Grain boundary segregation of an Al-Zn-Mg ternary alloy

Auger electron spectroscopy (AES) has been used to measure the grain boundary concentration profiles of alloy additions in an A1-5.5 pct Zn-2.5 pct Mg ternary in as-quenched, under-, peak-, and over-aged conditions. The AES depth profiles show marked segregation of Mg and Zn to the grain boundary, in contrast to that reported previously on similar A1 alloys. It is found that this apparent contradiction can be resolved by exploiting the plasmonloss features of the AES spectra to help elucidate the grain boundary segregation. With the AES/plasmon-loss measurements, one can determine not only the concentration of Mg and Zn at the grain boundary, but also the metallurgical environments surrounding the alloy additions. It is shown that, for over-aged specimens of the Al alloy, only a fraction of the total Mg at the grain boundary is incorporated in MgZn₂ precipitates, the remainder being segregated to within a few atomic layers of the boundary.     

Grain boundary segregation in a low alloy steel under tensile loading conditions

The influence of tensile stress on the grain boundary segregation behaviour in a NiCrMoV steel is elucidated through the measurement of grain boundary segregation isotherms and the determination of interaction processes amongst trace and alloying elements. The application of tensile stress accelerates and enhances the grain boundary coverage of elements P, N and S presumably through a stress-induced-diffusion process. The qualitative nature of the interaction processes which are operative in the absence of any applied stress remain unaltered on the application of the tensile stress. However, there is an enhancement of the interaction processes, in the presence of the applied stress; site-competitive, P + S, interaction process is particularly activated in the presence of a tensile stress.     

Grain boundary segregation in ferromagnetic alloys

The grain boundary segregation of Sb and S in Co-50%Ni alloys at temperatures between 600 and 1200°C has been examined by Auger electron spectroscopy. It has been found that the segregation of sulfur is more pronounced than that of antimony and the temperature dependence of segregation energy for both Sb and S changes abnormally near the Curie temperature. A thermodynamic analysis has been made on these results, taking the magnetic effect into account, and it has been ascertained that the magnetic transition intensifies the grain boundary segregation of Sb and S. The segregation data on P in Fe-P alloys by Grabke et al. have also been well explained by the same model. These findings are in good agreement with the predictions of Szkiarz and Wayman.     

Grain boundary segregation of carbon in iron

The distribution of C¹⁴ in a series of a-phase Fe-C alloys was studied autoradiographically. Excess carbon was present near grain boundaries in specimens equilibrated at 500‡ and 600‡C; the amount of carbon near the grain boundaries was found to decrease as the equilibration temperature increased. Electron microscopy of the specimens subsequent to the autoradiography showed that no grain boundary precipitates were present. The results are compared with the theories of nonequilibrium vacancy-aided segregation and Gibbsian adsorption, and also with McLean's model of grain boundary segregation. It is shown that the technique measures directly the quantity of thermodynamic interest, the total excess solute per unit area of grain boundary. Formerly Graduate Student, Henry Krumb School of Mines, Columbia University, New York, N. Y.. This paper is based on a thesis submitted by J. M. PAPAZIAN in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Columbia University.     

Grain boundary segregation of sulfur in iron

This paper reports a study of grain boundary segregation of sulfur in iron. The results show that if the amount of sulfur in the alloy is greater than the solubility limit, the amount of segregation increases with increasing ageing temperature. When the temperature is raised above the point where all sulfur goes into solution, the amount of segregation decreases with increasing temperature. These results can be explained quite straightforwardly by McLean's model for segregation. However, at high bulk concentrations. Auger analysis may be difficult because the signal obtained from a grain boundary facet will arise from a combination of segregated sulfur and precipitated sulfur.     

Grain boundary segregation of phosphorus and

This paper reports a study of grain boundary segregation of phosphorus and sulfur in types 304L and 316L stainless steel. Phosphorus was found to segregate in both alloys, but the amount of segregation depended on several factors. These included the time and temperature of the aging heat treatment, the total composition of the steel, and compositional banding. Sulfur was usually precipitated as chromium-rich sulfides and the amount of elemental segregation was low. The correlation between segregation of phosphorus and corrosion in the Huey test was not exact. This fact appeared to result from the contribution of other microstructural elements to the corrosion process.     

