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.     

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     

Influence of sulfur,phosphorus, and antimony segregation on the intergranular hydrogen embrittlement of nickel

The effectiveness of sulfur, phosphorus, and antimony in promoting the intergranular embrittlement of nickel was investigated using straining electrode tests in IN H₂SO₄ at cathodic potentials. Sulfur was found to be the critical grain boundary segregant due to its large enrichment at grain boundaries (10⁴ to 10⁵ times the bulk content) and the direct relationship between sulfur coverage and hydrogen-induced intergranular failure. Phosphorus was shown to be significantly less effective than sulfur or antimony in inducing the intergranular hydrogen embrittlement of nickel. The addition of phosphorus to nickel reduced the tendency for intergranular fracture and improved ductility because phosphorus segregated strongly to grain interfaces and limited sulfur enrichment. The hydrogen embrittling potency of antimony was also less than that of sulfur while its segregation propensity was considerably less. It was found that the effectiveness of segregated phosphorus and antimony in prompting intergranular embrittlementvs that of sulfur could be expressed in terms of an equivalent grain boundary sulfur coverage. The relative hydrogen embrittling potencies of sulfur, phosphorus, and antimony are discussed in reference to general mechanisms for the effect of impurity segregation on hydrogen-induced intergranular fracture.     

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.     

Hydrogen embrittlement of ultra-pure alloys of the inconel 600 type: Influence of the additions of elements (C,P, Sn,Sb)

The mechanical behavior of very high purity nickel base alloys of the Inconel 600 type that were simultaneously charged with hydrogen and deformed in tension was investigated. Experimental results show that this procedure decreases markedly the fracture strain of the pure 76 pct Ni-16 pct Cr-8 pct Fe alloy; cracks are observed after two to four pct elongation, and the fracture is completely intercrystalline. Hydrogen embrittlement appears as an intrinsic property of the Ni-Cr-Fe system in the sense that the grain boundary cohesion decreases when the purity of the alloy increases. The presence of carbon or phosphorus in the alloys increases grain boundary cohesion. The addition of metallic elements such as antimony or tin has relatively little effect on intergranular embrittlement.     

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.     

Effect of Sulfur and Antimony on the Intergranular Fracture of Iron at Cathodic Potentials

Antimony, segregated to grain boundaries of iron, was found to be five times more effective than sulfur in promoting intergranular fracture of iron when tested in IN H₂SO₄ at cathodic potentials. A decrease in the ductility of iron accompanied the fracture mode change at increasing cathodic potentials. The effectiveness of antimony relative to sulfur was determined from straining electrode tests on iron and iron + 250 appm antimony alloys heat treated at 800 °C and 600 °C to produce different grain boundary chemical compositions. Grain boundary compositions were determined by Auger Electron Spectroscopy (AES). Similar grain boundary sulfur concentrations of 0.2 monolayers were observed by AES for the iron and iron + 250 appm antimony alloy after an anneal of 240 hours at 600 °C, while 0.08 monolayers of antimony was observed for the iron + 250 appm antimony alloy. These results suggest that sulfur and antimony do not compete for grain boundary sites.     

Phosphorus segregation in austenite in Fe–P–C,Fe–P–B and Fe–P–C–B alloys

The equilibrium grain boundary segregation of phosphorus was investigated in Fe–P–C, Fe–P–B and Fe–P–C–B alloys after austenitising at temperatures ranging from 825–1100 °C. The grain boundary concentrations were determined by Auger electron spectroscopy on intergranular fracture surfaces. Phosphorus, carbon and boron segregate to the austenite grain boundaries. The segregation of P in austenite occurs mainly in equilibrium, but some additional segregation takes place during quenching. Boron and, in a lesser degree, carbon were found to decrease the grain boundary concentration of phosphorus. The results can be explained by assuming equilibrium segregation and mutual displacement of these elements in austenite.     

