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This report presents the results and recommendations of the deliberations of the U.S. Department of Energy (DOE) Fusion Energy Sciences Advisory Committee Panel on the Review of the Fusion Materials Research Program carried out during 1998. Metrics evaluated included evidence of recognition, publications per worker, new people attracted to the work and significance of recent accomplishments. [ABSTRACT FROM AUTHOR]

The effect of high-pressure hydrogen and temperature on crack growth was studied in wedge-opening-load (WOL) samples of a low-carbon steel. At temperatures above 280 ‡C, a hydrogen pressure of 3 ksi gave an increasing amount of acceleration in crack growth. These conditions approached but were below that needed to give hydrogen attack (HA) in the surrounding matrix. The value of da/dt increases exponentially with temperature, and Q is roughly equal to that for grain boundary diffusion. The growth is absent at K = 0 but varies little with K above 15 MPa√m. The value of da/dt increases steadily with hydrogen pressure in the range of 3 to 21 MPa. [ABSTRACT FROM AUTHOR]

Intergranular stress corrosion cracking (IGSCC) has been studied in two similar nickel-base alloys, alloys 600 and X-750, in deaerated steam at elevated temperatures (380 °C). In both cases, IGSCC occurs through the nucleation, growth, and linkup of grain boundary voids rather than by grain boundary dissolution. The rate of IGSCC is much faster in both alloys when they are heat-treated so that carbon is kept in solution instead of precipitating as grain boundary carbides. This higher carbon content allows the formation of a higher methane pressure in grain boundary voids. Slip impingement on the grain boundary plays a central role in nucleating a set of voids with a spacing of about 0.2 μm. A second set of more closely spaced bubbles develops between these through the action of stress-assisted diffusion of material from the first set of bubbles. [ABSTRACT FROM AUTHOR]

Intergranular stress corrosion cracking (IGSCC) of Alloy 600 in high-temperature, deaerated water or steam has been termed 'hydrogen-induced IGSCC.' We believe these cracks are initiated by the nucleation of a high density of bubbles on the grain boundary under the combined action of the applied stress and high pressure methane formed from carbon in solution reacting with hydrogen injected by corrosion. The bubbles then grow together by grain boundary diffusion to give local failure. This is supported by transmission electron microscope (TEM) observations of two-stage replicas, which show the subsurface formation of closely spaced (0.2 μm) bubbles along boundaries, and their growth into fine cracks before they open to communicate with the corroding atmosphere. The kinetics are examined and shown to be in quantitative agreement with several experimental observations. This mechanism involves no grain boundary dissolution of the metal, the only role of corrosion being the injection of hydrogen at a high fugacity. It predicts an activation energy roughly equal to that for grain boundary dψusion of nickel in Alloy 600. [ABSTRACT FROM AUTHOR]

Intergranular stress corrosion cracking (IGSCC) of Alloy 600 in high-temperature, deaerated water or steam has been termed 'hydrogen-induced IGSCC.' We believe these cracks are initiated by the nucleation of a high density of bubbles on the grain boundary under the combined action of the applied stress and high pressure methane formed from carbon in solution reacting with hydrogen injected by corrosion. The bubbles then grow together by grain boundary diffusion to give local failure. This is supported by transmission electron microscope (TEM) observations of two-stage replicas, which show the subsurface formation of closely spaced (0.2 μm) bubbles along boundaries, and their growth into fine cracks before they open to communicate with the corroding atmosphere. The kinetics are examined and shown to be in quantitative agreement with several experimental observations. This mechanism involves no grain boundary dissolution of the metal, the only role of corrosion being the injection of hydrogen at a high fugacity. It predicts an activation energy roughly equal to that for grain boundary dψusion of nickel in Alloy 600. [ABSTRACT FROM AUTHOR]

The rate of diffusion of Zn into Fe in solid Fe/Zn diffusion couples is found to be two to four orders of magnitude greater than would be predicted by lattice diffusion. The penetration depth varies markedly from sample to sample. It is achieved primarily in an initial transient, and then stops even though a steep concentration gradient persists in the ferrite. The maximum concentration of Zn in the ferrite surface layer often exceeds the published solubility of the first intermetallic phase, gamma, in ferrite. The initial Zn penetration seems to occur by DIGM. The mechanism for continued penetration is less clear. The variability in percent Zn in the ferrite, and possibly the variation in penetration depth, must stem from the delay in the nucleation of the most iron rich intermetallic phase, gamma. [ABSTRACT FROM AUTHOR]

