Site competition between sulfur and carbon at grain boundaries and their effects on the grain boundary cohesion in iron

Metallurgical and Materials Transactions A - Grain boundary segregation in iron-sulfur-carbon alloys containing up to 100 wt ppm sulfur and up to 90 wt ppm carbon has been investigated with Auger...     

Site competition between sulfur and carbon at grain boundaries and their effects on the grain boundary cohesion in iron

Grain boundary segregation in iron-sulfur-carbon alloys containing up to 100 wt ppm sulfur and up to 90 wt ppm carbon has been investigated with Auger electron spectroscopy (AES). The results show the site compctition on grain boundaries between the segregation of sulfur and carbon. The segregation energy of sulfur is estimated to be 75 kJ/mol. Impact tests of these alloys were carried out. Iron-sulfur alloys with less than 20 wt ppm carbon fractured by the intergranular mode with high ductile-brittle transition temperatures (DBTT’s). Addition of up to 90 wt ppm carbon to the binary alloys prevented the intergranular fracture caused by the grain boundary segregation of sulfur, and decreased the DBTT. Carbon, when segregated to grain boundaries, drives sulfur away from the boundaries and also increases the grain boundary cohesion. The DBTT values of the iron-sulfur-carbon alloys are analyzed in terms of the degree of grain boundary segregation of sulfur and carbon. It is shown that sulfur decreases the grain boundary cohesion of iron more severely than phosphorus if compared at the same degree of grain boundary segregation. Formerly Graduate Student     

The importance of grain boundary morphology and cohesion on intergranular strength

Abstract

The morphology and constituency at grain boundaries are important in determining the properties of a poly crystalline material. In the present work it is shown that sulphur contents greater than 20 at. ppm. can embrittle nickel and nickel-base superalloys when the sulphur segregates to the grain boundaries and reduces intergranular cohesive strength. Moreover, low ductility can occur in nickel-base superalloy structures with controlled impurity levels when high stress concentrations develop in the grain boundary regions, either because of massive grain boundary shear or because of concentrated slip near grain boundaries.

Résumé

La morphologie et la constitution des join ts de grains sont importantes pour la détermination des propriétés de matériaux poly cristallins. Dans cette étude les auteurs montrent que des concentrations de soufre supérieures à 20 ppm. at. peuvent fragiliser le nickel et des superalliages à base de nickel quand le soufre ségrège aux joints de grains et réduit la cohésion intergranulaire. De plus, dans les superalliages à concentration contrôlée d'impuretés, une faible ductilité peut apparaître quand de fortes concentrations de contraintes se développent aux joints de grains soit sous l'effet de cisaillement massif du joint soit sous l'effet d'un glissement concentré aux joints.     

Surface and grain boundary segregation on and in iron

The equilibrium segregation A (dissolved) = A (segregated), where A = C, Si, Sn, N, P, O, S … is studied for binary systems Fe-A in the stability range of the α-solid solution using surface-analytical methods. On the surfaces ordered structures were observed by LEED and surface concentrations were determined by AES – in dependence on bulk concentrations and temperature. The chemical binding state was characterized by XPS. In grain boundaries the segregation was determined by AES after intergranular fracture of samples. A survey is given of the existing results.     

Effect of grain boundary chemistry on the intergranular fracture of iron at cathodic potentials

The percent intergranular fracture and tensile ductility of iron tested at cathodic potentials in IN H₂SO₄ was found to depend primarily on the grain boundary sulfur concentration. Zone refined and vacuum melted iron with different bulk sulfur, oxygen, and nitrogen but similar carbon concentrations were evaluated. The grain boundary chemistry was measured by Auger Electron Spectroscopy and the fracture mode and ductility by uniaxial straining electrode tests at potentials of -0.60 to -2.0 V (SCE) in IN H₂SO₄. The tensile ductility, as measured by the total strain and reduction of area, of both irons decreased with increasing cathodic potentials. The fracture mode and ductility at a potential of - 0.6 V (SCE) was related to the grain boundary sulfur concentration with increasing sulfur resulting in an increasing percent intergranular fracture and a decreasing ductility. The fracture mode and ductility was not related to the grain boundary oxygen, nitrogen or carbon concentrations but large bulk nitrogen concentrations did promote cleavage and quasicleavage fracture.     

