Russell H. Jones

Tensile strength and fracture surface characterization of Hi-Nicalon SiC fibers

Abstract Dimensional, tensile strength and fracture surface characterizations were carried out for a particular batch of Hi-Nicalon™ SiC fiber. In general, filaments with larger cross-sectional areas (equivalent diameters) had lower strengths than filaments with smaller cross-sectional areas. A cyclic variation of fiber diameter along filament lengths was discovered with a repeat distance of about 16-cm and a maximum rate of change of about ±0.6 μm / cm . During tensile tests at room temperature, fracture originated at critical flaws that typically consisted of internal pores or carbonaceous inclusions. Well-demarcated mirror and hackle regions usually surrounded the critical flaws. With a few exceptions, the critical flaw size (ac) was linearly related to the mirror size (rm) by ac≈0.33rm. From fracture mechanics principles, values for the average mirror constant (Am) and effective fracture toughness for this batch of Hi-Nicalon™ fiber were estimated to be 2.99±0.33 and 1.1±0.2 MPa m1/2, respectively.

Hydrogen pressure dependence of the fracture mode transition in nickel

A relationship between fracture mode, grain boundary composition, and hydrogen pressure has been determined for nickel straining electrode samples tested at cathodic potentials. This relationship can be expressed asCs/* αCH2/−nwhereCs/* is the critical grain boundary sulfur concentration corresponding to 50 pct transgranular and 50 pct intergranular fracture andPH2 is the hydrogen pressure. The value ofn was found to be between 0.34 and 0.9. This expression was derived by relatingCs/* to the hydrogen overpotential with the Nernst equation. At a cathodic test potential of −0.3 V (SCE),Cs/* was equal to 0.20 monolayers of sulfur and at higher cathodic potentials or higher hydrogen pressures,Cs/* decreased such that at −0.72 V (SCE)Cs/* was equal to 0.045 monolayers of sulfur. The inverse hydrogen pressure dependence observed with cathodic hydrogen is similar to that for the hydrogen permeation rate or a critical hydrogen concentration derived by Gerberichet al.6 for gaseous hydrogen. This similarity between gaseous and cathodic hydrogen suggests that grain boundary impurities contribute to the hydrogen embrittlement process without altering the embrittlement process although this result does not indicate whether decohesion or plasticity dependent processes are responsible for the combined sulfur-hydrogen effect on the intergranular fracture of nickel.

Flow and fracture of alloys in the fusion environment

Abstract The present paper examines both ductile and brittle fracture models of steels and assesses the impact of the fusion reactor environment on the fracture processes. In particular, the connections between plastic flow properties and fracture modes are reviewed for both ductile and brittle crack propagation. Highly radiation-hardened materials exhibit extreme flow localization resulting in channel fracture. Physical models for this phenomon are developed and an estimate for the associated fracture toughness is given. The impact of radiation-hardening and ductility loss on fatigue crack growth is examined. Next, models describing the chemical effects on fatigue and fracture are briefly discussed. Finally, fracture design criteria are proposed for first wall structures in fusion reactors.

Effect of loading mode on the fracture toughness of a ferritic/martensitic stainless steel

The critical J integrals of mode I (JIC), mixed-mode I/III (JTC), and mode III (JIIIC) were examined for a ferritic stainless steel (F-82H) at ambient temperature. A determination of JTC was made using modified compact-tension specimens. Different ratios of tension/shear stress were achieved by varying the principal axis of the crack plane between 0 and 55 deg from the load line. The value for JIC was determined by means of specially designed specimens. The results showed that F-82H steel had high fracture toughness. Both JIC and JIIIC were about 500 kJ/m2, and the mode I tearing modulus JIda) was about 360 (kJ/m2)/mm. However, JTC and mixed-mode tearing modulus (dJT/da) values varied with the crack angles and were lower than their mode I and mode III counterparts. Both the minimum JTC and dJT/da values occurred at a crack angle between 40 and 50 deg, at which the load ratio of σiii/σ, was 0.84 to 1.2. The Jmin was 240 kJ/m2, and ratios of JlC/Jmin and JIIICJmin were 2.1 and 1.9, respectively. The morphology of the fracture surfaces was consistent with the change in JTC and dJT/da values. While the upper shelf-fracture toughness of F-82H depended on loading mode, the Jmin value remained high. Other important considerations include the effect of mixed-mode loading on the ductile-brittle-transition temperature and effects of hydrogen and irradiation on J^.

