Gary S. Was

Deformation and fracture in microlaminates

Abstract The utility of microlaminates in engineering applications depends ultimately on their strength and toughness. While the properties of monolithic films and coatings can be controlled through crystal structure and microstructure, the properties of microlaminates are a sensitive function of the interfaces. It is the large number of interfaces in a microlaminate that determines the unique behavior of this special type of composite. This review begins with a property-based definition of a microlaminate. The mechanisms by which microlaminates deform plastically are reviewed and evaluated in the context of data on metal-metal, metal-intermetallic, metal-ceramic and ceramic-ceramic systems. It is evident that in addition to layer geometry, the layer microstructure plays a major role in determining the operative deformation mechanism. The fracture processes in a microlaminate are examined in the context of the layer strength, microstructure, defects and crack-tip-dislocation processes. High toughnesses in microlaminate materials can be attained through a combination of mechanisms, and their effectiveness depends critically on the ability to affect the magnitude and shape of the stress field at the tip of the crack. The study of deformation and fracture in microlaminates is still a relatively young field in materials science. However, while our understanding of these processes is still quite incomplete, it is improving rapidly with advances in experiment, theory and modeling capability.

Radiation-induced material changes and susceptibility to intergranular failure of light-water-reactor core internals

Abstract Current understanding of radiation-induced material changes that occur in light-water-reactor (LWR) core components is critically reviewed and linked to intergranular failure processes. Although the basic science of radiation damage processes in metals is reasonably well established, accurate prediction of microstructures, microchemistries and mechanical property changes in complex stainless alloys during irradiation at LWR temperatures is not possible at present. Mechanistic understanding of these radiation-induced changes in commercial alloys is considered to be of paramount importance for the mitigation of the intergranular environmental cracking that occurs in service. Fundamental research is needed to define defect–solute interactions and microstructural evolution at intermediate temperatures and dose rates pertinent to LWRs where transient effects often dominate behavior. In addition, it is essential that radiation effects on matrix microstructure and microchemistry and grain boundary microchemistry be understood. Finally, a stronger emphasis on accurately quantifying radiation effects on environmental cracking mechanisms and kinetics is needed.

Ion irradiation-induced phase transformation of pyrochlore and zirconolite

Abstract Pyrochlore (Gd2Ti2O7) and zirconolite (CaZrTi2O7) were irradiated with 1.5 MeV Xe+, 1.0 MeV Kr+, and 0.6 MeV Ar+ at temperatures from 20 to 1073 K. Critical amorphization dose increased with increasing temperature. Heavier ion irradiation increased the critical temperature. The extrapolated critical temperatures for amorphization were calculated as: 1300, 1100 and 950 K for pyrochlore irradiated with 1.5 MeV Xe+, 1.0 MeV Kr+ and 0.6 MeV Ar+, respectively; 710 and 654 K for zirconolite irradiated with 1.5 MeV Xe+ and 1.0 MeV Kr+, respectively. At the early stage of irradiation, pyrochlore transformed to a disordered fluorite structure; monoclinic zirconolite first transformed to partially disordered cubic pyrochlore structure followed by a disordered fluorite structure.

Microstructural and microchemical mechanisms controlling intergranular stress corrosion cracking in light-water-reactor systems

This review paper examines mechanisms controlling IGSCC in selected LWR components. Emphasis is placed on identifying material microstructures and microchemistries which promote susceptibility to premature failure. Two important examples are evaluated in some detail: stainless steel pipe cracking and primary-side SCC of alloy 600 steam generator tubing. In each case, grain boundary segregation and precipitation phenomena in these materials are reviewed and assessed relative to the mechanisms of IGSCC. This paper summarizes materials presented at the 1993 International Summer School on the Fundamentals of Radiation Damage held at the University of Illinois. A more comprehensive overview of SCC mechanisms and LWR examples was provided at the school, but will not be included in this article. Microstructural and microchemical aspects controlling IGSCC described here serve as a lead-in to the following paper focussing on how irradiation influences SCC resistance of reactor core components.

