Chris Torbet - Academia.edu (original) (raw)

Papers by Chris Torbet

$Research paper thumbnail of In Situ Characterization of Deformation Twinning in Magnesium During Cyclic Loading via Electron Backscatter Diffraction$

International Journal of Fatigue, 2015

Abstract In situ ultrasonic fatigue with a cyclic frequency of 20 kHz was employed in an environm... more Abstract In situ ultrasonic fatigue with a cyclic frequency of 20 kHz was employed in an environmental scanning electron microscope (ESEM) to characterize fatigue crack formation and growth in the near alpha titanium alloy Ti–6242S. The role of environment on small fatigue crack initiation and growth was investigated in vacuum and in variable pressures of saturated water vapor, as well as in laboratory air. Small crack growth behavior from cracks initiated at FIB-produced micro-notches indicated a significant environmental dependence, with fatigue crack growth rates increasing with increasing partial pressures of water vapor. Environment also influenced crack initiation lifetime in that cracks initiated earlier in laboratory air than in vacuum or saturated water vapor environments. Transgranular, crystallographic crack growth was observed in each environment, with the crack path in primary α grains producing facets parallel to basal planes when crack size was small. Small crack growth resistance had a marked sensitivity to microstructural features, such as α/α grain boundaries with high misorientation and α/α + β boundaries. These initial investigations demonstrate the usefulness of in situ ultrasonic fatigue instrumentation (UF-SEM) as a new tool for the characterization of environmental and microstructural influences on very high cycle fatigue (VHCF) behavior.

Acta Materialia, Sep 1, 2018

In polycrystalline metallic materials, quantitative and statistical assessment of the plasticity ... more In polycrystalline metallic materials, quantitative and statistical assessment of the plasticity in relation to the microstructure is necessary to understand the deformation processes during mechanical loading. Plastic deformation often localizes into physical slip bands at the sub-grain scale. Detrimental microstructural configurations that result in the formation and evolution of slip bands during loading require advanced strain mapping techniques for the identification of these atomically sharp discontinuities. A new discontinuity-tolerant DIC method, Heaviside-DIC, has been developed to account for discontinuities in the displacement field. Displacement fields have been measured at the scale of the physical slip bands over large areas in nickel-based superalloys by high resolution scanning electron microscopy digital image correlation (SEM DIC). However, conventional DIC methods cannot quantitatively measure plastic localization in the presence of discontinuous kinematic fields such as those produced by slip bands. The Heaviside-DIC technique can autonomously detect discontinuities, providing information about their location, inclination, and identify slip systems (in combination with orientation mapping). Using Heaviside-DIC, discontinuities are physically evaluated as sharp shear-localization events, allowing for the quantitative measure of strain amplitude nearby the discontinuities. Measurements using the new Heaviside-DIC technique are compared to conventional DIC methods for identical materials and imaging conditions.

The minerals, metals & materials series, 2020

Nature Communications, Oct 2, 2020

Additive manufacturing promises a major transformation of the production of high economic value m... more Additive manufacturing promises a major transformation of the production of high economic value metallic materials, enabling innovative, geometrically complex designs with minimal material waste. The overarching challenge is to design alloys that are compatible with the unique additive processing conditions while maintaining material properties sufficient for the challenging environments encountered in energy, space, and nuclear applications. Here we describe a class of high strength, defect-resistant 3D printable superalloys containing approximately equal parts of Co and Ni along with Al, Cr, Ta and W that possess strengths in excess of 1.1 GPa in as-printed and post-processed forms and tensile ductilities of greater than 13% at room temperature. These alloys are amenable to crack-free 3D printing via electron beam melting (EBM) with preheat as well as selective laser melting (SLM) with limited preheat. Alloy design principles are described along with the structure and properties of EBM and SLM CoNi-base materials.

