Behavior of grain boundary chemistry and precipitates upon thermal treatment of controlled purity alloy 690

Grain boundary composition and carbide composition and structure were characterized for various microstructures of controlled purity alloy 690. Heat treatments produced varying degrees of grain boundary chromium depletion and precipitate distributions which were characterizedvia scanning transmission electron microscopy (STEM). Convergent beam electron diffraction revealed that the dominant carbide is M₂₃C₆, and energy dispersive X-ray analysis (EDAX) determined that the metallic content was about 90 at. pct chromium. A discontinuous precipitation reaction was observed and is attributed to a high degree of carbon supersaturation. Grain boundary composition measurements confirm that chromium depletion is controlled by volume diffusion of chromium to chromium-rich grain boundary carbides in the temperature range of 873 to 1073 K. Grain boundary chromium levels as low as 18.8 at. pct were obtained by thermal treatment at 873 K for 250 hours and 973 K for 1 hour. A thermodynamic and kinetic model developed for alloy 600 was modified to describe the development of the chromium depletion profile in alloy 690 during thermal treatment. Experimentally measured chromium profiles agree well with the model results for the dependence of the chromium depletion zone width and depth on various input parameters. The establishment of the model for alloy 690 allows the chromium depletion zone width and depth to be computed as a function of alloy composition, grain size, and temperature. The chromium depletion profiles and the precipitate structure and composition of controlled purity 690 are compared to those of controlled purity 600. A thermodynamic analysis of the carbide stability indicates that other factors, such as favorable orientation relationships, play an important role in controlling the precipitation of Cr₂₃C₆ in nickel-base alloys.     

The effects of heat treatment on the sensitization and SCC behavior of INCONEL 600 alloy

A series of heat treatments were performed to study the sensitization effect on the stress corrosion cracking (SCC) behavior of INCONEL 600 alloy. The microstructural evolution and the chromium depletion near the grain boundary were carefully studied using analytical electron microscopy (AEM). Comparable constant load tests in the solution containing various concentrations of sodium thiosulfate (Na₂S₂O₃) were, also performed in order to characterize the critical chromium concentration under the test conditions. It is observed that thermal treatment has a great effect on the microstructure and the chromium depletion profile of this alloy. Most specimens contained precipitates which were formed along grain boundaries in the semicontinuous form and were identified as M₇C₃ with a hexagonal crystal structure (a ₀=1.398 nm,C ₀=0.45 nm). Some intragranular precipitates which were identified as M₂₃C₆ with an fec crystal structure (a ₀=1.06 nm) were observed in the specimens aged for a longer period of time. The results of the constant load test showed that the susceptibility to SCC is sensitive to the chromium concentration at the grain boundary, and the minimum value to prevent SCC failure, under the test conditions, is approximately 8 wt pct.     

抗晶间腐蚀奥氏体不锈钢的晶界工程

Sensitization by chromium depletion due to chromium carbide precipitation at grain boundaries in austenitic stainless steels can not be prevented perfectly only by previous conventional techniques, such as reduction of carbon content, stabilization-treatment, local solution-heat-treatment, etc. Recent studies on grain boundary structure have revealed that the sensitization depends strongly on grain boundary character and atomic structure, and that low energy grain boundaries such a~ coincidence-site-lattice (CSL) boundaries have strong resistance to intergranular corrosion. The concept of grain boundary design and control has been developed as grain boundary engineering (GBE). GBEed materials are characterized by high frequencies of CSL boundaries which are resistant to intergranular deterioration of materials, such as intergranular corrosion. A thermomechanical treatment was tried to improve the resistance to the sensitization by GBE. A type 304 austenitic stainless steel was cold-rolled and solution-heat-treated, and then sensitization-heat-treated. The grain boundary character distribution was examined by orientation imaging microscopy (OIM). The intergranular corrosion resistance was evaluated by electrochemical potentiokinetic reactivation (EPR) and ferric sulfate-sulfuric acid tests. The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction The frequency of CSL boundaries indicated a maximum at the small reduction. The ferric sulfate-sulfuric acid test showed much smaller corrosion rate in the thermomechanical-treated specimen than in the base material. A high density of annealing twins were observed in the thermomechanical-treated specimen. The results suggdst that the therrmomechanical treatment can introduce low energy segments in the grain boundary network by annealing twins and can arrest the percolation of intergranular corrosion from the surface. The effects of carbon content and other minor elements on optimization in grain boundary character distribution (GBCD) and thermomechanical parameters were also examined during GBE.     

