Christopher J. Torbet

A new TriBeam system for three-dimensional multimodal materials analysis

The unique capabilities of ultrashort pulse femtosecond lasers have been integrated with a focused ion beam (FIB) platform to create a new system for rapid 3D materials analysis. The femtosecond laser allows for in situ layer-by-layer material ablation with high material removal rates. The high pulse frequency (1 kHz) of ultrashort (150 fs) laser pulses can induce material ablation with virtually no thermal damage to the surrounding area, permitting high resolution imaging, as well as crystallographic and elemental analysis, without intermediate surface preparation or removal of the sample from the chamber. The TriBeam system combines the high resolution and broad detector capabilities of the DualBeam(TM) microscope with the high material removal rates of the femtosecond laser, allowing 3D datasets to be acquired at rates 4-6 orders of magnitude faster than 3D FIB datasets. Design features that permit coupling of laser and electron optics systems and positioning of a stage in the multiple analysis positions are discussed. Initial in situ multilayer data are presented.

Nonlinear ultrasonics for in situ damage detection during high frequency fatigue

In this paper, we report the use of the feedback signal of an ultrasonic fatigue system to dynamically deduce fatigue damage accumulation via changes in the nonlinear ultrasonic parameter. The applicability of this parameter in comparison to the resonant frequency for assessment of fatigue damage accumulation in a wrought aluminum alloy has been demonstrated, without the need for coupling fluids or independent generation of incident ultrasonic waves. The ultrasonic nonlinearity increased and the resonant frequency of the system decreased with initiation and propagation of the major crack. The nonlinear ultrasonic parameter shows greater sensitivity to damage accumulation than the resonant frequency. The number of cycles for crack propagation, estimated based on the changes in the nonlinear ultrasonic parameter, is in very good agreement with calculated crack growth rates based on the fractography studies.

In Situ Imaging of High Cycle Fatigue Crack Growth in Single Crystal Nickel-Base Superalloys by Synchrotron X-Radiation

A novel X-ray synchrotron radiation approach is described for real-time imaging of the initiation and growth of fatigue cracks during ultrasonic fatigue (f=20 kHz). We report here on new insights on single crystal nickel-base superalloys gained with this approach. A portable ultrasonic fatigue instrument has been designed that can be installed at a high-brilliance X-ray beamline. With a load line and fatigue specimen configuration, this instrument produces stable fatigue crack propagation for specimens as thin as 150 {mu}m. The in situ cyclic loading/imaging system has been used initially to image real-time crystallographic fatigue and crack growth under positive mean axial stress in the turbine blade alloy CMSX-4.

A facility for conducting high-temperature oxidation experiments of alloys in helium environments containing part per million levels of impurities

An experimental facility was constructed to study the corrosion of alloys in helium containing part per million (ppm) levels of CO, CO2 ,C H4 and H2 as impurities, relevant to the environment in the heat exchanger of the Very High Temperature Gas Cooled Reactor. The system provides the capability of exposing multiple specimens in up to seven separate helium environments, simultaneously, for durations of >1000 h and temperatures up to 1200 ◦ C. Impurity concentrations are controlled down to 1 ppm accuracy and analyzed using a discharge ionization detector gas chromatograph. The utility and reliability of the facility in quantitatively accounting for the masses of reactants and products involved in the oxidation of alloy 617 at 900 ◦ C and 1000 ◦ C in the helium gas containing 15 ppm CO and 1.5 ppm CO2 is confirmed by the weight-gain measurements, gas-phase analysis and post-test microstructural analysis.