@conference {1611, title = {Creep, Fatigue, and Oxidation Interactions During High and Very High Cycle Fatigue at Elevated Temperature of Nickel-Based Single Crystal Superalloys}, booktitle = {Superalloys 2020}, year = {2020}, abstract = {

High-temperature fatigue of Ni-based single crystal superalloys is studied at 1000 \°C in a wide range of loading conditions (\−1 ? R ? 0.8) and number of cycles (103 \− 109). Under fully reversed conditions, a competition between crack initiations from the surface\— assisted by oxidation\—and from internal metallurgical defects\—mostly large casting pores\—is observed. Increasing the testing frequency shifts the competition to a higher number of cycles. Conversely, decreasing the casting pore size or coating the specimen promotes surface initiations. When a positive mean stress is added (R ? 0), a creep deformation/damage mechanism mainly controls fatigue life, despite fracture surfaces presenting a variety of initiation types. Fatigue life can be predicted by a simple creep law if the contribution of the alternating stress is considered. A linear damage summation method that considers pure fatigue and pure creep damage is used to predict the fatigue lives, and Haigh diagrams for different alloys are presented.

}, keywords = {Creep, fatigue, High temperature, Superalloy}, isbn = {9783030518349}, doi = {10.1007/978-3-030-51834-9}, url = {http://dx.doi.org/10.1007/978-3-030-51834-9_65}, author = {Cervellon, A. and Yi, J. Z. and Corpace, F. and Hervier, Z. and Rigney, J. and Wright, P. K. and Torbet, C. J. and Cormier, J. and Jones, J. W. and Pollock, T. M.}, editor = {Tin, Sammy and Hardy, Mark and Clews, Justin and Cormier, Jonathan and Feng, Qiang and Marcin, John and O{\textquoteright}Brien, Chris and Suzuki, Akane} } @article {1701, title = {Design and Tailoring of Alloys for Additive Manufacturing}, journal = {Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science}, volume = {51}, year = {2020}, pages = {6000{\textendash}6019}, abstract = {

Additive manufacturing (AM) promises a major transformation for manufacturing of metallic components for aerospace, medical, nuclear, and energy applications. This perspective paper addresses some of the opportunities for alloy and feedstock design to achieve site-specific and enhanced properties not attainable by conventional manufacturing processes. This paper provides a brief overview of the role of powders, as well as solidification and solid-state phase transformation phenomena typically encountered during fusion-based AM. Three case studies are discussed that leverage the above to arrive at microstructure control. The first case study focuses on approaches to modify the solidification characteristics by in-situ alloying. The second case study focuses on the need for concurrent design of alloys and processing conditions to arrive at the columnar to equiaxed transition during solidification. The third case study focuses on the design of a cobalt alloy for AM, with emphasis on tailoring liquid and solid state phase transformations. The need for comprehensive knowledge of processing conditions during AM, in-situ and ex-situ probing of microstructure development under AM conditions, and post-print processing, characterization, and qualification are articulated for the design of future alloys and component geometries built by AM.

}, issn = {10735623}, doi = {10.1007/s11661-020-06009-3}, url = {https://doi.org/10.1007/s11661-020-06009-3}, author = {Pollock, T. M. and Clarke, A. J. and Babu, S. S.} } @article {1411, title = {Direct measurements of slip irreversibility in a nickel-based superalloy using high resolution digital image correlation}, journal = {Acta Materialia}, volume = {186}, year = {2020}, pages = {172{\textendash}189}, abstract = {

Fatigue crack nucleation in crystalline materials typically develops due to highly localized cyclic slip. During a fatigue cycle, reverse slip differs locally from slip in the forward direction particularly in precipitate-containing materials such as superalloys. In this paper we report the first direct measurements of irreversibility at the scale of individual slip bands by high-resolution digital image correlation (DIC) in a polycrystalline nickel-based superalloy. Quantitative measurements of the slip irreversibility are challenging for regions of material that have a size that captures the microstructure and its variability. High spatial resolution at the nanometer scale during experimental measurements is needed to observe slip localization during deformation. Moreover, large fields are also needed to obtain the material response over statistically representative populations of microstructural configurations. Recently, high resolution scanning electron microscope (SEM) digital image correlation (DIC) has been extended for quantitative analysis of discontinuities induced by slip events using the Heaviside-DIC method. This novel method provides quantitative measurements of slip localization at the specimen surface. In this paper, the Heaviside-DIC method is used to measure slip irreversibility and plastic strain accumulation in a nickel-based superalloy. The method detects bands with high levels of irreversibility early in cycling that ultimately form fatigue cracks upon further cycling. The local microstructural configurations that induce large amounts of plasticity and slip irreversibility are correlated to crack nucleation locations.