A 1020 carbon steel and a sensitive dilatometer have been used to study the changes in the kinetics, and morphology, of methane bubbles with temperature, methane pressure, and bubble size. The transition from roughly spherical bubbles at low methane pressure to lenticular ones at high pressure, as predicted by theory, has been demonstrated along with the predicted change in pressure exponent and activation energy. That is, at high pressures the rate of bubble growth increases as ( P and exhibits an activation energy characteristic of surface diffusion, while at quite low pressures the pressure exponent is about 3/2 with an activation energy between that for grain boundary and lattice diffusion. When the lenticular bubbles grow to a diameter roughly equal to that of the grains, their growth kinetics change to be limited by the power law creep of the matrix. [ABSTRACT FROM AUTHOR]

The hydrogen attack of steel involves the formation of methane bubbles along the grain boundaries and their subsequent link-up to form fissures. This paper presents a detailed model for the kinetics of growth of such methane bubbles. The model considers two parallel processes which can control the growth-one involving the bubble growth by direct power-law creep and the other involving combinations of surface diffusion, grain boundary diffusion and matrix accommodation processes. The proposed model is more general and complete than the earlier ones and considers for the first time the possibility of bubble growth being controlled by surface diffusion and accommodation processes. The predictions of the model are shown to compare well with the experimental results obtained in our lab and with the literature data. The model also indicates the relative importance of lower carbon activity and increased creep stength of steel to its hydrogen attack resistance. [ABSTRACT FROM AUTHOR]

The amount and mechanisms of metal loss has been studied for oblique and normal impact on copper, 70/30 brass, and precipitation hardened Cu-2 pct Be, using steel ball bearings and velocities from 138 to 300 m/s. The weight losses are roughly the same for the three alloys though the mecahnisms of flow and fracture change from highly plastic for copper to pronounced adiabatic shear localization for the Cu-Be alloy. At these high velocities and ball sizes the deformation is adiabatic, thus localizing the flow and enhancing loss compared to a slow isothermal indentation. Also, inertial forces are large at the base of any extruded lip, thus playing a central role in removing metal. For normal impacts the principle weight loss occurs where the impacts partially overlap, and stems from the concentration of strain at local irregularities in the surface. [ABSTRACT FROM AUTHOR]

The kinetics of the initial stages of hydrogen attack in a commercial 0.3 pct C steel (grade A516) were investigated using an in situ dilatometer. The time, temperature and hydrogen pressure dependences of the rate of sample expansion were measured at hydrogen pressures from 1 to 20 MPa, and temperatures from 350 to 475 °C for sample strains of 10 to 10. Sample expansion began shortly after hydrogen exposure and proceeded at a nearly constant rate throughout the 'incubation period' preceding rapid attack. At high temperatures and low pressures, this rate was proportional to P and had an apparent activation energy of 115 ± 9 kJ. At high pressures and low temperatures, the rate was proportional to PP and showed an apparent activation energy of 210 ± 13 kJ. This suggests that bubble growth during the incubation period occurs predominantly by grain boundary diffusion and is driven by near-equilibrium internal methane pressures. Sample expansion in the subsequent stages of accelerating growth probably is controlled by creep and methane generation. [ABSTRACT FROM AUTHOR]

A silicon killed 1020 steel was deformed to a maximum of 15 pct elongation in three point bending and exposed to hydrogen at 6.5 MPa and 450 °C for 44 to 220 h to develop hydrogen attack. In the undeformed material methane nucleated primarily on manganese sulphides and maganese silicate inclusions. With time fissures form along the entire length of the inclusions creating large fissures (>500 microns). Cold working the steel greatly enhanced the rate of fissure formation on the grain boundaries as well as the fraction of boundaries attacked. These fissures interconnect, venting methane to the surface. As a result of this venting the fissures in the deformed area swell much less and the material decarburizes at a more rapid rate than the undeformed. The degree of decarburization was independent the sign of the plastic strain. However, the direction of the fissures changed, being parallel to the free surface on the tensile side and perpendicular to the surface on the compressive side. The effect of cold work on HA was completely eliminated by an anneal at 650 °C for 1 h. [ABSTRACT FROM AUTHOR]

