Thomas M. Angeliu

Misorientation Mapping for Visualization of Plastic Deformation via Electron Back-Scattered Diffraction

The ability to map plastic deformation around high strain gradient microstructural features is central in studying phenomena such as fatigue and stress corrosion cracking. A method for the visualization of plastic deformation in electron back-scattered diffraction (EBSD) data has been developed and is described in this article. This technique is based on mapping the intragrain misorientation in polycrystalline metals. The algorithm maps the scalar misorientation between a local minimum misorientation reference pixel and every other pixel within an individual grain. A map around the corner of a Vickers indentation in 304 stainless steel was used as a test case. Several algorithms for EBSD mapping were then applied to the deformation distributions around air fatigue and stress corrosion cracks in 304 stainless steel. Using this technique, clear visualization of a deformation zone around high strain gradient microstructural features (crack tips, indentations, etc.) is possible with standard EBSD data.

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.

Repassivation and Crack Propagation of Alloy 600 in 288°C Water

Abstract Polarization and repassivation behaviors of alloy 600 (UNS N06600) were evaluated at 288°C in 0.1 M boric acid (H3BO3) titrated with sodium hydroxide (NaOH, pH25°C = 7.9) as a function of dissolved hydrogen (0 cm3/kg to 48 cm3/kg [0 ppm to 2.7 ppm] H2) and Zn (0 wppb and 60 wppb). Potentiodynamic scans measured polarization behavior, while a combination of drop-weight straining and cathodic reduction/potential pulse techniques measured repassivation behavior. Potentiodynamic scans revealed larger current densities, especially over the range from −800 mVSHE to −550 mVSHE, with addition of H2. At 0 cm3/kg H2, dissolved Zn at 60 wppb reduced the current density at ∼ −650 mVSHE. However, 60 wppb Zn did not affect repassivation kinetics at 0 cm3/kg and 48 cm3/kg H2. Repassivation kinetics experiments conducted slightly above the open-circuit potential (EOC) revealed a monotonic decrease in the oxidation current transient with increasing H2 at short times. Reduced current transients at higher levels of...

The Influence of Dissolved hydrogen on Nickel Alloy SCC: A Window to Fundamental Insight

Prior stress corrosion crack growth rate (SCCGR) testing of nickel alloys as a function of the aqueous hydrogen concentration (i.e., the concentration of hydrogen dissolved in the water) has identified different functionalities at 338 and 360 C. These SCCGR dependencies have been uniquely explained in terms of the stability of nickel oxide. The present work evaluates whether the influence of aqueous hydrogen concentration on SCCGR is fundamentally due to effects on hydrogen absorption and/or corrosion kinetics. Hydrogen permeation tests were conducted to measure hydrogen pickup in and transport through the metal. Repassivation tests were performed in an attempt to quantify the corrosion kinetics. The aqueous hydrogen concentration dependency of these fundamental parameters (hydrogen permeation, repassivation) has been used to qualitatively evaluate the film-rupture/oxidation (FRO) and hydrogen assisted cracking (HAC) SCC mechanisms. This paper discusses the conditions that must be imposed upon these mechanisms to describe the known nickel alloy SCCGR aqueous hydrogen concentration functionality. Specifically, the buildup of hydrogen within Alloy 600 (measured through permeability) does not exhibit the same functionality as SCC with respect to the aqueous hydrogen concentration. This result implies that if HAC is the dominant SCC mechanism, then corrosion at isolated active path regions (i.e., surface initiation sites or cracks) must be the source of localized elevated detrimental hydrogen. Repassivation tests showed little temperature sensitivity over the range of 204 to 360 C. This result implies that for either the FRO or the HAC mechanism, corrosion processes (e.g., at a crack tip, in the crack wake, or on surfaces external to the crack) cannot by themselves explain the strong temperature dependence of nickel alloy SCC.