Grain-boundary penetration of type 316 stainless steel exposed to cesium or cesium and tellurium

When 20 pct cold-worked Type 316 stainless steel is exposed to Cs at 700°C under controlled oxygen-chemical potential environment, Cs penetration into the stainless steel grain boundaries occurs at oxygen potentials ΔG_o2 ≥ -96 kcal per mole. At lower oxygen potentials (~ΔG_o2 ≤ —110 kcal per mole), no corrosion occurs. Under the same experimental conditions, when the stainless steel is exposed to Cs:Te (2:1, atomic), corrosion occurs and penetration morphology appears to depend strongly on the oxygen-potential environment. The stainless steel suffers intergranular corrosion by Te (in the presence of Cs-Te) under conditions where chromium oxidation is not expected to occur. The kinetics of grain-boundary penetration by Te have been studied at temperatures between 550 and 700°C. The depth of the penetrated zone varies as (time)^1/2, and the process has an activation energy of 34 kcal per mole. The results are discussed, and the effects of stainless steel microstructure and externally applied stress on corrosion reactions are also described.     

The microstructural response of mill-annealed and solution-annealed INCONEL 600 to heat treatment

Samples of INCONEL* 600 were examined in the mill-annealed and solution-annealed states, and after isothermal annealing at 400 °C and 650 °C. The corrosion behavior of the samples was examined, analytical electron microscopy was used to determine the microstructures present and the chemistry of grain boundaries, and Auger electron spectroscopy was used to measure grain boundary segregation. Samples of different alloys in the mill-annealed state were found to have quite different microstructures, with Cr-rich M₇C₃ carbides occurring either along grain boundaries or in intragranular sheets. The corrosion behavior of the samples correlated well with the occurrence of grain boundary chromium depletion. Solution annealing at 1190 °C caused dissolution of all carbides, whereas at 1100 °C the carbides either dissolved or the grain boundaries moved away from the carbides, depending upon alloy carbon content. Low-temperature annealing at 400 °C had little effect on millannealed or fully solutionized samples, but in samples with intragranular carbides present, the grain boundaries moved until intersecting or adjacent to the carbides. Isothermal annealing at 650 °C caused carbide nucleation and growth at grain boundaries in fully solutionized samples. Chromium depletion at grain boundaries accompanied carbide precipitation, with a minimum chromium level of 6 wt pct achieved after 5 hours. Healing was found to occur after 100 hours. Solution-annealed samples with intragranular carbides present had more rapid corrosion kinetics since the grain boundaries moved back to the existing carbides. Thermodynamic analysis of the chromium-depletion process showed good agreement with experimental measurements. The Auger results found only boron present at grain boundaries in the mill-annealed state. Aged samples had boron, nitrogen, and phosphorus present, with phosphorus and nitrogen segregating to the greatest extent. The kinetics of phosphorus segregation are much slower at 400 °C compared with 650 °C.     

