Measurement and evaluation of hydrogen trapping in thoria dispersed nickel

The trapping of hydrogen in nickel-2 vol pct thoria has been measured by permeation techniques. The theory of McNabb and Foster was used to determine the density of trap sites, the rate constant for trapping, and the rate constant for untrapping. Annealed material has trap sites associated with the thoria-nickel interface with the density of sites equal to about a monolayer of hydrogen at the interface. Cold-rolling up to 59 pct reduction in thickness increases the trap site density, probably by opening more sites at the nickel-thoria interfaces. The trapping energy was determined to be about 7.1 kcal/mol H. The trapping rate constant has an activation energy similar to the activation energy for hydrogen diffusion. It was shown that trapping models assuming equilibrium between lattice hydrogen and trapped hydrogen do not apply to the nickel-thoria system. For this system trapping is relatively rapid and untrapping is slow, particularly at low temperatures.     

High-temperature nitridation of Ni-Cr alloys

The nitriding behavior of nickel-chromium alloys was investigated at 1398 K over the range 1 to 6000 bar of external nitrogen pressure. The morphology of the nitrided zone depends on the concentration of chromium in the initial alloy and the N₂ pressure (fugacity) applied upon the system. The transition from CrN to Cr₂N precipitation was observed within the reaction zone after nitriding at 100 to 6000 bar of N₂ when the chromium content in the initial alloys was 28.0 at. pct or higher. It is shown that the ternary phase π (Cr₁₀Ni₇N₃) is formed in this system at 1273 K. through a peritectoid reaction between Cr₂N and nickel solid solution and becomes unstable above 1373 K. The thermodynamic evaluation of the Ni-Cr-N system was performed and phase equilibria calculated. Evidence for “up hill” diffusion of nitrogen near the reaction front during the internal nitridation of Ni-Cr alloys at 1398 K was found. It was attributed to the relative instability of chromium nitrides and strong Cr-N interaction in the matrix of the Ni-based solid solution within the nitrided zone.     

Diffusional creep and creep-degradation in dispersion-strengthened Ni-Cr base alloys

Dispersoid-free regions were observed in the dispersion-strengthened alloy TD-NiCr (Ni-20 Cr-2 ThO₂) after, slow strain rate testing (stress rupture, creep, and fatigue) in air from 1145 to 1590 K. Formation of the dispersoid-free regions appears to be the result of diffusional creep. The net effect of creep in TD-NiCr is the degradation of the alloy to a duplex microstructure. Creep degradation of TD-NiCr is further enhanced by the formation of voids and intergranular oxidation in the dispersoid-free bands. Void formation was observed after as litte as 0.13 pct creep deformation at 1255 K. The dispersoid-free regions apparently provide sites for void formation and oxide growth since the strength and oxidation resistance of Ni-20 Cr is much less than Ni-20 Cr-2 ThO₂. This localized oxidation does not appear to reduce the static load bearing capacity of TD-NiCr since long stress-rupture lives were observed even with heavily oxidized microstructures, but this oxidation does significantly reduce the ductility and impact resistance of the material. Dispersoid-free bands and voids also were observed in two other dispersion-strengthened alloys, TD-NiCrAl (Ni-16Cr-4 Al-2 ThO₂) and IN-853 (Ni-20 Cr-2.5 Ti-1.5 Al-1.3 Y₂O₃). Thus, it appears that diffusional creep is characteristic of dispersion-strengthened alloys and can play a major role in the creep degradation of these materials.     

Growth kinetics of grain boundary ferrite allotriomorphs in Fe-C-X alloys

Parabolic rate constants for the thickening (α) and lengthening (β) kinetics of grain boundary allotriomorphs of proeutectoid ferrite have been measured as a function of isothermal transformation temperature in several Fe-C-X’ alloys whereX = Si, Ni, Mn, and Cr. These constants have been corrected approximately for the growth inhibition produced by facets on the allotriomorphs. The corrected α values are compared with those calculated on the basis of three models: equilibrium at α:γ boundaries with partition ofX, local equilibrium with “pile-up” ofX rather than bulk partition, and paraequilibrium. Values calculated from both the paraequilibrium and the “pile-up” models were in order of magnitude or better agreement with the corrected experimental α’s. Similar levels of agreement were obtained for the equilibrium model in the Si and Cr alloys and also in one Ni alloy at lower reaction temperatures. However, an estimate of the maximum possible diffusion distance of alloying element into austenite during growth supported only the paraequilibrium model under nearly all conditions investigated. Even for this model, however, measured rate constants are significantly less than those calculated for Fe-C-Mn and Fe-C-Cr and greater for Fe-C-Si and the higher Ni, Fe-C-Ni alloy. The Mn and Cr discrepancies seem best explained at present by a solute drag-like effect; an accompanying paper indicates that interphase boundary precipitation of carbides is involved in the Si and Ni alloys, though an inverse solute drag-like effect may also be operative. Formerly graduate student, Department of Metallurgical Engineering, Michigan Technological University. Formerly Professor at Michigan Technological University.     

