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.     

Temper embrittlement and intergranular segregation of antimony: A quantitative analysis performed with the backscattering of energetic ions

The intergranular segregation of antimony associated with temper embrittlement in a low carbon manganese steel was quantitatively studied through the backscattering of MeV¹²C¹²⁺ and¹²¹4N¹²⁺ ions. The principles of the technique and its application to interface segregation problems are briefly explained and its main advantages discussed. The influence of various heat treatments was investigated and shown to strongly influence the segregation taking place in the α field. Segregation could not be detected in the γ field. The kinetics of the phenomenon in the critical range (400° to 600°C) is described. The role of the micro-structure was studied and it is shown that segregation does not occur only at the previous austenitic grain boundaries but at all the disordered high angle boundaries of the structure. The grain boundary Sb content after a reversion and a resegregation treatment was also studied. The results are interpreted in terms of a reversible type of segregation taking place entirely in the α phase.     

On the mechanism of intergranular embrittlement by phosphorus in transformer steel

Transformer steel (Fe-6 at. pct Si) was doped with varying amounts of phosphorus and given an embrittling step-cool heat treatment. Auger electron spectroscopy was used to determine that large increases in intergranular phosphorus concentration occurred in approximate proportion to the bulk phosphorus level through an equilibrium segregation mechanism. Bicrystals of this material were fractured at 300, 77 and 4.2 K. Grain boundary fracture energy, γ^gb was determined as a function of intergranular phosphorus concentration at 4.2 K. An analysis of γ^gband fracture mode, as a function of temperature, was used to evaluate the relative merits of intergranular fracture models based on reduced interatomic separation energy (Gibbs-Griffith model) and reduced interatomic cohesive strength (Seah model). It was found that the reduced interatomic separation energy model best fits the experimental findings. Formerly of Clemson University.     

Gas phase embrittlement of nickel by sulfur

Nickel and nickel-base alloys are extremely sensitive to embrittlement and enhanced crack propagation due to sulfur, which may contaminate during melting, processing, or service. This paper reports the results of a systematic study of the effect of sulfur partial pressure and exposure temperature on intergranular penetration and tensile embrittlement in Ni270. For exposures between 450 °C and 900 °C, a maximum embrittlement was observed at about 700 °C. Partial pressures of sulfur exceeding about 5 × 10^-8 atm were embrittling. Kinetic measurements, based on the depth of postexposure intergranular fracture, gave an activation energy of 74 kJ/mol in the temperature range 450 °C to 800 °C. This value is considerably less than the value reported for matrix sulfur diffusion and also less than that reported for intergranular oxygen diffusion. Auger spectroscopy confirmed high local concentrations of sulfur at grain boundaries. In only one case, under the most severe sulfur exposure, was indirect evidence of sulfide formation observed; clearly, the embrittlement was a consequence of intergranular diffusion and segregation of elemental sulfur. A simple composite model for the tensile strength and ductility of partially embrittled specimens, in terms of sulfur penetration distance, satisfactorily accounted for the experimental measurements. Formerly with the Materials Engineering Department, Rensselaer Polytechnic Institute.     

Factors affecting the intergranular hydrogen embrittlement of Co3Ti

The influence of intergranular hydrogen embrittlement on the mechanical behavior and fracture behavior of Co₃Ti and 0.1 wt% B-doped Co₃Ti compounds were studied. The significant environmental effect was investigated at ambient temperatures. The elongation and tensile strength showed lower values in the sequence of vacuum, air and hydrogen conditions though the yield strength was insensitive to the environment. The remarkable strain rate dependence was also investigated. As the strain rate decreased, the elongation and tensile strength decreased in a sigmoidal form and again the yield strength remained constant. The decreases of the elongation and tensile strength were investigated with decreasing test temperature below 400°C at the lower strain rate. These embrittlements were severer in the nearstoichiometric alloys of Co-23 at.% Ti than in the off-stoichiometric alloys of Co-21 at.% Ti. The addition of 0.1 wt% B did not actually affect the mechanical behavior of undoped Co₃Ti compounds. Fracture behavior was fairly consistent with the mechanical behavior; there was a change in fracture mode from the transgranular fracture mode, through a mixed mode, to the intergranular fracture mode as the strain rate decreases, by the alternation from vacuum to hydrogen conditions and as the Co atoms become excess. The reduction of ductility and tensile strength occurred in the present Co₃Ti compound was suggested to be due to hydrogen-assisted intergranular embrittlement. The dynamic and atomistic mechanism by which the cohesive strength of grain boundary was affected by the hydrogen was proposed as the probable explanation.     