Investigation of Carbide Precipitation Process and Chromium Depletion during Thermal Treatment of Alloy 690

For the purpose of studying the effect of heat treatment on carbide morphology and chromium concentration distribution, which are critical to the resistance of alloy 690 to stress corrosion cracking (SCC), a series of thermal treatments was performed. A model taking into account the intercorrelated dynamic process between the carbide precipitation and chemical diffusion of the chromium atom from matrix to grain boundary (GB) was constructed on the basis of classical nucleation theory, Kolmogorov–Johnson–Mehl–Avrami law, and diffusion theory. The validity of this model was evaluated by comparing the simulated results of the carbide average size and chromium concentration near the GB with the corresponding measured results. A discontinuous factor was introduced based on the relation linking the interdistance between the carbides and the carbide average size; thus, the carbide morphology and chromium concentration could be predicted by this model. According to the results of the experiments and simulations, a carbide discontinuous factor smaller than 2.2 together with the chromium concentration at the GB higher than a critical value (21 wt pct) were essential for the corrosion resistance ability of the alloy, and then some proper heat-treatment conditions were obtained through predicting the value of the two variables. In addition, the effects of the grain size and composition variation on the carbide discontinuous factor and chromium concentration profile were simulated. The results indicated that an intermediate grain size of approximately 31.8 to ~63.5 μm was beneficial for effectively improving the resistance of the alloy to SCC. Simultaneously, the carbon content should be adjusted near 0.02 pct, and the chromium content should be the highest possible in its chemical composition scale.     

Double loop electrochemical potentiokinetic reactivation test optimization in checking of duplex stainless steel intergranular corrosion susceptibility

The double loop electrochemical potentiokinetic reactivation (DL-EPR) test using an electrolyte of 33 pct H₂SO₄ solution with 0.3 pct HCl, at room temperature and at a potential scan rate dE/dt of about 2.5 mV/s, was chosen to evaluate the sensitization of austeno-ferritic duplex stainless steels (DSS). Reproducible and optimal test responses and high test selectivity in detecting integranular corrosion (IGC) susceptibility were verified for four DSS differing in their method of fabrication (cast or wrought) and their ferrite phase content (44 to 57 pct). The test was successfully used to analyze the interactions between precipitation, chromium depletion, and IGC sensitization of the UNS S31250 steel, which was aged between 6 minutes and 120 hours at temperatures varying from 500 °C to 900 °C. The eutectoid decomposition of the ferrite, at different aging temperatures, was investigated using various techniques. The chromium depletion was analyzed qualitatively by X-ray mapping in a scanning transmission electronic microscope (STEM) and quantitatively by analytical calculation based on the chromium diffusion in the ferrite. It was shown that the chromium content in the ferrite can decrease from 30 to 7.5 pct by weight during aging before total decomposition occurs. The interactions between precipitation and IGC sensitization during DSS aging were clearly shown by superimposing the time-temperature-start of precipitation (TTP) and time-temperature-sensitization (TTS) diagrams obtained from the DL-EPR tests performed for various levels of sensitization.     

Characterization and corrosion behavior of high-chromium white cast irons

Erosion-resistant high-chromium white cast irons (CWIs) are widely used in hydrotransport components, particularly in oil-sand operations. Due to the acceptance that corrosion processes can accentuate material degradation by erosion processes andvice versa, it is important to understand the corrosion resistance of these materials in the environments in which they are used. Three CWI alloys with different chemical compositions—chromium (26 to 40 wt pct) and carbon (2.5 to 4.3 wt pct)—were investigated in this study. Electrochemical DC potentiodynamic polarization and potentiostatic tests were carried out in a solution of 1000 ppm Cl⁻ at a pH of 8.5 (obtained by adding NaOH) that simulates a recycle cooling water. A detailed characterization of the microstructures was also performed. There are significant effects of microstructural features and alloy composition on the corrosion behavior of CWIs. Two key factors have been shown to determine the corrosion behavior: the primary carbide area fraction and the amount of chromium as well as other elements in the matrix. The corrosion resistance of the CWI alloys strongly depends on the ratio of chromium content in the M₇C₃ carbide to that in the matrix (Cr_M7C3/Cr_matrix).     