Cavity nucleation at particles on sliding grain boundaries. A shear crack model for grain boundary sliding in creeping polycrystals

The paper summarizes the results of classical nucleation theory applied to the nucleation of creep cavities at hard second-phase particles. Stress concentrations at particles in sliding grain boundaries are analysed for the cases that the particles can be circumvented by diffusion and power-law creep and for purely elastic material response. The shear stress which is effectively transmitted through a sliding grain boundary in a polycrystal is calculated using a shear crack model. Comparison of the results with finite element calculations for a planar, hexagonal array of grains shows that the accuracy of the model is satisfactory. The duration of elastic transients is estimated and it is found that, in the numerical examples considered, stress relaxation times are shorter than the incubation time for cavity nucleation. From the calculated stress concentration factors it is concluded that cavities cannot be nucleated by the considered mechanism of vacancy condensation unless the nuclei have a narrow, rather than spherical, shape.     

Development of (110) [001] texture in iron by diffusion of sulfur into grain boundaries

Secondary recrystallization selective to a (110)[00l] texture was obtained in 0.15 mm thick iron sheet processed in the α-phase and having final cold reductions in the range between 50 to 70 pct. Grain boundary inhibition was induced by the addition of elemental sulfur to an inert separator. The diffusion of sulfur, being more rapid in the primary grain boundaries than in bulk iron, produced inhibition by solute-induced grain boundary restraint. Improved magnetic properties were obtained by comparison to iron textured by the conventional MnS process.     

Hydrogen detrapping from grain boundaries and dislocations in high purity iron

Hydrogen detrapping in high purity iron was studied by measuring evolution rates of quenched-in hydrogen from 80 to 800 K using a quadrupole mass spectrometer in an ultra high vacuum system. The peak of the evolution rate was observed at 395 K in single crystal specimens and 415 K in polycrystalline specimens with a heating rate of 1 K min⁻¹. Effects of grain size and deformation on the evolution rate was also studied. It was shown that the results are consistent with the evolution rates calculated with the binding energy B = 0.51 ± 0.02 eV and the trap density term γC_{T = (4 ∼ 15) × 10⁻⁵} in polycrystalline iron, and B = 0.47 ± 0.02 eVand γC_T = (2 ∼ 13) × 10⁻⁵ in single crystal iron. The dominant traps are considered to be grain boundaries in polycrystalline specimens and dislocations in single crystal specimens.     

Grain boundary segregation of sulfur and antimony in iron alloys

A correlation between sulfur and antimony grain boundary segregation has been observed on inter-granular surfaces of iron by Auger electron spectroscopy (AES). The slope of a plot of S/Sb indicated a ratio of two antimony atoms per sulfur atom arriving at the grain boundary, while the ratio for the total S/Sb at the grain boundary was about 1.2. These results were obtained with Fe, Fe + 0.07Mn, Fe + 0.03Sb, Fe + 0.1Mn + 0.02Sb, and Fe + 0.1Mn + 0.05Sb (at. pct) alloys. Possible expla-nations for this correlated segregation, such as cosegregation of sulfur and antimony, precipitation of a thin layer of antimony sulfide, and compctitive segregation with carbon and nitrogen, were evalu-ated using AES, X-ray photoelectron spectroscopy (XPS), and scanning transmission electron mi-croscopy with energy-dispersive X-ray (STEM-EDS). The results of these analyses indicated that there was no resolvable antimony sulfide phase in the grain boundary and that S and Sb were not chemically bound at the grain boundary in a two-dimensional phase. The S was shown to be strongly bound to the iron in a chemical state close to that of an iron sulfide, but the Sb was in the elemental state. Nor could this correlated segregation be satisfactorily explained by a cosegregation process nor by compctitive segregation with other elements. The most plausible explanation appears to involve the effect of sulfur on the activity/solubility of antimony or antimony on the activity/solubility of sul-fur, as explained by an increase in the ratioX _c /X _Co in the Brunauer-Emmett-Teller (BET) adsorption isotherm adapted for equilibrium segregation in solids.     

Striation and faceting of grain boundaries in nickel due to sulfur and other elements

As is the case for free surfaces, the grain boundaries of metals may show striation and faceting as a result of the effect of certain elements. This is true for nickel containing sulfur, tellurium and bismuth, and for copper containing bismuth. In all of the cases, the phenomenon occurs only when the grain boundary concentration of the element concerned approaches saturation, and material transport is taking place towards or away from the grain boundaries. Thus, intergranular striation and faceting are telltale signs of a non-equilibrium segregation. Formerly Research Associate, Département de Métallurgie, Ecole Nationale Supérieure des Mines de Saint-Etienne Formerly Professor, Département de Métallurgie, Ecole Nationale Supérieure des Mines de Saint-Etienne.     