Raw and groomed (room temp and high temp) RUS peaks data for the SB-CoNi-10+ specimen. Groomed da... more Raw and groomed (room temp and high temp) RUS peaks data for the SB-CoNi-10+ specimen. Groomed data are supplied as a summary Excel file and raw RUS peaks data are supplied as .txt files with the following column header definitions: FQ = Average measured frequency recorded from the two transducers, units of kHz. AMPL = Sum measured amplitude recorded from the two transducers, arbitrary units, (% transducer 1, % transducer 2) D2 = Calculated 2nd derivative of amplitude as a function of frequency for the selected peak, arbitrary units, used for peak picking ZW = Calculated zero crossing width for the selected peak, arbitrary units, used for peak picking All frequencies listed in the groomed summary file have units of kHz, all dimensions have units of mm, mass has units of grams. See different tabs for the 5 RT spectra (averaged together), and the individual HT spectra for each temperature.

Ultrasonics, May 1, 2017

Finite element (FE) modeling has been coupled with resonant ultrasound spectroscopy (RUS) for non... more Finite element (FE) modeling has been coupled with resonant ultrasound spectroscopy (RUS) for nondestructive evaluation (NDE) of high temperature damage induced by mechanical loading. Forward FE models predict mode-specific changes in resonance frequencies (∆f R), inform RUS measurements of mode-type, and identify diagnostic resonance modes sensitive to individual or multiple concurrent damage mechanisms. The magnitude of modeled ∆f R correlate very well with the magnitude of measured ∆f R from RUS, affording quantitative assessments of damage. This approach was employed to study creep damage in a polycrystalline Ni-based superalloy (Mar-M247) at 950°C. After iterative applications of creep strains up to 8.8%, RUS measurements recorded ∆f R that correspond to the accumulation of plastic deformation and cracks in the gauge section of a cylindrical dog-bone specimen. Of the first 50 resonance modes that occur, ranging from 3 to 220 kHz, modes classified as longitudinal bending were most sensitive to creep damage while transverse bending modes were found to be largely unaffected. Measure to model comparisons of ∆f R show that the deformation experienced by the specimen during creep, specifically uniform elongation of the gauge section, is responsible for a majority of the measured ∆f R until at least 6.1% creep strain. After 8.8% strain considerable surface cracking along the gauge section of the dog-bone was observed, for which FE models indicate low-frequency longitudinal bending modes are significantly affected. Key differences between historical implementations of RUS for NDE and the FE model-based framework developed herein are discussed, with attention to general implementation of a FE model-based framework for NDE of damage.

Metallurgical and Materials Transactions, Jan 21, 2016

The formation of abnormally large grains has been investigated in the polycrystalline nickel-base... more The formation of abnormally large grains has been investigated in the polycrystalline nickel-based superalloy Rene´88DT. Cylindrical specimens with a 15 lm grain size were compressed to plastic strains up to 11.0 pct and subsequently rapidly heated to above the c¢ solvus at 1423 K (1150 C) and held for 60 seconds. All deformed samples partially recrystallized during the heat treatment, with the recrystallized grain size varying with the degree of deformation. The largest final grain size occurred in samples deformed to approximately 2 pct strain, with isolated grains as large as 700 lm in diameter observed. It is proposed that abnormally large grains appear due to nucleation-limited recrystallization, not abnormal grain growth, based on the high boundary velocities measured and the observed reduction in grain orientation spread.

Acta Materialia, Aug 1, 2009

ABSTRACT Microstructural evolution and the onset of failure in a NiCoCrAlY bond coat (BC) on a si... more ABSTRACT Microstructural evolution and the onset of failure in a NiCoCrAlY bond coat (BC) on a single-crystal superalloy substrate have been investigated in a newly developed “hot spot” apparatus that imposes a temperature gradient along the length of a coated, cylindrical specimen. Local spallation events were observed in the “hottest spot” of the coated specimens after 50–60 oxidation cycles with a peak temperature of 1050 °C. The thickness of the thermally grown oxide (TGO) was in the range of 2–3 μm when spalls were initiated. The failure surface contained a significant density of embedded oxides and the remnant TGO exhibited surface cracks indicative of a buckling delamination failure mode. The failed surface morphology and the cross-section microstructure of the BC have been characterized, including morphological imperfections in the TGO. The role of these microstructural features in the failure process is addressed.