Growth and overall transformation kinetics above the bay temperature in Fe-C-Mo alloys

The kinetics and morphology of isothermal transformation in the vicinity of the time-temperaturetransformation (TTT) diagram bay have been investigated with optical and transmission electron microscopy (TEM) in 19 Fe-C-Mo alloys at three levels of carbon concentration (approximately 0.15, 0.20, and 0.25 wt pct) and at Mo concentrations from 2.3 to 4.3 wt pct, essentially always at temperatures above or at that of the bay,T _b . Quantitative metallography yielded no evidence for incomplete transformation (stasis) in any of these alloys atT > T _b . Measurements of the thickening kinetics of grain boundary ferrite allotriomorphs (invariably containing either interphase boundary or fibrous Mo₂C) demonstrated four different patterns of behavior. The customary parabolic time law for allotriomorph thickening in Fe-C and in many Fe-C-X systems was obtained only at higher temperatures and in the more dilute Fe-C-Mo alloys studied. With decreasing temperature and increasing solute concentrations, a two-stage and then two successive variants of a three-stage thickening process are found. In the most concentrated alloys and at temperatures nearest the bay, the second stage of the three-stage thickening process corresponds to “growth stasis”—the cessation of allotriomorph thickening. Sufficient prolongation of growth stasis presumably leads to “transformation stasis.” A number of models for growth of the carbide-containing allotriomorphs were investigated during attempts to explain the observed kinetics. It was concluded that their growth is controlled by carbon diffusion in austenite but with a driving force drastically reduced by a very strong solute drag-like effect (SDLE) induced by Mo segregation at disordered-type austenite: ferrite boundaries. Carbide growth in the fibrous structure appears to be fed by diffusion of Mo along austenite: ferrite boundaries, whereas carbides in the interphase boundary structure grow primarily by volume diffusion of Mo through austenite. Formerly Republic Steel Corporation Fellow, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI, and Visiting Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA. Formerly Professor, Michigan Technological University. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Growth kinetics of grain boundary ferrite allotriomorphs in Fe−C alloys

The kinetics of lengthening and thickening of grain boundary allotriomorphs of proeutectoid ferrite were measured at several temperatures in high purity Fe−C alloys containing 0.11 pct, 0.23 pct and 0.42 pct C. These measurements were conducted by measuring the length of the longest and the width of the widest allotriomorph in each specimen. All specimens were austenitized so as to make the grain boundaries perpendicular to the plane of polish. This measurement technique appreciably reduced the scatter in the parabolic rate constant data previously encountered in thermionic emission microscopy measurements. Parabolic rate constants for lengthening and thickening were calculated, using the experimental aspect ratio, by means of the Atkinson analysis for oblate ellipsoids. The ratio of the measured to the calculated constants was in all cases less than unity. The previously made suggestion that these slow growth kinetics are due to faceting was supported through the observation of facets on allotriomorphs by means of scanning and transmission electron microscopy. The aspect ratio of ferrite allotriomorphs was shown to beca 1/3, independent of reaction time temperature and carbon content. The dihedral angle of the allotriomorphs was found to be 100±5 deg, as compared with a published angle for recrystallized and equilibrated specimens ofca 115 deg. Several possible explanations for the aspect ratio and dihedral angle findings are considered. M. RIGSBEE, formerly Postdoctoral Research Associate with Michigan Technological University     