Irradiation-enhanced creep in SiC: data summary and planned experiments

Silicon carbide composites are under consideration for structural applications in proposed magnetic fusion energy systems. The limited data available suggest that creep of silicon carbide, like that of metals, is enhanced in a neutron environment. Existing irradiation-creep data on silicon carbide are reviewed and the ramifications of these data are discussed. Two experimental approaches to determine the irradiation creep of a silicon carbide composite are described. One of these utilizes a flat, reduced gage section specimen loaded in tension and the other utilizes a pressurized cylinder.

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 ratioXc/XCo in the Brunauer-Emmett-Teller (BET) adsorption isotherm adapted for equilibrium segregation in solids.

Subcritical intergranular crack growth rates and thresholds of Fe and Fe + Sb

The addition of 0.06 monolayers of antimony to the grain boundaries of iron with 0.3 monolayers of sulfur was found to have no effect on the fracture toughness or subcritical crack growth behavior at cathodic potentials. Tests were conducted using compact tension type samples tested in 1N H2SO4 at cathodic potentials of −0.6V (SCE) to −1.25V (SCE). The absence of any effect of antimony on the fracture toughness was related to iron being in a “minimum” fracture toughness condition such that further segregation of an embrittling element had no effect. Also, the subcritical intergranular crack growth threshold was found to decrease with increasing cathodic potential consistent with results reported by others for transgranular fracture of steels in gaseous hydrogen.

Hydrogen-Induced subcritical crack growth of a 12 Cr-1 Mo ferritic stainless steel

Subcritical crack growth and tensile ductility measurements have been made on a 12 Cr-1 Mo ferritic stainless steel at cathodic potentials in a 1 N H2SO4 solution at 25 °C. The tensile ductility was found to be a minimum at −600 mV (SCE) and both the subcritical crack growth behavior and tensile ductility were similar for material in the tempered (760 °C/2.5 h) or tempered-plus-segregated (540 °C/240 h) condition. A rising-load crack growth threshold of 20 MPa √m was measured and a rising-load fracture toughness of 110 MPa √m was determined from extrapolation of the stage III crack growth curve. A K-independent stage II was observed and a stage II crack growth rate of about 1 × 10−5 mm/s was measured. The fracture mode was a mixture of intergranular and quasi-cleavage for both heat treatments and for subcritical and tensile fracture tests. Impact fracture properties were independent of heat treatment and grain boundary composition with the fracture mode predominantly transgranular. The difference in the fracture mode for hydrogen-induced crack growth and dynamic crack growth was explained by a difference in the relationship between their stress profiles and the maximum grain boundary segregation distribution.

Some ion probe mass spectrometer observations of nickel activated recrystallization of doped tungsten and TZM

The study was initiated to utilize the elemental imaging capabilities of a secondary ion mass spectrometer to help elucidate the chemically activated recrystallization mechanisms.

Acoustic Emission During Intergranular Stress Corrosion Cracking

The physical and chemical processes taking place during intergranular stress corrosion cracking (IGSCC), in particular the effects of impurities on cracking mechanisms, have been the subjects of a research program sponsored by the Division of Materials Science, Office of Basic Energy Science, U. S. Department of Energy at Pacific Northwest Laboratory (PNL), operated by Battelle Memorial Institute. Acoustic emission (AE) was brought into the program because of the unique ability of AE methods to detect dynamic microscopic fracture processes. In this paper, the results of these tests are presented.