Grain boundary segregation of antimony and nickel in iron

This paper reports a study of grain boundary segregation of antimony and nickel in iron. These results show that nickel additions to the matrix increase antimony segregation. This observation is in agreement with previously reported work. Antimony additions to the matrix or segregation to the grain boundary have no measurable effect on nickel segregation. Decarburization increases segregation of both antimony and nickel.     

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.     

Tempered martensite embrittlement and intergranular fracture in an ultra-high strength sulfur doped steel

This paper reports a study of tempered martensite embrittlement in a Ni-Cr steel doped with 0.01 wt pct S. The segregation of sulfur to the grain boundaries and the associated embrittlement of this material are very dependent upon the austenitizing temperature. If the austenitizing temperature is below 1050 °C very little embrittlement and very little intergranular fracture are observed because sulfur remains precipitated as chromium sulfide. At higher austenitizing temperatures the sulfides dissolve and sulfur segregates to the grain boundaries. Because of the high bulk content, the sulfur concentration at the grain boundaries becomes great enough for the sulfides to reprecipitate there. This leads to low energy intergranular ductile fracture. However, some sulfur remains unprecipitated at the boundary and can lower the cohesive strength across the boundary. When plate-like cementite precipitates at the grain boundary during tempering heat treatments at 300 to 400 °C, the combination of the carbides and the unprecipitated sulfur causes intergranular fracture and tempered martensite embrittlement.     

The effect of stress state on the hydrogen embrittlement of nickel

The ductility of nickel sheet subjected to in situ cathodic hydrogen charging has been investigated over a range of multiaxial stress states including uniaxial, plane-strain, and equibiaxial tension. The data show that the extent of ductility loss due to the presence of hydrogen increases as the stress state tends from uniaxial to equibiaxial tension. In all instances, the hydrogen embrittlement is characterized by intergranular fracture with failure occurring due to microcrack formation, microcrack link-up, and macrocrack growth. The increased susceptibility of nickel to intergranular hydrogen embrittlement with increasing biaxiality of stress state is shown to be a consequence of an enhanced rate of the link-up of strain-induced intergranular microcracks.     

The effect of phosphorus segregation on the intermediate-temperature embrittlement of ferritic,spheroidal graphite cast iron

The objective of this article is to study the effect of phosphorus segregation on the fracture modes of the intermediate-temperature intergranular embrittlement which occur in ferritic, spheroidal graphite cast iron. The specimens were quenched from 820 °C and 500 °C during the furnace-cooling period of ferritization annealing in order to vary the degree of phosphorus segregation, then deformed in tension at various temperatures between 20 °C and 520 °C with a constant crosshead speed of 0.01 mm/s. These two kinds of specimens were also fractured by impact at about -50 °C in the vacuum chamber of a scanning Auger microscope in order to analyze the phosphorus segregation and compare the fracture modes. The results show that the fracture mode of the intermediate-temperature embrittlement is influenced by the history of heat treatment prior to tension. When the specimens were held at 500 °C and quenched from this temperature, the fracture was intergranular. However, the specimens quenched from 820 °C revealed cleavage fracture with cracks propagating radially from a central region with magnesium-rich particles. Identified by transmission electron microscopy (TEM), the particles were MgO. Grain-boundary segregation of phosphorus in the specimen held at 500 °C was confirmed by Auger analysis of the impact fracture surface. Segregation of phosphorus must play an important role in the fracture mode of the intermediate-temperature intergranular embrittlement.     

The effects of applied stress on the intergranular phosphorus segregation in a chromium steel

The mean Auger peak ratio (APR) of phosphorus, averaged over the intergranular fracture surface of each sample, has been measured on smooth and notched types of specimens, which were subjected to stress agings at 773 K for times up to 15 h under given stresses of tension or compression after a long term aging at 773 K under no stress. When the stress level for aging was smaller than a critical value, the mean APR of P of the smooth specimen under tension rapidly increased during the first l h aging and then decreased to around the initial value in 15 h of aging, whereas that of the compressed specimen decreased, nearly by the same magnitude as in the case of tension, during the first l h and then similarly approached to the initial value in 15 h of aging. The mean P-APR in the vicinity of the notch root was significantly small compared with those at positions distant from the notch root in a stress aged notched specimen.     