High temperature embrittlement of NI and Ni-Cr alloys by trace elements

The effects of Sb, Sn, and Zr additions on the creep properties of Ni and Ni + 20 pct Cr are reported. Antimony and tin additions (~1 wt pct) induce extensive grain boundary cavitation in nickel, while smaller antimony additions had little effect on Ni + 20 pct Cr. Addition of 0.11 pct Zr to Ni + 20 pct Cr greatly inhibited grain boundary cavitation and reduced its Coble creep rate. Auger electron spectroscopy of cavitated specimens provided direct evidence of impurity segregation to cavity surfaces. Residual sulfur segregated most strongly, and was observed on cavity surfaces in all cavitated specimens. Tin segregated somewhat less intensely than sulfur, and antimony segregated only slightly. Segregation of antimony and sulfur to uncavitated portions of Ni + 1 pct Sb grain boundaries was also observed. These results are discussed in terms of segregation effects on energetic and transport properties of the grain boundaries and cavity surfaces. This paper is based on a presentation made at the symposium “The Role of Trace Elements and Interfaces in Creep Failure” held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorship of The Mechanical Metallurgy Committee of TMS-AIME.     

Growth kinetics of grain boundary allotriomorphs of proeutectoid ferrite in Fe-C-Mn-X2 alloys

The parabolic rate constant for the thickening of grain boundary ferrite allotriomorphs at the faces of austenite grain boundaries was measured as a function of isothermal transformation temperature in three Fe-C-X₁-X₂ alloys where X₁ is Mn and X₂ is successively Si, Ni, and Co. The results were compared with the predictions of the local equilibrium model for multi-component systems and with those derived from the theory of growth under paraequilibrium conditions. The distribution of Mn and Si in ferrite and austenite in the Fe-C-Mn-Si alloy was also measured as a function of reaction temperature with transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). The observed temperature below which alloying element partition ceased was in good agreement with the local equilibrium model. Whereas the parabolic rate constant for thickening was considerably larger than the amount predicted by this theory in the alloying element diffusion-controlled regime, the opposite was true in the carbon diffusion-controlled regime. Similarly, the calculated paraequilibrium constant was usually considerably larger than that measured experimentally. Synergistic enhancements of the effects of Mn and X₂ in diminishing thickening kinetics were observed for each X₂. The time-temperature-transformation (TTT) curves for the beginning of transformation were calculated from a modified Cahn analysis for the overall kinetics of grain-boundary-nucleated reactions using values of the nucleation rate and the parabolic growth rate constant computed from various models and compared with experimentally determined TTT curves. Substantial discrepancies between the calculated and measured curves were ascribed to synergistic effects of Mn and X₂ upon nucleation and growth kinetics. Formerly Graduate Student, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA Formerly Mehl Professor Emeritus at Carnegie Mellon University.     

Ni8Ta in nickel-rich Ni−Ta alloys

An ordering reaction has been observed in the nickel-rich binary alloys of the Ni?Ta system that are aged below 570°C following quenching from elevated temperatures. This reaction has been investigated by employing electron diffraction microscopy, X-ray diffraction, and electrical resistivity techniques. The ordered phase has been identified as Ni₈Ta and its structure is identical to that of the Ni₈Nb (Cb) reported earlier. In the stoichiometric Ni₈Ta alloy (Ni-11.1 at. pct Ta) three variants of the Ni₈Ta phase are nucleated apparently “homogeneously” throughout the matrix. The precipitate or domain morphology has been identified as a cuboid elongated in one direction such that the cube faces are parallel to the {100} planes of the matrix. The cuboid morphology of the precipitates tends to disappear as the precipitates coalesce and the particle size approaches 1000Å.     

Structure-property relationships in thermomechanically treated beryllia dispersed nickel alloys

BeO dispersed nickel alloys, produced by powder metallurgy techniques, were studied extensively in stress rupture at 815, 982, and 1093°C (1088, 1255, and 1366 K) and by transmission electron microscopy. The alloys were subjected to a variety of thermomechanical treatments (TMT) to determine the benefits of TMT on properties. It is shown that the use of intermediate annealing treatments after 10 pct reduction steps is highly beneficial on both low and high temperature properties. It is indicated that the high temperature strength is not primarily dependent on the grain aspect ratio or texture but depends strongly on the dislocation density and distribution of dislocations in a stable substructure which is pinned by the fine oxide dispersion.     

Structure-property relationships in thermomechanically treated beryllia dispersed nickel alloys

BeO dispersed nickel alloys, produced by powder metallurgy techniques, were studied extensively in stress rupture at 815, 982, and 1093°C (1088, 1255, and 1366 K) and by transmission electron microscopy. The alloys were subjected to a variety of thermomechanical treatments (TMT) to determine the benefits of TMT on properties. It is shown that the use of intermediate annealing treatments after 10 pct reduction steps is highly beneficial on both low and high temperature properties. It is indicated that the high temperature strength is not primarily dependent on the grain aspect ratio or texture but depends strongly on the dislocation density and distribution of dislocations in a stable substructure which is pinned by the fine oxide dispersion.     