High resolution scanning auger microscopic investigation of intergranular fracture in As-quenched Fe-12mn

Previous research in this laboratory led to the conclusion that the low temperature intergranular fracture mode in Fe-Mn alloys is microstructurally determined, and does not require metalloid segregation or other chemical contamination. That conclusion was tested in the present investigation, which used high resolution scanning Auger microscopy to study the intergranular fracture surfaces. The fracture mode at liquid nitrogen temperature was found to be intergranular fracture whenever the alloy was quenched from the austenite field, irrespective of the austenization time or temperature. High resolution chemical analyses of the intergranular fracture surfaces failed to reveal any consistent segregation of P, S, O, or N. The occasional appearance of sulfur or oxygen on the fracture surface was found to be due to a low density precipitation of MnS and MnO₂ along the prior austenite grain boundaries. Excepting these dispersed precipitates, there was no evidence of manganese enrichment of the prior austenite grain boundaries. A slight segregation of carbon was found along the grain boundaries, but does not appear to be implicated in the tendency toward intergranular fracture. The present results hence reinforce the conclusion that the low temperature intergranular fracture of Fe-12Mn is microstructurally determined.     

Auger electron spectroscopy study of grain boundary segregation in alloy K-500: Part I. Behavior in as-processed state

Increased interest has been paid to grain boundary segregation in alloy K-500 due to severe intergranular cracking recently observed in forged bars. However, little systematic study of this segregation has been performed so far. A detailed auger electron spectroscopy (AES) study of grain boundary segregation in alloy K-500 has been carried out as a function of alloy chemistry. To determine C segregation, the C and O contamination rates in a vacuum chamber were measured and the necessary condition for C grain boundary segregation determination was established. It has been found that severe C, Al, and Cu segregation to grain boundaries occurred and depended on alloy chemistry. High bulk Ni and low bulk Al promoted C and Al grain boundary segregation, and low bulk Ni and high bulk Al significantly enhanced Cu segregation to grain boundaries. The depth profiles of intergranularly segregated elements also showed different features for high and low Ni content alloys. In high Ni alloys, C and Al levels dropped continuously as a function of distance from the grain boundaries but the Cu level dropped only slightly. In low Ni alloys, the Al and C levels rose from relatively low grain boundary levels to a peak at a certain distance from the grain boundary where the high grain boundary Cu level dramatically dropped. Transmission electron microscope (TEM) observation revealed a grain boundaryγ′-depleted zone followed by a region with coarser and denserγ′ particles in low Ni and high Al alloys but quite uniformly distributedγ′ particles with no depleted zone in high Ni and low Al alloys. These can be explained by the observed segregation behavior. The occurrence of Cu segregation is explained according to available theories about surface segregation in binary Ni-Cu alloys, and the segregation of C and Al to grain boundaries is suggested to be probably due to their interaction with Ni and Cu.     

Effects of Residual Elements Arsenic,Antimony, and Tin on Surface Hot Shortness

Scrap-based electric arc furnace (EAF) steelmaking is limited by a surface cracking problem in the recycled steel products, which is known as surface hot shortness. This problem originates from the excessive amount of copper (Cu) in the steel scrap, which enriches during the oxidation of iron (Fe) and consequently melts and penetrates into the austenite grain boundaries. In this article, the effects of arsenic (As), antimony (Sb), and tin (Sn) on surface hot shortness were investigated. A series of Fe-0.3 wt pct Cu-x wt pct (As, Sb, or Sn) alloys with x content ranging from 0.06 to 0.10 wt pct was oxidized in air at 1423 K (1150 °C) for 60, 300, and 600 seconds inside the chamber of a thermogravimety analyzer (TGA) where heat is supplied through infrared radiation. Scanning electron microscopy (SEM) investigations show that (1) the presence of Sb and Sn results in severe grain boundary cracking, whereas the presence of As does not, (2) open cracks with Fe oxides were found beneath the oxide/metal interface in the Sb and Sn alloys, and (3) the oxide/metal interfaces for all As, Sb, and Sn alloys are planar. Penetration experiments of pure Cu and Cu-30 wt pct Sn liquid were also conducted in the chamber of a hot-stage confocal laser scanning microscopy (CLSM) in nonoxidizing atmosphere: (1) on the Fe-35 wt pct manganese (Mn) alloys to study the correlation between cracking and grain boundary characters, and (2) on the pure Fe substrates to exclude the bulk segregation effects of Sn on grain boundary cracking. It was found that grain boundary cracking rarely took place on low-energy grain boundaries. The results also suggest that the bulk segregation of Sn in the substrate is not necessary to promote significant grain boundary cracking, and as long as the liquid phase contains Sn, it will be highly embrittling.     