The influence of thermal treatment on the chemistry and structure of grain boundaries in inconel 600

Although it is widely accepted that certain heat treatments result in carbide precipitation accompanied by chromium depletion at the grain boundaries, no direct evidence of this phenomenon exists for Inconel 600. Using the Scanning Transmission Electron Microscope (STEM), the extent of grain boundary chromium depletion is quantitatively determined as a function of thermal treatment time at 700 °C following a 30 min solution anneal at 1100 °C. Results confirm the presence of grain boundary chromium depletion that varies in extent with time at temperature, the chromium concentration falling to values as low as 3 wt pct. The chromium depletion volume is characterized by a depletion parameter which is correlated with intergranular corrosion test results to determine a self-healing (desensitization) chromium concentration of 9 wt pct. Trace element segregation at grain boundaries is measured by Auger Electron Spectroscopy (AES) as a function of aging treatment. Results show that after thermally treating samples for various times at 700 °C, phosphorus is always present at the grain boundaries. Intergranular corrosion behavior as a function of thermal treatment appears to be governed more strongly by chromium depletion than trace element segregation. G. S. WAS, formerly Research Assistant, Nuclear Engineering Dept., Massachusetts Institute of Technology H. H. TISCHNER, formerly Postdoctoral Associate, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Sensitization and Intergranular Corrosion Behavior of High Nitrogen Type 304LN Stainless Steels for Reprocessing and Waste Management Applications

High nitrogen 304LN stainless steels (SS) intended for chloride and nitric acid environments in spent nuclear fuel reprocessing and waste management applications were evaluated for their sensitization and intergranular corrosion (IGC) resistance. For this purpose, high nitrogen (0.132 pct, 0.193 pct and 0.406 pct) containing, impurity-controlled, vanadium-added 304LN SS alloys were developed. For comparison, 304L SS, which is currently used in reprocessing plants, was also studied. These stainless steels were subjected to heat treatment at 948 K (675 °C) for various durations ranging from 1 to 1000 hours and tested for susceptibility to IGC as per ASTM A262 Practice A and E tests. The degree of sensitization was estimated with the double loop electrochemical potentiokinetic reactivation technique. The increase in nitrogen content resulted in higher hardness and finer grain size. Based on the detailed microstructural and corrosion studies, it was determined that an addition of 0.132 pct and 0.193 pct nitrogen showed better IGC resistance and an additional increase in nitrogen resulted in deterioration resulting from chromium nitride precipitation, which was confirmed by electrochemical phase separation and X-ray diffraction studies. The onset of desensitization was faster for the alloy with 0.132 pct nitrogen as well as 0.406 pct nitrogen because of the lower nitrogen content in the former case and the finer grain size in the latter case. The higher hardness and superior IGC resistance of 0.132 pct and 0.193 pct nitrogen containing Type 304LN SS suggests the suitability of this alloy for nitric acid- and chloride-containing environments of reprocessing and waste management plants.     

The mechanism of intergranular cracking of Ni-Cr-Fe alloys in sodium tetrathionate

A series of electrochemical, immersion, and constant extension rate tests was conducted on samples of Ni-16Cr-9Fe in sodium tetrathionate at room temperature. Samples were heat treated to produce severe chromium depletion at the grain boundaries. Titrimetric analysis of the tetrathionate solution, before and after exposure to a sensitized alloy, under an applied cathodic current shows that the tetrathionate ion is reduced. The species primarily responsible for the observed IGA in immersion tests and IG cracking in constant extension rate tests is the tetrathionate ion, S₄O₆ ^-, although elemental S also causes shallow IGA. The mechanism responsible for the observed IGA and IG cracking in sensitized Ni-Cr-Fe alloys is stress assisted intergranular attack with the effect of stress being purely mechanical in nature. The degree of IGA and IG cracking is directly related to the grain boundary chromium content. Samples with less than 5 wt pct Cr at the grain boundary are rapidly attacked while those with 8 wt pct Cr are less susceptible and 12 wt pct Cr renders the grain boundary immune to attack. Lower extension rates and higher Na₂S₄O₆ concentrations represent more aggressive conditions for attack.     