The Role of grain boundary misorientation in intergranular cracking of Ni-16Cr-9Fe in 360 °C argon and high-Purity water

The effect of grain boundary misorientation on the intergranular cracking behavior of pure Ni-16Cr-9Fe was assessed by determining if low-angle boundaries (LABs) or coincident site lattice boundaries (CSLBs) are more crack resistant than general high-angle boundaries (GHABs) in argon and high-purity water. Cracking susceptibility of boundary types was determined using constant extension rate tensile tests (CERTs) in 360 °C argon and in deaerated, high-purity water. Annealed samples contained 12 to 20 pct CSLBs, while CSLB-enhanced samples contained 27 to 44 pct CSLBs; GHAB proportions varied accordingly. Cracked boundary fractions for CSLB-enhanced samples tested in either environment ranged from 0.01 to 0.08, while those for annealed samples ranged from 0.07 to 0.10, indicating that samples with increased proportions of CSLBs are more crack resistant. No LABs cracked in either environment. In annealed samples, the proportion of CSLBs that cracked in water was 6.7 pct compared to 1.5 pct in argon; the proportion of GHABs that cracked in water was 9.3 pct compared to 6.6 pct for argon. Thus, CSLBs are more crack resistant than GHABs in either environment, and both are more crack resistant in argon than in water. The higher amounts of cracking and the higher CSLB cracking susceptibility in high-purity water indicate the presence of an environmental effect on cracking behavior. The beneficial effect of LABs and CSLBs is likely due to the ability of these boundaries to induce slip in neighboring grains by either transmitting or absorbing and re-emitting lattice dislocations, thereby reducing grain boundary stresses and the propensity for crack initiation. The results indicate that control of grain boundary proportions can improve the intergranular stress corrosion cracking susceptibility of pure Ni-16Cr-9Fe.

A THERMODYNAMIC AND KINETIC BASIS FOR UNDERSTANDING CHROMIUM DEPLETION IN Ni-Cr-Fe ALLOYS

Abstract Thermodynamic and kinetic models are constructed to describe the development of the chromium depleted zone in Ni-Cr-Fe alloys heated in the range 773–1173 K. The models are interactive and constitute a computer program called DEPLETE. The thermodynamic model is constructed using the Kohler method for the description of the free energy of a multi-component system. It provides the chromium concentration at the carbide-matrix interface as a function of alloy composition and temperature. The kinetic model tracks the shape of the chromium profile as a function of time at temperature and grain size. Model results show that the interfacial chromium concentration decreases for increasing carbon concentration and decreasing heat treatment temperature. Experimental verification of the model is made using high resolution energy despersive X-ray analysis via STEM. Measured results agree well with model results for the dependence of chromium depletion on various input parameters as well as the magnitude and shape of the chromium depleted zone. Experimental measurements also show that the grain boundary carbides are of the form M7C3 where M is about 96% chromium. Results confirm that carbide precipitation at the grain boundary is controlled by volume diffusion of chromium in the matrix and that in the temperature range 873 to 1073 K the chromium concentration in the grain boundary accurately approximates the carbide-matrix interfacial concentration.

Effects of irradiation on intergranular stress corrosion cracking

Abstract Intergranular stress corrosion cracking (IASCC) is a pervasive and generic problem in current light water reactor and advanced reactor designs that can lead to widespread component failure. IASCC is believed to be due to either to changes in the grain boundary composition, the microstructure or the water chemistry and corrosion potential. Of greatest interest are the changes in composition and microstructure since IASCC exhibits a well-defined, although not invariant, dose threshold. Changes in grain boundary composition are a result of radiation-induced segregation (RIS) and result in enrichment of nickel, depletion of chromium as well as changes in the impurity element compositions at the grain boundary. Although the basic theory of RIS is believed to be understood, quantitative descriptions of observed changes are not yet possible and hinder the correlation between RIS and IASCC. Changes in the microstructure are intimately linked to the strength and ductility of the irradiated alloy and strong correlations between IASCC and irradiated yield strength have been found. However, a fundamental understanding of the deformation mechanisms and the way in which deformation is coupled to IG cracking in alloys irradiated under LWR conditions (250–360°C, 1–5 dpa) is lacking. Finally, although radiation is known to affect IGSCC through changes in water chemistry and corrosion potential, it is not a necessary condition. Overshadowing and slowing progress on this important problem is a lack of well-defined-data from properly irradiated and properly characterized materials, due principally to inherent experimental and financial difficulties. As such, the specific mechanism(s) of IASCC remain unknown.