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     

A study of the chemical composition of grain boundaries and creep cavity surfaces in a Cu-Sb alloy

Chemical compositions of grain boundaries and creep cavity surfaces in Cu + 0.5 at. pct Sb have been measured quantitatively using Auger Electron Spectroscopy. The grain boundary enrichment ratio for antimony due to segregation was found to be greater than 16. The distribution of antimony on the fracture surface was very homogeneous, with concentrations of about 7 at. pct on the grain boundaries and 9.2 at. pct on the cavity surfaces. The ratio of grain boundary segregation to surface segregation was lower than expected and possible reasons for this discrepancy are discussed. Other impurities (C, S, and O) were more inhomogeneously distributed. Carbon was found mainly on the walls of the cavities and on the grain boundaries while sulfur preferentially concentrated at steps on the cavity surfaces. The distribution of oxygen is closely related to the morphology of the fracture surface. It is concluded that oxygen arises from contamination of intergranular microcracks which extend to the surface of the sample and were exposed to the atmosphere. The highly reproducible Auger spectra clearly indicates that all other impurities were present in the material prior to microanalysis.     

再结晶钼合金的晶界偏析及其对脆性晶间断裂的影响

业已确定，再结晶钼合金的碳和氧偏析与温度的关系并不是单调的，这是一系列组织结构变化的结果，位错亚组织破碎，晶粒长大和晶界迁移，晶粒边界粒子的析出和溶解，近边界区内杂质元素之间的化学相互作用。建立了晶界化学和冷脆性温度之间的关系，造成晶间脆性的因素是一定温度下的氧偏析增加，杂质原子导致的近边界区强化，沿着晶界的扁平碳化物形成。     

Grain boundary fracture of L12 type intermetallic compound Ni3Ai

AES analysis of intergranular fracture surfaces of Ni₃Al showed that grain boundaries are free from any detectable amount of impurity segregation. From this finding it was suggested that grain boundary brittleness in Ni₃Al is not due to the segregation of harmful elements. X-ray diffraction and SEM observation of the fracture surfaces detected a plastically strained layer of which thickness is compa-rable to grain size.     

A grain boundary etching method for the analysis of intergranular P-segregation in iron-based alloys

The effectiveness of a grain boundary etching method for the non-destructive analysis of intergranular segregation of P in iron-based alloys was examined by using a saturated aqueous solution of picric acid containing sodium tridecylbenzene sulfonate. Among all the alloys examined only those doped with P suffered selective etching attack against grain boundaries. The degree of the etching attack in a P-doped NiCr steel was found to have a linear relation with the concentration of grain boundary P as measured by Auger electron spectroscopy. From these evidences, the grain boundary etching method was concluded to be useful as a technique analyzing the grain boundary concentration of P in NiCr steels. The application of the method was successfully made to CrMo and C-free NiCr steels which are hard to exhibit perfect intergranular fracture. The application was not successful to Fe-40 pct Ni and Fe-3 pct Si alloys whose surfaces were unstable electrochemically compared with those of NiCr and CrMo steels.     