The role of sulfur in the air embrittlement of nickel and its alloys

A mechanism leading to the embrittlement of nickel and its alloys following high temperature air exposure is proposed. This mechanism involves the internal oxidation of sulfides to oxides, accompanied by a release of embrittling sulfur onto the grain boundaries. The mechanism is shown to work in a model system of nickel containing MnS precipitates, in which a ring of internal oxidation 250 μm in depth forms during 200 hours air exposure at 1000 °C. Auger analysis shows very high sulfur levels on grain boundaries within this region, but also reveals considerable sulfur concentrations beyond it. This massive release of free sulfur had the effect of rendering the alloy brittle over the entire temperature range investigated (25 to 1000 °C). The contribution of this mechanism to the known air embrittlement of pure nickel (Ni270) and a nickel base superalloy (IN738) is investigated. Although enhanced O/Ni peak height ratios were observed in the air exposed samples of both materials, the only significant sulfur concentrations were observed on the surfaces of grain boundary cavities formed in Ni270. However, the starting sulfur levels were extremely low in both cases, and the mechanism may contribute to high temperature air embrittlement in other systems. R.A. MULFORD, formerly with the General Electric Corporate Research and Development Laboratory, Schenectady     

The effect of aging on hydrogen embrittlement of a nickel alloy

The effect of aging at 500° C on the hydrogen embrittlement tendency of a cold worked Ni-base superalloy was investigated in a series of experiments which included hydrogen charging studies, mechanical tests in hydrogen and in air, and fractographic and slip line investigations. Embrittlement tendency increased (time-to-failure decreased) markedly during the first hour of aging and then remained constant until about 1000 h aging time, whereupon it increased rapidly again. The short-time embrittlement could be accounted for either by a mechanism involving segregation of P to grain boundaries or by one involving planar slip induced by short-range order. The hydrogen charging studies indicated that hydrogen uptake decreases during aging, a result which is not consistent with the P segregation hypothesis. The increase in embrittlement at long aging times is most readily explained in terms of planar slip induced by long range order. Tensile tests over a range of strain rates suggested that accelerated transport of hydrogen by dislocation dragging of hydrogen atmospheres is involved in embrittlement. Formerly Graduate Assistant, Dept. of Met. Eng. & Mat. Sci., University of Notre Dame     

Effects of grain-boundary segregation and precipitation on the hydrogen susceptibility of nickel

The straining electrode tensile test was employed to study hydrogen embrittlement in three grades of nickel. Aging at intermediate temperatures (400 to 800 °C) after solution annealing caused sulfur segregation to grain boundaries in the materials. This sulfur segregation was found to influence intergranular hydrogen embrittlement, and the effect was most prominent when the surface supply of hydrogen was low and when the grain boundaries were free of graphite precipitates. Aging also induced graphite precipitation at the grain boundaries of one grade of nickel (Nickel 200). The presence of these grain-boundary particles reduced susceptibility to hydrogen embrittlement, but at the same time caused a form of aging embrittlement. The effect of the second-phase precipitate, when present, masked the effect of sulfur segregation on hydrogen susceptibility. Formerly with Bell-Northern Research, Ottawa, ON, Canada.     

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,     

Surface segregation of phosphorus, carbon, and sulfur in commercial low-carbon grades of steel

Surface segregation behavior of solute atoms has been studied on low-carbon steels used in producing galvannealed sheet steels for automotive body panel applications. Samples of cold-rolled low-carbon steels with different amounts of carbon and phosphorus in solution were heated in a vacuum chamber and their surface chemistries analyzed by Auger electron spectroscopy. For the steels studied here, one or more of the elements carbon, phosphorus, and sulfur accumulated significantly at the surface within a temperature window of 300 to 973 K. As the temperature was increased, carbon appeared on the surface first, followed by phosphorus, and then sulfur. Each succeeding segregating element displaced the previous one from the surface. The free solute concentration in the bulk and the temperature were critical factors controlling the amount of solute accumulation at the surface. Once segregated, the solute atoms remained on the surface as the samples cooled. Carbon and/or boron in steels retarded the transport of phosphorus to the surface. The implications of these findings in understanding the galvannealing behavior of these steels are discussed.