Chromium depletion and martensite formation at grain boundaries in sensitised austenitic stainless steel

A microstructural and compositional investigation of grain boundary precipitation and martensite formation in sensitised 304 stainless steel has been conducted. Grain boundary depletion of chromium has been quantified in terms of sensitisation time, temperature and boundary type by energy dispersive X-ray microanalysis in the transmission electron microscope. Chromium depleted profiles measured in grain boundary vicinities are sometimes asymmetrical and correlate with the expected profiles generated by growth of semicoherent and incoherent carbide interfaces. The depletion of chromium promotes martensite formation within near-grain boundary regions and this transformation has been directly studied by in situ cold stage microscopy down to − 150°C. Transformation occurs at the most severely depleted boundaries and initiation is favoured at slip band-boundary intersection points and along grain boundaries whose plane orientation matches that of the martensite habit plane. The preferential formation of grain boundary martensite could be an important factor in the stress corrosion and environment sensitive failure of this material.     

Chromium depletion in the vicinity of carbides in sensitized austenitic stainless steels

The analytical electron microscope (AEM) was used to examine the microstructure of type 316LN stainless steel alloys which had been annealed for 50 to 300 hours in the temperature range 600 to 700 °C. The M₂₃C₆ carbide chemistry and distribution are described as a function of heat treatment.X-ray spectroscopy in the AEM revealed significant chromium depletion at grain boundaries in the vicinity of carbides for samples aged at 50 and 100 hours at 650 °C and 100 and 300 hours at 700 °C, with lower grain boundary chromium values observed at 650 °C than at 700 °C. The width of the chromium depleted zone normal to the grain boundaries increased with increasing annealing time and/or temperature. Measurements of chromium concentration along the grain boundaries away from a carbide were made after aging at 700 °C for 100 hours, and the chromium level rose steadily until the bulk value was reached at a distance of ~3μm from the carbide. The width of the chromium depleted zone normal to the boundaries in the same sample was an order of magnitude less. Some molybdenum depletion was also found at the grain boundaries, and the Mo-depletion profiles were in form and extent similar to the chromium results. Simple thermodynamic models were used to calculate the equilibrium value of chromium at the carbide-matrix interface, and the chromium distribution along and normal to the grain boundaries. The results of these models agreed well with the AEM results, and the agreement can be improved by considering the effect of electron probe configuration on the AEM measurements. The calculated thermodynamic data and the AEM results were related to the corrosion behavior of the alloys. The occurrence of severe asymmetries in some concentration profiles normal to the grain boundaries, which increased with increasing annealing temperature or time, was shown to be due to boundary movement during the discontinuous precipitation of M₂₃C₆ carbides.     

The effect of grain boundary chemistry on Intergranular stress corrosion cracking of Ni-Cr-Fe alloys in 50 Pct NaOH at 140 °C

The role of chromium, carbon, chromium carbides, and phosphorus on the intergranular stress corrosion cracking (IGSCC) resistance of Ni-Cr-Fe alloys in 50 pct NaOH at 140 °C is studied using controlled-purity alloys. The effect of carbon is studied using heats in which the carbon level is varied between 0.002 and 0.063 wt pct while the Cr level is fixed at 16.8 wt pct. The effect of Cr is studied using alloys with Cr concentrations between 5 and 30 wt pct. The effect of grain boundary Cr and C together is studied by heat-treating the nominal alloy composition of Ni-16Cr-9Fe-0.035C, and the effect of P is studied using a high-purity, P-doped alloy and a carbon-containing, P-doped alloy. Constant extension rate tensile (CERT) results show that the crack depth increases with decreasing alloy Cr content and increasing alloy C content. Crack- ing severity also correlates inversely with thermal treatment time at 700 °C, during which the grain boundary Cr content rises and the grain boundary C content falls. Phosphorus is found to have a slightly beneficial effect on IG cracking susceptibility. Potentiodynamic polarization and potentiostatic current decay experiments confirm that Cr depletion or grain boundary C enhances the dissolution at the grain boundary. Results support a film rupture-anodic dissolution model in which Cr depletion or grain boundary C (independently or additively) enhances dissolution of nickel from the grain boundary region and leads to increased IG cracking.     