Four normalized carbon steels made in different ways (Si-killed, Al-killed, REM-treated, and electroslag refined) were studied to determine the role of differing fine inclusions on the early stages of hydrogen attack (HA). Hydrogen exposures were made at 450°C (6.5 MPa) and 375°C (7.6 MPa). The first stage of HA is shown to be the development of a closely spaced (1 to 2 µm) array of small bubbles over the ferrite/pearlite, or occasionally the ferrite/ferrite boundaries. These bubbles grew together to form tears, primarily in the rolling plane, leading to more rapid expansion normal to this plane than parallel to it. Fracture planes followed high solute layers in banded steel but only rarely if ever did the fine bubbles form on inclusions. At 450°C REM-treated steel was attacked the fastest and Al-killed steel required exposures two to four times as long for attack. [ABSTRACT FROM AUTHOR]

A model is developed for the kinetics of nucleation and growth of methane bubbles in the hydrogen attack of carbon steel. It is concluded that at high temperatures the time to incubate fissuring along grain boundaries is determined by the rate of iron diffusion away from microscopic growing bubbles. At lower temperatures and/or higher hydrogen pressures carbon supply is limiting. The equations fit the observed incubation times if the bubble density is high (~10/cm) and essentially independent of temperature ( T) and hydrogen pressure ( P) over a wide range. It is postulated that the number of growing bubbles is limited at high nucleation rates (low T and high P) by carbon starvation. At hi T and low P chemisorption to lower the solid-vapor surface energy or fine inclusions are required to aid nucleation. A quantitative analysis of these processes leads to several predictions which can be checked experimentally. [ABSTRACT FROM AUTHOR]

Crack growth in 2.25Cr-lMo steels exposed to 3000 psi hydrogen has been investigated in the temperature range 440 °C to 500 °C, using modified wedge-opening loaded specimens to vary stress intensity. Under conditions of temperature and hydrogen pressure, where general hydrogen attack does not occur, the crack propagated by the growth and coalescence of a high density of methane bubbles on grain boundaries, driven by the synergistic influence of internal methane pressure and applied stress. Crack growth rates were measured in base metal, and the heat-affected zones (HAZs) of welds were tempered to different strength levels. The crack growth rate increased with material strength. Above a threshold of about K = 20 MPa√m (at 480 °C), the crack growth rate increased rapidly with stress intensity, increasing as roughly K . Because of better creep resistance, stronger materials can sustain higher levels of stress intensity to drive crack growth and nucleate the high density of voids necessary for crack growth. Stress relaxation by creep reduces the stress intensity, and thus the growth rate, especially in weaker materials. The crack growth rate in the heat-affected zone was found to be substantially faster than in the base metal of the welds. Analysis indicates that K rather than C* is the appropriate crack-tip loading parameter in the specimen used here and in a thick-walled pressure vessel. The DC potential drop technique met with limited success in this application due to the spatially discontinuous manner of crack growth and limited crack-tip opening displacement. [ABSTRACT FROM AUTHOR]

The dissolution of FeC in ferrite has been shown to be controlled by a slow first order interfacial reaction. The rate constant, K, is Arrhenius in behavior and involves two activation enthalpies; one applies to a jump process with a short relaxation time, another to a long relaxation time process which is tentatively identified as a change in kink density on ledges in the interface. The heat of solution of FeC in ferrite also appears in the exponential. A single process, activated jump theory is not adequate to explain the behavior of K with temperature; a dissolution model based on a ledge-double kink mechanism appears to be in agreement with experiment. These features point to significant differences between the mechanism of interphase boundary migration and grain boundary migration. In addition, the FeC-ferrite interface is felt to be at equilibrium with respect to carbon but not iron, leading to small departures from stoichiometry in the FeC at the interface. [ABSTRACT FROM AUTHOR]

The rate of dissolution of spheroidized FeC in ferrite in both high purity and commercial purity plain carbon eutectoid steels has been investigated. The experiments consisted of equilibrating wire specimens of spheroidized eutectoid steels at 650°C or 700°C and then jumping the temperature (in 2 to 7 m-sec) 4 to 32 C deg above the equilibration temperature. After heating, the specimens were maintained constant at the new temperature, and the dissolution of FeC was followed by observing the changes in electrical resistivity. The data are correlated to changes in the average carbon concentration in ferrite and are compared to the predictions of a generalized phenomenological model. Two important facts have been determined. First, the FeC ferrite interfaces are not in a state of local equilibrium during dissolution; the deviations from equilibrium in all cases were large. Second, the kinetics of dissolution are first order and the log of the reaction rate constants are a linear function of (1/ T), where T is the absolute temperature. These results lead to the conclusion that under the present experimental conditions, the dissolution of FeC in ferrite is controlled entirely by an interfacial reaction. A rather detailed description of the experimental procedure is included in this paper because the technique is new, and it is broadly applicable in the study of many metallurgical phenomena. [ABSTRACT FROM AUTHOR]