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.     

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.     

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.     

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.     

The embrittlement and de-embrittlement of grain boundaries in an Fe-Mn-Ni alloy due to grain boundary segregation of Mn

A ductile-brittle-ductile (DBD) transition behavior in an age-hardenable Fe-8Mn-7Ni alloy has been analyzed in light of segregation and desegregation of alloying elements at prior austenite grain boundaries. The DBD transition in the alloy can be distinguished by two C-type curves: one corresponding to the start of zero tensile elongation and the other to the finish. The activation energies for ductile-to-brittle and brittle-to-ductile transitions are in close agreement with that for age hardening. Manganese content at the prior austenite grain boundaries was analyzed by Auger electron spectroscopy, and intergranular fracture strength at the brittle fracture region showed inverse trends with Mn concentration at the grain boundaries. All these observations strongly suggest that manganese segregation and its desegregation are responsible for the DBD transition of this alloy. Formerly Graduate Student, Department of Metallurgical Engineering, Seoul National University     

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.     

The influence of grain boundary sulfur concentration on the intergranular brittleness of nickel of different purities

When systematically combined, tensile tests at -150 °C and analysis by AES (Auger Electron Spectroscopy) or by electron probe reveal the mechanical and chemical effects of sulfur in the grain boundaries of nickel of different purities. The sulfur concentration in the grain boundaries of nickels containing globally less than 450 ppm S appears to be the essential factor in the grain boundary brittleness of the metal. Other elements may however interfere with the sulfur in the mechanical characteristics of the metal, either by increasing or decreasing the brittleness according to the particular case. They also interfere with the sulfur by slowing down the kinetics of the approach to equilibrium. Formerly Research Associate, Département de Métallurgie, Ecole Nationale Supérieure des Mines de Saint-Etiénne Formerly Professor, Département de Métallurgie, Formerly Professor, Département de Métallurgie,     

The role of grain boundary solutes and structure on the yielding and intergranular cracking of iron

The effect of interstitial solute concentration and partitioning, and grain boundary extrinsic structure on the magnitude of the Petch parameters and frequency of hydrogen-induced intergranular cracking was systematically investigated in both decarburized iron and Fe-0.15 pct Ti. It was established that grain boundary interstitial solutes increase the value of the petch slope and the presence of these solutes in the lattice increases the value of the “friction” stress. Their presence in the grain boundary also increases the frequency of intergranular cracking. This is not true for extrinsic grain boundary ledges, whose presence does not appear to effect either the Petch parameters or hydrogen-induced cracking. The observed results were rationalized by the behavior of interstitial solute complexes at the boundary and the ability of such complexes to react with hydrogen.     

Equilibria between rare earth elements and sulfur in molten iron

The equilibrium constants of Ce-S, La-S, and Nd-S in molten iron and the related interaction coefficients were determined experimentally and the following results obtained: K_CeS = 2.80 × 10^-6 e_S ^Ce = −1.88 K_LaS = 7.41 × 10^-7 e_S ^La= −1.51 K_NdS = 2.57 × 10^-6 e_Nd ^S= −1.51 In these experiments a radioactive isotope technique was used to determine the extremely low contents of the rare earth elements in the metal phase at equilibrium. In order to avoid errors caused by any partial inclusion of RE content of nonmetallic inclusions in the analytical results for the dissolved RE, the metal samples were electrolyzed in an organic electrolyte at low temperature, and the dissolved rare earth contents determined by measuring the radioactivity of the electrolyte. The dependence of the equilibrium values (concentration products) Ce-S on [pct C] in the system Fe-Ce-S-C was also determined. The interaction coefficient e_Ce ^C was determined to be −0.43. Earlier workers have obtained higher values for the Ce-S and La-S equilibrium constants than the present authors. Some explanation is offered for the difference.     

Kinetics of sulphur segregation at grain boundaries and the mechanical properties of nickel

Abstract

The prior Petch study by the authors of segregated Ni-S alloys containing <1-50 ppm S is extended here to examination of the kinetics of grain boundary hardening and of the effects of thermal history in these same alloys. It is demonstrated that the kinetics of segregation as inferred from boundary hardening are quite complex and much too rapid to be accoun ted for by normal volume diffusion. However, segregation kinetics become sufficiently slow below room temperature that quenching to, and testing at, cryogenic temperatures permit comparison of the tensile properties of bulk specimens of the nickel alloys in the condition with sulphur dispersed and that with sulphur segregated near the grain boundaries. Even the purest nickel tested (<1 ppm S) exhibits a significant increase in reduction-in-area and an increase in the .2% flow stress in the dispersed state relative to the segregated state. The present results are shown to be most fully consistent with a vacancy assisted, non-equilibrium segregation model rather than one of precipitation or equilibrium segregation.

Résumé

Pour faire suite à leur précédente étude de la relation de Petch dans les alliages Ni-S contenant 0–50 ppm, les auteurs ont étudié la cinétique de durcissement des joints de grains et les effets de l'histoire thermique dans ces meme alliages. La cinétique de ségrégation telle que déduite du durcissement du joint est quelque peu complexe et beaucoup trop rapide pour être interprétée en termes de diffusion volumique normale. Cependant, au-dessous de la température ambiante, la cinétique de ségrégation devient suffisamrtlent lente pour que, par trempe et essais à des tempéraptures cryogéniques, une comparaison puisse être établie entre les propriétés en traction D'échantillons d'alliages de nickel soit con tenant du soufre sous forme dispersée soit contenant du soufre ségrégé prés des join ts de grains. Même le nickel le plus pur (S < 1 ppm) révèle une augmentation significative de la réduction de section et une augmentation de la limite d'élasticité à 0.2% quand le soufre est à l'etat disperse plutot qu'a l'état ségrégé. Ces résultats se révèlent beaucoup plus compatibles avec un modèle de ségrégation de non-équilibre assistée par les lacunes plutôt qu'avec un modèle de ségrégation d'équilibre ou un modèle de précipitation.