Grain boundary segregation of boron and sulfur and its effect on ductility in rapidly solidified Ni-base L12 compounds

This paper reports a study of boron segregation in Ni₃Al, Ni₃Si, Ni₃Ge, and Ni₃Ga. Sulfur segregation in Ni₃Al is also considered. The results show that boron segregates in Ni₃Al and that the amount of segregation tends to increase as the bulk concentration of boron in the alloy increases. However, because the samples had not reached equilibrium during the heat treatment, there was some scatter in this correlation. Boron also segregated in Ni₃Si and Ni₃Ge, but there appeared to be no dependence of the amount of segregation on the bulk concentration. This result is not surprising because in these alloys the boron concentration was above the solubility limit. Boron segregation also occurred in Ni₃Ga and it appeared to increase with increasing bulk concentration. However, only a small amount of data was obtained for this system. Sulfur segregated in Ni₃Al and its concentration on the grain boundaries increased with increasing bulk concentration. It did not appear to compete with boron for grain boundary sites. Aluminum also segregated in Ni₃Al, but there was a large amount of scatter in the data. The plastic strain to failure measured for the samples of Ni₃Al did not correlate with the amount of boron segregation. In particular, we could not explain the fact that boron additions enhance grain boundary cohesion more effectively in Ni-rich alloys by an increase in boron segregation in these alloys. Stoichiometric alloys and Ni-poor alloys that were very brittle had boron segregation in equivalent amounts to that found in ductile Ni-rich alloys.     

Grain boundary segregation of boron in INCONEL 718

The segregation behavior of boron at grain boundaries in two INCONEL 718⁺ based alloys with different B concentrations was studied. The alloys, one containing 11 ppm of B and the other 43 ppm, were homogenized at 1200 °C for 2 hours followed by water quenching and air cooling. A strong segregation of boron at grain boundaries was observed using secondary ion mass spectrometry after the heat treatment in both the alloys. The segregation was found mainly to be of nonequilibrium type. The homogenized samples were also annealed at 1050 °C for various lengths of time. During annealing, boride particles were observed to first form at grain boundaries and then to dissolve on continued annealing at 1050 °C. The mechanisms of segregation and desegregation of B are discussed.     

Grain boundary diffusion and grain boundary segregation of tellurium in silver

Using the radiotracers ¹²¹Te and ¹²³Te with high specific activity, grain boundary (GB) diffusion of Te in Ag has been studied in a wide temperature interval of about 600 K. At low temperatures (378–504 K) Harrison's type-C regime has been realized, which has enabled us to perform a number of direct measurements of the GB diffusion (GBD) coefficient D_GB. The activation energy and the preexponential factor of GBD are equal to H_GB = 86.75 kJ/mol and D_{GB = 1.01 × 10⁻⁴m²/s}, respectively. Combining the obtained D_GB-values with the P = sD_GB°-values measured at higher temperatures 650–970 K) in the B-regime, the GB-segregation factor s has been estimated taking the GB width of δ = 0.5 nm. The resultant s-values predict an enthalpy of GB-segregation of H_s ≅ −43.29 kJ/mol. This is the first study with C-regime measurements of impurity GBD and accurate estimations of the GB-segregation factor from the GBD-data. Comparing the present results with the results of our recent C-regime measurements of GB self-diffusion in silver [J. appl. Phys.72, 2758 (1992)], a possibility of 2D-phase formation during Te GBD in Ag is discussed.     

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.     

Grain boundary structure and chromium segregation in a 316 stainless steel

Analytical electron microscopy (AEM) has been used to examine the relationship between grain boundary structure and the segregation of chromium in a sensitised AISI 316 stainless steel. Fifty grain boundaries have been analysed. Among the boundaries there was a threefold difference in full width half maximum (FWHM) of the chromium concentration profiles. This variation is interpreted in terms of the different long-range stress fields created by different combinations of structural units in each boundary. From the narrowness of their profiles, it is deduced that the σ = 3, 11, 13a, 13b and 2ga boundaries are favoured while the σ = 9 boundary is non-favoured. There was no correlation between FWHM and grain boundary chromium concentration. Additionally, we observe that it is not pre-requisite for the boundary plane to be close to {111} for a chromium carbide to nucleate. There is also no correlation between the boundary normal and either the FWHM of the chromium concentration profile or the grain boundary chromium concentration.     

Grain boundary segregation in Ni and binary Ni alloys doped with sulfur

The kinetics and temperature dependence of sulfur segregation in Ni and binary alloys of Ni with Cu, Al, Cr, Mo, W, and Hf, all containing between 70 and 100 atomic ppm sulfur, have been measured using Auger spectroscopy over the temperature range 500 to 1000 °C. No evidence for cosegregation of these alloying elements with sulfur is found. The alloying elements do influence the precipitation of sulfides, however, and this influences the amount of segregation which occurs. Comparison with theoretical models of grain boundary segregation allows the sulfur solubility in the various alloys to be determined.