Creep and intergranular cracking of Ni-Cr-Fe-C in 360 °C argon

The influence of carbon and chromium on the creep and intergranular (IG) cracking behavior of controlled-purity Ni-xCr-9Fe-yC alloys in 360 °C argon was investigated using constant extension rate tension (CERT) and constant load tension (CLT) testing. The CERT test results at 360 °C show that the degree of IG cracking increases with decreasing bulk chromium or carbon content. The CLT test results at 360 °C and 430 °C reveal that, as the amounts of chromium and carbon in solution decrease, the steady-state creep rate increases. The occurrence of severe IG cracking correlates with a high steady-state creep rate, suggesting that creep plays a role in the IG cracking behavior in argon at 360 °C. The failure mode of IG cracking and the deformation mode of creep are coupled through the formation of grain boundary voids that interlink to form grain boundary cavities, resulting in eventual failure by IG cavitation and ductile overload of the remaining ligaments. Grain boundary sliding may be enhancing grain boundary cavitation by redistributing the stress from inclined to more perpendicular boundaries and concentrating stress at discontinuities for the boundaries oriented 45 deg with respect to the tensile axis. Additions of carbon or chromium, which reduce the creep rate over all stress levels, also reduce the amount of IG fracture in CERT experiments. A damage accumulation model was formulated and applied to CERT tests to determine whether creep damage during a CERT test controls failure. Results show that, while creep plays a significant role in CERT experiments, failure is likely controlled by ductile overload caused by reduction in area resulting from grain boundary void formation and interlinkage.

Ab initio calculations for industrial materials engineering: successes and challenges

Computational materials science based on ab initio calculations has become an important partner to experiment. This is demonstrated here for the effect of impurities and alloying elements on the strength of a Zr twist grain boundary, the dissociative adsorption and diffusion of iodine on a zirconium surface, the diffusion of oxygen atoms in a Ni twist grain boundary and in bulk Ni, and the dependence of the work function of a TiN-HfO(2) junction on the replacement of N by O atoms. In all of these cases, computations provide atomic-scale understanding as well as quantitative materials property data of value to industrial research and development. There are two key challenges in applying ab initio calculations, namely a higher accuracy in the electronic energy and the efficient exploration of large parts of the configurational space. While progress in these areas is fueled by advances in computer hardware, innovative theoretical concepts combined with systematic large-scale computations will be needed to realize the full potential of ab initio calculations for industrial applications.

Electron Back-Scattered Diffraction Misorientation Mapping Applied to Stress Corrosion Cracking of Stainless Steels

Stress corrosion cracking (SCC) is a persistent problem that limits metallic component life in a wide variety of environments. Because SCC is largely influenced by the crack tip oxide rupture rate, life prediction models require characterization of the crack tip strain rate for SCC cracks. Crack tip strain rate may be mathematically derived from crack tip strain; however, crack tip strain is difficult to characterize due to the required small scale of the measurement and its highly localized nature [1]. Measuring the spatial gradient in crack tip strain requires both sensitivity to plastic strain and high spatial resolution. EBSD and related techniques, such as electron channeling patterns, have been used with some success to measure plastic strain around cracks in Fe-3Si single crystals [2], Cu single crystals [3], and Ni-based superalloys [4], among others. In this paper, we will discuss the use of misorientation mapping as a means for visualizing plastic deformation around crack tips in air fatigue and stress corrosion cracks.

Development of Nanostructured and Nanoparticle Dispersion-Reinforced Metallic Systems - Progress and Challenges

Investigations on nanostructured metallic systems have shown that exceptional property enhancements are potentially achievable through structural refinement to the nano-scale. While dramatic property improvements have been reported on these materials in the past, significant technical challenges exist in processing, as well as in retention of microstructural stability and useful properties at elevated temperatures. This presentation will summarize the key challenges in developing nanostructured metallic systems, and highlight our progress and selected successes in the understanding of structure-property relationships in bulk nanostructures as well as nanostructured multilayered systems. Work in nanoparticle dispersion-strengthened metallic systems will also be highlighted in this presentation.