Combined effect of special grain boundaries and grain boundary carbides on IGSCC of Ni–16Cr–9Fe–xC alloys

Abstract Susceptibility to intergranular stress corrosion cracking in Ni–16Cr–9Fe– x C alloys in 360°C primary water is reduced with increasing fraction of special grain boundaries, i.e. coincident site lattice boundaries (CSLB) and low angle boundaries, and grain boundary carbides. Intergranular stress corrosion cracking (IGSCC) was investigated using interrupted constant extension rate tensile tests in a primary water environment at 360°C. Thermal–mechanical treatments were used to increase the fraction of special boundaries from approximately 20–25% to between 30 and 40%. In a carbon-doped heat, further heat treating was used to precipitate grain boundary carbides preferentially on high-angle boundaries (HAB). Orientation imaging microscopy was used to determine the relative grain misorientations and scanning electron microscopy (SEM) was used to identify specific grain boundaries after each interruption. After each strain increment, the same regions in each sample were examined for cracking. Results showed that irrespective of the microstructure condition, CSLBs always cracked less than HABs. Results also showed that IGSCC is reduced with increasing solution carbon content, and for the same amount of carbon in solution, the addition of grain boundary carbides reduced IGSCC still further. The best microstructure was the one consisting of an enhanced CSLB fraction and chromium carbides precipitated preferentially on high-angle boundaries.

Isolating the effect of radiation-induced segregation in irradiation-assisted stress corrosion cracking of austenitic stainless steels

Post-irradiation annealing was used to help identify the role of radiation-induced segregation (RIS) in irradiation-assisted stress corrosion cracking (IASCC) by preferentially removing dislocation loop damage from proton-irradiated austenitic stainless steels while leaving the RIS of major and minor alloying elements largely unchanged. The goal of this study is to better understand the underlying mechanisms of IASCC. Simulations of post-irradiation annealing of RIS and dislocation loop microstructure predicted that dislocation loops would be removed preferentially over RIS due to both thermodynamic and kinetic considerations. To verify the simulation predictions, a series of post-irradiation annealing experiments were performed. Both a high purity 304L (HP-304L) and a commercial purity 304 (CP-304) stainless steel alloy were irradiated with 3.2 MeV protons at 360 °C to doses of 1.0 and 2.5 dpa. Following irradiation, post-irradiation anneals were performed at temperatures ranging from 400 to 650 °C for times between 45 and 90 min. Grain boundary composition was measured using scanning transmission electron microscopy with energy-dispersive spectrometry in both as-irradiated and annealed samples. The dislocation loop population and radiation-induced hardness were also measured in as-irradiated and annealed specimens. At all annealing temperatures above 500 °C, the hardness and dislocation densities decreased with increasing annealing time or temperature much faster than RIS. Annealing at 600 °C for 90 min removed virtually all dislocation loops while leaving RIS virtually unchanged. Cracking susceptibility in the CP-304 alloy was mitigated rapidly during post-irradiation annealing, faster than RIS, dislocation loop density or hardening. That the cracking susceptibility changed while the grain boundary chromium composition remained essentially unchanged indicates that Cr depletion is not the primary determinator for IASCC susceptibility. For the same reason, the visible dislocation microstructure and radiation-induced hardening are also not sufficient to cause IASCC alone.

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.