Novel hafnium containing steels for power generation

Abstract

There is clear evidence that creep damage in power plant steels is associated with grain boundary precipitates. These particles provide favourable nucleation sites for grain boundary cavities and microcracks. The formation of M₂₃C₆ carbides as grain boundary precipitates can also lead to grain boundary chromium depleted zones which are susceptible to corrosive attack. Such precipitates are the causing loss of creep life in the later stages of creep because of their very high coarsening rate. Through Monte Carlo based grain boundary precipitation kinetics models, combined with continuum creep damage modelling it is predicted that improvements in creep behaviour of power plant steels can be achieved by increasing the proportion of MX type particles. Studies of a Hf containing steel have produced improvements in both creep and corrosion properties of 9%Cr steels. Hf has been ion implanted into thin foils of a 9 wt-%Cr ferritic steel to study its effect on precipitation. Two new types of precipitates are formed, Hf carbide, (an MX type precipitate) and a Cr–V rich nitride, with the formula M₂N. The Hf carbide particles were identified using convergent beam diffraction techniques, and micro-analysis. The nanosized particles are present in much higher volume fractions when compared to VN volume fractions in conventional power plant ferritic steels. Furthermore it is confirmed that the Hf causes the removal of M₂₃C₆ grain boundary precipitates. This has led to an increased concentration of Cr within the matrix, reduced chromium depleted zones at grain boundaries, and increased resistance to intergranular corrosion cracking.     

The effect of sheet processing on the microstructure, tensile, and creep behavior of INCONEL alloy 718

The grain size, grain boundary character distribution (GBCD), creep, and tensile behavior of INCONEL alloy 718 (IN 718) were characterized to identify processing-microstructure-property relationships. The alloy was sequentially cold rolled (CR) to 0, 10, 20, 30, 40, 60, and 80 pct followed by annealing at temperatures between 954 °C and 1050 °C and the traditional aging schedule used for this alloy. In addition, this alloy can be superplastically formed (IN 718SPF) to a significantly finer grain size and the corresponding microstructure and mechanical behavior were evaluated. The creep behavior was evaluated in the applied stress (σ _a ) range of 300 to 758 MPa and the temperature range of 638 °C to 670 °C. Constant-load tensile creep experiments were used to measure the values of the steady-state creep rate and the consecutive load reduction method was used to determine the values of backstress (σ₀). The values for the effective stress exponent and activation energy suggested that the transition between the rate-controlling creep mechanisms was dependent on effective stresses (σ _e =σ _a σ₀) and the transition occurred at σ _e ≅ 135 MPa. The 10 to 40 pct CR samples exhibited the greatest 650 °C strength, while IN 718SPF exhibited the greatest room-temperature (RT) tensile strength (>1550 MPa) and ductility (ε _f >16 pct). After the 954 °C annealing treatment, the 20 pct CR and 30 pct CR microstructures exhibited the most attractive combination of elevated-temperature tensile and creep strength, while the most severely cold-rolled materials exhibited the poorest elevated-temperature properties. After the 1050 °C annealing treatment, the IN 718SPF material exhibited the greatest backstress and best creep resistance. Electron backscattered diffraction was performed to identify the GBCD as a function of CR and annealing. The data indicated that annealing above 1010 °C increased the grain size and resulted in a greater fraction of twin boundaries, which in turn increased the fraction of coincident site lattice boundaries. This result is discussed in light of the potential to grain boundary engineer this alloy. INCONEL is a registered trademark of Special Metals Corp., Huntington, WV. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.     

