fuzzy logic model were evaluated and validated against a mathematical model and experimental data.

Les méthodes traditionnelles d’extraction du métal à partir de minéraux sulfureux engendrent une pollution environnementale. Dans les dernières décennies, on a porté une plus grande attention à la réduction directe de l’hydrogène en présence de chaux, parmi les méthodes de séparation du métal-sulfure. Cette étude avait pour but de représenter une technique de rechange pour la modélisation du procédé de réduction directe de l’hydrogène, augmentée par la chaux, au moyen d’un modèle de logique floue. Le point central de cet article est de trouver une relation bien définie parmi les variables vitales, incluant la température globale, le rapport des quantités molaires des réactants solides, le diamètre des boulettes, la porosité, la taille de particule du sulfure, la perte de poids de la boulette, la fixation du soufre dans la boulette et le temps d’exécution de la réaction en utilisant un schéma de modèle de logique floue. Finalement, on a évalué et validé les résultats prédits par le modèle de logique floue contre un modèle mathématique et des données expérimentales.