Microscopy and Microanalysis, Aug 1, 2022

Materials Characterization, Oct 1, 2022

Measurement Science and Technology, Jul 30, 2009

An experimental facility was constructed to study the corrosion of alloys in helium containing pa... more An experimental facility was constructed to study the corrosion of alloys in helium containing part per million (ppm) levels of CO, CO 2 , CH 4 and H 2 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 CO 2 is confirmed by the weight-gain measurements, gas-phase analysis and post-test microstructural analysis.

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, Sep 1, 2021

Abstract The anisotropic fatigue properties of three Ni-based SX superalloys (AM1, CMSX-4 and MC-... more Abstract The anisotropic fatigue properties of three Ni-based SX superalloys (AM1, CMSX-4 and MC-NG) were investigated in the very high cycle fatigue regime at 20 kHz, R = −1 and 750 °C. The fatigue testing shows that the [111] orientation has higher fatigue lives at high stresses and lower fatigue lives at very low stresses compared to the standard [001] orientation. At high stresses, the resolved shear stress on the octahedral slip systems explains the anisotropy effect. At low stresses, pseudo-cube slip in the matrix becomes the prominent deformation mechanism over γ′-shearing in the [111] orientation, resulting in comparatively lower fatigue properties.

Materials Characterization, Sep 1, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Review of Scientific Instruments, Feb 1, 2012

The unique capabilities of ultrashort pulse femtosecond lasers have been integrated with a focuse... more 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 DualBeamTM 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.

Acta Materialia, Nov 1, 2021

Abstract Bayesian inference with sequential Monte Carlo is used to quantify the orientation distr... more Abstract Bayesian inference with sequential Monte Carlo is used to quantify the orientation distribution function coefficients and to calculate the fully anisotropic elastic constants of additively manufactured specimens from only the experimentally-measured resonant frequencies. The parallelizable and open-source SMCPy Python package enabled Bayesian inference within this new modeling framework, resulting in an order of magnitude reduction of the computation time for an 8-core machine. Residual stress-induced shifts on the resonant frequencies were explicitly accounted for during the Bayesian inference, enabling the estimation of their effect on the resonant frequencies without a stress-relief heat treatment. Additively manufactured cobalt-nickel-base superalloy (SB-CoNi-10C) specimens were sectioned at multiple inclinations relative to the build direction and scanned with resonant ultrasound spectroscopy to demonstrate characterization of any arbitrarily textured cubic microstructure through the resonant frequencies. The orientation distribution function coefficients of the textured polycrystalline microstructure were estimated in tensorial form to calculate both the 2nd order Hashin-Shtrikman bounds and the self-consistent estimate of the elastic constants, enabling accurate determination of all 21 possible independent elastic constants through the convergence constraints of the texture. Pole figures generated directly from the calculated texture coefficients showed good agreement with experimentally measured textures.

Applied Physics Letters, Oct 29, 2012

John Wiley & Sons, Inc. eBooks, Feb 14, 2014

International Journal of Fatigue, Mar 1, 2022

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2021

Abstract A novel experimental setup combining induction heating, contactless (infrared thermomete... more Abstract A novel experimental setup combining induction heating, contactless (infrared thermometer) temperature measurements, and custom 15 cm length SiC-tipped piezoelectric transducers has been developed to rapidly collect resonant ultrasound spectroscopy (RUS) data from metallic rectangular parallelepiped (RP) specimens heated in air. This setup is used to collect over 70 of the lowest-frequency resonance modes from a single crystal of the novel CoNi-based superalloy SB-CoNi-10+—an alloy identified for further study due to a promising set of high temperature properties. RUS spectra from room temperature (RT) to 927 °C (1200 K) along with dilatometry measurements of the thermal expansion behavior are supplied to a robust, open-sourced, Bayesian inference code for simultaneous estimation of: three single crystal elastic constants, three crystal axis misorientation parameters, and a noise parameter. This study is the first application of the Bayesian RUS methodology at elevated temperatures, and the first to characterize SB-CoNi-10+. A monotonic decrease in elastic constants ( C 11 , C 12 , C 44 ) from (227.6, 148.5, 126.0) GPa to (190.7, 138.1, 95.2) GPa and a 15% increase in elastic anisotropy from A = 3.185 → 3.618 is observed upon heating from RT to 927 °C, with an increased rate of softening above 600 °C (873 K).