The effects of heat treatment on the chromium depletion,precipitate evolution,and corrosion resistance of INCONEL alloy 690

A series of heat treatments were performed to study the sensitization and the stress corrosion cracking (SCC) behavior of INCONEL Alloy 690. The microstructural evaluation and the chromium depletion near grain boundaries were carefully studied using analytical electron microscopy (AEM). The measured chromium depletion profiles were matched well to the calculated results from a thermodynamic/kinetic model. The constant extension rate test (CERT) was performed in the solution containing 0.001 M sodium thiosulfate (Na₂S₂O₃) to study the SCC resistance of this alloy. The Huey test was also performed in a boiling 65 pct HNO₃ solution for 48 hours to study the intergranular attack (IGA) resistance of this alloy. Both tests showed that INCONEL 690 has very good corrosion resistance. It is believed that the superior IGA and SCC resistances of this alloy are due to the high chromium concentration (≈30 wt pct). It is concluded in this study that INCONEL 690 may be a better alloy than INCONEL 600 for use as the steam generator (S/G) tubing material for pressurized water reactors (PWR's)     

Low temperature sensitization of type 304 stainless steel pipe weld heat affected zone

Large-diameter Type 304 stainless steel pipe weld heat-affected zone (HAZ) was investigated to determine the rate at which low temperature sensitization (LTS) can occur in weld HAZ at nuclear reactor operating temperatures and to determine the effects of LTS on the initiation and propagation of intergranular stress corrosion cracks (IGSCC). The level of sensitization was determined with the electrochemical potentiokinetic reactivation (EPR) test, and IGSCC susceptibility was determined with constant extension rate tests (CERT) and actively loaded compact tension (CT) tests. Substructural changes and carbide compositions were analyzed by electron microscopy. Weld HAZ was found to be susceptible to IGSCC in the as-welded condition for tests conducted in 8-ppm-oxygen, high-purity water at 288 °C. For low oxygen environments (i.e., 288 °C/0.2 ppm O₂ or 180 °C/1.0 ppm O₂), IGSCC susceptibility was detected only in weld HAZ that had been sensitized at temperatures from 385 °C to 500 °C. Lower temperature heat treatments did not produce IGSCC. The microscopy studies indicate that the lack of IGSCC susceptibility from LTS heat treatments below 385 °C is a result of the low chromium-to-iron ratio in the carbide particles formed at grain boundaries. Without chromium enrichment of carbides, no chromium depleted zone is produced to enhance IGSCC susceptibility.     

Thermodynamics and kinetics of δ→ α martensitic transformation in Pu alloys

A regular solution model is fit to experimental equilibrium temperatures(T ₀) and pressures(P ₀) for diffusionlessδ→α transformations in Pu-Ga and Pu-Al alloys, in order to define the chemical free energy change ΔG forδ→ α transformation. Analysis of reported isothermalδ→ αtransformation rate data in terms of nucleation-controlled martensitic kinetics gives a nucleation activation energyQ which is a nonlinear function of ΔG. The activation volumeV _* defined byδQ/δΔG is of the order of 10 to 40 atomic volumes, suggestive of rate control by an interfacial Peierls barrier. The grain size dependence of the transformation-start temperature at a fixed cooling rate of 2.08 × 10_-2 Ks_-1 is measured in a Pu-1.7 at. pct Ga alloy, revealing an inhibition of transformation at fine grain sizes. The overall kinetic behavior is characteristic of a martensitic mechanism.     

Density measurements as an assessment of creep damage and cavity growth

The change in density during creep of several polycrystalline metals may be correlated through the expression —Δρ/ρ =B(∈t/d)(σ/G)^q exp (—Q _gb /RT) where —Δρ is the density change, p is the original density, e is the strain,t is the time,d is the linear intercept grain size,σ is the applied stress,G is the shear modulus,Q _gb is the activation energy for grain boundary diffusion,R is the gas constant,T is the absolute temperature, andB andq are constants withq ≃2 to 3. This expression is consistent with the theory of unconstrained grain boundary diffusion growth of cavities provided there is also concomitant strain-dependent nucleation. The expression does not support the power-law growth of cavities, growth by surface diffusion, or constrained grain boundary diffusion growth. Formerly Research Associate, University of Southern California.