Procedure of the identifiable analysis of dispersed precipitations in alloys

Formulas and analytical and cumulative probabilistic distributions to perform a system analysis of dispersed precipitations in alloys and other materials are proposed. For this purpose, a method for establishing the similarity and difference (identification) between the characteristics of the experimental histogram and theoretical size distribution of the particles is developed. This method of identifying the distributions makes it possible to determine the correlation between the features of transformation of experimental distributions and intrasystem processes.     

Microstructure and mechanical properties of ternary intermetallic compound dispersed P/M Al-Mn-X-Zr (x=Cu,Ni) alloys

Heat-resistant aluminum alloys are generally developed by dispersing stable intermetallic compounds by adding transition metals (TM) whose diffusion coefficient in aluminum alloys is low even at high temperatures. Commonly used intermetallic compounds include Al-TM binary intermetallic compounds, for example, Al₆Fe, Al₃Ti and Al₃Ni. By contrast, multicomponent intermetallic compounds are hardly used. The present study focuses on Al-Mn-Cu and Al-Mn-Ni ternary intermetallic compounds, and by finely dispersing these intermetallic compounds, attempts to develop heat-resistant alloys. Through the atomization method, Al-(4.96–5.96)Mn-(6.82–7.53)Cu-0.4Zr and Al-(5.48–8.76)Mn-(2.23–4.32)Ni-0.4Zr (in mass%) powders were fabricated, and by degassing these powders at 773 K, intermetallic compounds were precipitated. These powders were then solidified into extrudates by hot extrusion at 773 K. The microstructural characterization of powders and exrudates was carried out by XRD analysis, SEM/EDX and TEM. The mechanical properties of extrudates were determined at room temperature, 523 K and 573 K. In Al-Mn-Cu alloys, while a small amount of Al₂Cu was crystallized, precipitated Al₂₀Mn₃Cu₂ intermetallic compounds were mainly dispersed. In Al-Mn-Ni alloys, while a small amount of Al₆Mn intermetallic compounds was precipitated, the precipitated A₆₀Mn₁₁Ni₄ intermetallic compounds were mainly dispersed. Both ternary intermetallic compounds were about 200 nm in size. The compounds were elliptical, and their longitudinal direction was oriented along the extrusion direction. In the Al-Mn-Cu alloys, since the work hardening at room temperature was high, the tensile strength became 569 MPa. At elevated temperatures, since hardly any work hardening was observed, the tensile strength decreased markedly. However, in Al-Mn-Ni alloys, since the work hardening is low even at room temperature, the roomtemperature strength is not high. Thus, the decrease in tensile strength at elevated temperatures is relatively small and a high strength was obtained at 523 K and 573 K: 276 MPa and 207 MPa, respectively.     

Characterization of dispersed intermetallic phases in rapidly quenched Al-Ti-Ce alloys

The microstructures of Al-3Ti-lCe (wt pct) and Al-5Ti-5Ce alloys melt-spun under controlled He atmosphere have been characterized using analytical electron microscopy. The rapidly solidified microstructures comprise uniform, fine-scale dispersions of intermetallic phase in an aluminum matrix, and particular attention has been given to identification of the dispersed phases. In the Al-3Ti-lCe alloy, the dispersed particles are polycrystalline with a complex twinned substructure and a diamond cubic crystal structure(a _o =1.44 ±0.01 nm) and composition consistent with the ternary compound Al₂₀Ti₂Ce (Al₁₈Cr₂Mg₃ structure type, space group Fd3m). In the Al-5Ti-5Ce alloy, there is, in addition to the dispersed ternary phase, a separate uniform array of fine-scale particles of the binary compound Al₁₁Ce₃. The majority of such particles have the body-centered orthorhombic structure of the low-temperature polymorph, α-Al₁₁Ce₃, but there is evidence to suggest that at least some particles developvia initial formation of the high-temperature body-centered tetragonal phase, β-Al₁₁Ce₃. The accumulated evidence suggests that both binary and ternary particles formed as primary phases directly from the melt during rapid solidification, leaving only small concentrations of solute in aluminum matrix solid solution. Both phases are observed to be resistant to coarsening for up to 240 hours at 400 °C. Formerly Research Fellow, Department of Materials Engineering, Monash University.     

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.