Dissociation of grain boundaries induced by changes of composition,the ejection of dislocations from grain boundaries,and the nucleation of diffusion induced grain boundary migration

We present various observations of the dissociation of large and small angle grain boundaries in copper during the alloying of the specimens with zinc. In all large angle cases, this dissociation takes the form of the large angle boundary reacting to form another large angle boundary and a small angle boundary. This process may be of great importance in understanding the nucleation of diffusion induced grain boundary migration. Small angle boundaries can undergo complete disintegration upon the addition of zinc, and the magnitude of the stress involved in such a disintegration is estimated.     

The effects of hydrogen and segregation on fatigue crack nucleation at defined grain boundaries in nickel bicrystals

In order to explore the role of impurity segregation in intergranular fatigue crack initiation and propagation, tests have been made on nickel bicrystals, variously heat-treated to induce increasingly severe degrees of sulfur segregation, and using air, vacuum and hydrogen as the environment. The grains of the bicrystals were oriented to yield two types of boundaries: type I, with strong elastic-plastic incompatibility and type II, a compatible tilt boundary. If the boundaries were clean, persistent slip band cracking in the grains occurred in preference to intergranular cracking. Although equal degrees of sulfur segregation, as measured by Auger-spectroscopy, could be produced at the two types of boundaries by the heat treatments, the incompatible one was much more susceptible to intergranular cracking than the other, which could be made to crack intergranularly only by high partial pressures of hydrogen. The results show that high stresses associated with incompatibility coupled with a lowering of cohesive forces at the boundary produced by the segregant are the main factors controlling intergranular fracture.     

Precipitation at grain boundaries in the commercial alloy Al 7075

Grain boundary precipitation in a commercial alloy 7075 Al has been studied using transmission electron microscopy. The crystallography of the precipitates and the matrix is characterized in terms of the misorientation between the grains and the orientation of the grain boundary. The only particles observed at both low and high angle boundaries were precipitates of the equilibrium η phase. Precipitation tends to occur by the establishment of a crystallographic orientation relationship with respect to one grain. A single variant is normally found at a boundary at which copious precipitation occurs; this is frequently the case at low angle boundaries. The grain boundary orientation plays the decisive role on the nucleation of the precipitates. Whenever the boundary plane is close to the habit plane of particular variant, copious nucleation of that variant occurs. The grain boundary orientation also plays an important role in precipitate growth through its influence on the mobility of the nucleus-matrix interface, which strongly depends on the orientation of the interface.     

Equilibrium between sulfur and titanium in liquid iron

This investigation of liquid Fe-Ti-S alloys has involved experimental measurements of the titanium sulfide solubility product over the temperature range 1560 °C to 1640 °C. In addition, the sulfur-titanium interaction parameter was measured and the sulfide stoichiometry was determined. Using literature values for the sulfur and titanium self-interaction parameters, the solubility product for the equilibrium sulfide, TiS, was determined to be 0.411 at 1600 °C. The sulfur-titanium interaction parameter was determined to be −0.20 at 1600 °C. Comparisons with previously reported studies are presented.     

Plastic creep flow effects in the diffusive cavitation of grain boundaries

We analyze the growth of cavities along grain interfaces by the combined processes of grain boundary diffusion and plastic dislocation creep in the adjoining grains. It is shown that the coupling between the processes can be expressed in terms of a parameter L, which has the dimensions of length and which is a function of material properties, temperature and applied stress; L decreases with increasing temperature and stress and has, e.g., values in the range of 0.25 to 25 μm for various pure metals when stressed to 10⁻³ × shear modulus at 0.5 T_m. The contribution of dislocation creep to the cavity growth rate is shown to be negligible when L is comparable to or larger than the cavity spacing, but significant interactions occur, leading to growth rates very much in excess of those predicted on the basis of boundary diffusion alone, when L is comparable to or smaller than the cavity size. The coupling occurs because extensive dislocation creep allows local accommodation of matter diffused into the grain boundary from the cavity walls.The cavity growth rate is analyzed by formulating a new variational principle that governs combined processes of grain boundary diffusion and non-linear viscous creep, and by implementing this principle through the finite-element method to obtain numerical solutions. Results for the cavity growth rate are presented for a wide range of ratios of L to cavity spacing, and of cavity radius to spacing. Also, results are presented for the total growth time of cavities from an initial size to final coalescence.