On the mechanisms of high-temperature intergranular embrittlements of Ni3Al-Zr Alloys

Recent studies on the room-temperature fracture behavior of Ni₃Al-Zr alloys after preexposure at elevated temperatures show various types of intergranular failure. In the presently studied Ni₇₈Al₂₁Zr₁B_0.2 alloy, a strong intergranular fracture tendency at room temperature has been found after preexposure at 750 °C, which is caused by the grain boundary precipitation in this alloy. After short-term exposure above 1200 °C and bending fracture at room temperature, the alloy also suffers intergranular embrittlement due to grain boundary melting. The intergranular fracture appearance is quite different from that observed in a previous study for a Ni_77.4Al₂₂Zr_0.6B_0.2 alloy after air exposure for 100 hours at 1200 °C. In that case, the intergranular fracture was accompanied by grain boundary diffusion (invasion) and segregation of oxygen. The mechanisms of these types of grain boundary failure are discussed. Formerly Doctoral Candidate, Institute of Materials Science and Engineering, National Taiwan University.     

Hydrogen diffusion and its relevance to intergranular cracking in nickel

This investigation examines the effect of hydrogen precharging upon the fracture mode of pure nickel at 77 K. Specific attention is given to the diffusion coefficient of hydrogen along grain boundaries,D _g , the critical hydrogen concentration to cause intergranular fracture, C _g ^* , and the binding energy of grain boundaries with hydrogen,E _b . Both scanning electron microscope (SEM) observations and a newly developed grain boundary diffusion model indicate that the fracture mode changes from transgranular (TG) to intergranular (IG) when the hydrogen concentration in grain boundaries reaches a critical value, in the range of 6.5 to 9.8 at. pct. The experimental results further show that the observed increment of IG cracking depth with the precharging time could be accounted for by lattice diffusion alone, thus implying that hydrogen transport in this material is not enhanced by grain boundaries. Finally, the binding energy of grain boundaries with hydrogen is found to be 11.3 to 12.3 kJ/mol.     

High temperature brittle intergranular cracking in high strength nickel alloys undoped and doped with S,Zr, and/or B—II. Solute segregation analysis

In order to clarify the mechanism of high temperature brittle intergranular cracking in high strength nickel alloys, solute segregation at grain boundaries and on high temperature fracture surfaces has been examined by scanning Auger microscopy. Intergranular S segregation producing global embrittling effects was found to increase in the following order: alloys with low S or high S and Zr, high S and B, and high S. The grain boundaries extensively contained Ti or Zr rich sulfides in the S-doped alloys and B segregation inducing intergranular toughening in the alloy with high S and B. The alloys with low S, and high S and B showed more strongly S segregation on high temperature fracture surfaces, which was much greater compared with at grain boundaries, than with high S, and high S and Zr. The local stress intensification did not produce a remarkable S enrichment at grain boundaries except near sulfides. It is proposed that S fluxes from crack surfaces and stressed sulfides to the crack tip induce local embrittlement. The composition and temperature effects on the brittle cracking behavior are discussed in terms of the global and local embrittling effects.     

Grain boundary segregation in the Ni-base alloy 182

Grain boundary segregation is often considered to play a role in the various types of intergranular failures observed in austenitic alloys. However, there has been little direct study of this segregation because it is usually very difficult to obtain intergranular fracture in these alloys in the high vacuum required for surface analysis. This paper reports an Auger electron spectroscopy study of grain boundary segregation in the nickel-base Alloy 182. This alloy was used because it would easily fracture along its grain boundaries in high vacuum and because it has a very complex microstructure, as do many nickel-base alloys used in engineering applications. Furthermore, the alloy is widely used as a weld filler metal to join nickel-base alloys to one another or to stainless steels. Phosphorus was found to be the only impurity element that segregated to the grain boundaries. There was considerable variability in segregation from grain boundary to grain boundary and also on a single grain facet. It is suggested that these variations arise primarily from variations in grain boundary structure, in the density and types of precipitates in a grain boundary, and in the consequent variety of precipitate matrix interfaces present at the grain boundary. It is also suggested that quantitative Auger analysis on such a material would be very difficult.