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.     

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 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 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,     

Effects of the additions of B,P, dSn,and Sb on oxygen assisted hydrogen embrittlement of nickel

Effects of additions of B, P, Sn, and Sb on hydrogen embrittlement of nickel were examined on specimens cathodically charged with hydrogen. Tensile specimens were annealed either in vacuum or in a dry hydrogen gas. Undoped, Sn-doped, and Sb-doped materials annealed in vacuum revealed high susceptibility to intergranular hydrogen embrittlement, while the susceptibility was greatly reduced when the materials were annealed in hydrogen. The deleterious effect of vacuum annealing was proposed to result from grain boundary penetration of oxygen from the annealing environment. Additions of a small amount of B and P almost completely suppressed the IGHE of specimens annealed in vacuum. It is suggested that the effect of these elements on grain boundary penetration of oxygen is an important factor for their beneficial effects on IGHE. Addition of Sn somewhat increased the ductility of hydrogen charged specimens annealed both in vacuum and in hydrogen, indicating that this element is not deleterious. Addition of Sb increased the embrittlement susceptibility of specimens annealed in hydrogen. However, the deleterious effect of Sb was not serious in spite of the high concentration of Sb that was examined.     

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.     

Effect of boron on the grain boundary segregation of phosphorus and intergranular fracture in high-purity Fe-0.2 Pct P-B alloys

The effect of boron on the grain boundary segregation of phosphorus in a high-purity Fe-0.2 pct P alloy has been investigated by Auger electron spectroscopy (AES). The segregation of phosphorus decreases markedly with the segregation of boron; phosphorus atoms are replaced by boron atoms at grain boundaries. The free energy of segregation of boron at 1073 K is determined to be 100 kJ/mol. The effect of boron on the phosphorus-induced intergranular fracture (IGF) has been examined with impact testing, and the fractography has been studied with scanning electron microscopy (SEM). Addition of 12.5 wt · ppm boron completely prevents the IGF induced by the segregation of phosphorus and decreases the ductile-brittle transition temperature (DBTT) by about 170 K when quenched from 1073 K. The suppression of the IGF due to the addition of boron is caused by two mechanisms. One is the increased grain boundary cohesion of iron caused by the segregated boron as its inherent effect. The other is the decrease in the segregation of phosphorus caused by the segregation of boron. The former has been shown to be more effective than the latter in suppressing the IGF. Formerly Graduate Student, Postdoctoral Fellow Formerly Research Student, Institute for Materials Research, Tohoku University Formerly with the Institute for Materials Research, Tohoku University     

The influence of oxygen and sulfur on the intergranular brittleness of iron

The effect of oxygen on the intergranular brittleness of high purity iron has been determined by means of the following two different methods: 1) by normalized impact tests on specimens of different compositions after a standard heat treatment; and 2) by slow tensile tests at low temperatures on specimens of constant composition after different heat treatments. The results, after comparison with previously published studies, show that the effects of sulfur and carbon on the intergranular brittleness of iron are preponderant, while the effect of oxygen, in the conditions of this study, is nil. where C. Pichard was formerly affiliated.     

The influence of oxygen and sulfur on the intergranular brittleness of iron

The effect of oxygen on the intergranular brittleness of high purity iron has been determined by means of the following two different methods: 1) by normalized impact tests on specimens of different compositions after a standard heat treatment; and 2) by slow tensile tests at low temperatures on specimens of constant composition after different heat treatments. The results, after comparison with previously published studies, show that the effects of sulfur and carbon on the intergranular brittleness of iron are preponderant, while the effect of oxygen, in the conditions of this study, is nil. Formerly affiliated.