}, keywords = {Heaviside-digital image correlation, High resolution digital image correlation, Low cycle fatigue, Polycrystalline Ren{\'e} 88DT nickel-based superalloy, Scanning electron microscopy digital image correlation, Slip irreversibility, Strain localization and accumulation}, issn = {13596454}, doi = {10.1016/j.actamat.2019.12.009}, url = {https://doi.org/10.1016/j.actamat.2019.12.009}, author = {Stinville, J. C. and Callahan, P. G. and Charpagne, M. A. and Echlin, M. P. and Valle, V. and Pollock, T. M.} } @article {1581, title = {Microscale characterization of damage accumulation in CMCs}, journal = {Journal of the European Ceramic Society}, year = {2020}, abstract = {

The developing roles of damage mechanisms in the failure response of SiC/SiC minicomposites was investigated by the characterization of microscale damage accumulation with respect to microstructure. A multi-modal approach combining spatially resolved acoustic emission (AE) with tensile testing in-SEM (scanning electron microscope) was used to simultaneously examine surface (observed in-SEM) and bulk damage (monitored via AE). Strong agreement was shown between the evolving crack density estimated by AE and in-SEM measurements. The following were observed: (i) in-plane matrix content and distribution impacted crack growth; (ii) spatially-distributed matrix cracks generated varying stress-dependent AE; and (iii) certain individual cracks became more probable failure locations due to unique combinations of damage mechanisms that drove their growth. This approach enabled characterizing potential failure determinants and suggests that early damage behavior is related to certain microstructural features (e.g. surface flaws), while subsequent damage behavior is coupled to interactions of local mechanisms evolving with stress.

}, keywords = {Acoustic emission, Ceramic matrix composite, Cracking, Damage initiation, Silicon carbide}, issn = {1873619X}, doi = {10.1016/j.jeurceramsoc.2020.05.077}, url = {https://doi.org/10.1016/j.jeurceramsoc.2020.05.077}, author = {Swaminathan, B. and McCarthy, N. R. and Almansour, A. S. and Sevener, K. and Pollock, T. M. and Kiser, J. D. and Daly, S.} } @conference {1616, title = {Tuning Strain Localization in Polycrystalline Nickel-Based Superalloys by Thermomechanical Processing}, booktitle = {Superalloys 2020}, year = {2020}, publisher = {Springer International Publishing}, organization = {Springer International Publishing}, abstract = {

Thermomechanical processing routes are used to produce microstructures that minimize plastic strain localization at the sub-grain scale in a polycrystalline \γ \− \γ\&$\#$39; nickel- based superalloy. This novel approach is made possible by the use of innovative experimental tools and statistical data analysis that capture slip events over large represen- tative fields of view. Results are correlated to conventional observations of fatigue crack initiation and early stage of propagation. The effect of coherent twin boundaries and primary \γ\&$\#$39; precipitates on fatigue properties and plastic localization is detailed.

}, keywords = {3D microstructure, Annealing twin boundaries, Nickel-based superalloy, Primary γ{\textquoteright} precipitates, Slip bands}, isbn = {9783030518349}, doi = {10.1007/978-3-030-51834-9}, url = {http://dx.doi.org/10.1007/978-3-030-51834-9_65}, author = {Charpagne, M. A. and Stinville, J. C. and Polonsky, A. T. and Echlin, M. P. and Murray, S. P. and Chen, Z. and Bozzolo, N. and Cormier, J. and Valle, V. and Pollock, T. M.}, editor = {Tin, Sammy and Hardy, Mark and Clews, Justin and Cormier, Jonathan and Feng, Qiang and Marcin, John and O{\textquoteright}Brien, Chris and Suzuki, Akane} } @article {1511, title = {A 3D analysis of the onset of slip activity in relation to the degree of micro-texture in Ti{\textendash}6Al{\textendash}4V}, journal = {Acta Materialia}, volume = {181}, year = {2019}, pages = {36{\textendash}48}, abstract = {

The mechanical properties of titanium alloys result from their complex multi-scale microstructural features, including micron scale precipitates and millimeter scale microtextured regions (MTRs). While previous investigations have revealed that the presence of mm-scale MTRs can degrade mechanical properties, particularly fatigue, the accompanying strain localization processes that operate at the microscale within the \α grains in MTRs are not well understood. The present work is a mechanistic investigation of MTRs using crystal plasticity simulations of mm3-scale experimentally captured and synthetically generated 3D microstructure datasets. The explicit modeling of both the \α grains and MTRs in Ti\–6Al\–4V enables assessment of the effect of microtexture and local structure variations within the MTR on overall deformation behavior and the onset of plastic slip in MTRs. The presence of MTRs with a dominant [0001] orientation results in both stress and plastic strain hotspots during the early stages of straining. Crystal plasticity predictions are compared to previous digital image correlation studies on early strain localization. The influence of MTRs on the local stress and strain fields is discussed with regard to the monotonic tension, fatigue and dwell-fatigue behavior of titanium alloys.

}, keywords = {3D EBSD, Crystal plasticity, Macrozones, Microtexture, Titanium alloys}, issn = {13596454}, doi = {10.1016/j.actamat.2019.09.028}, url = {https://doi.org/10.1016/j.actamat.2019.09.028}, author = {H{\'e}mery, S. and Na{\"\i}t-Ali, A. and Gu{\'e}guen, M. and Wendorf, J. and Polonsky, A. T. and Echlin, M. P. and Stinville, J. C. and Pollock, T. M. and Villechaise, P.} } @article {1506, title = {Alloy Design and Processing Design of Magnesium Alloys Using 2nd Phases}, journal = {JOM}, volume = {71}, year = {2019}, pages = {2219{\textendash}2226}, abstract = {

Two phases dominate the performance of commercial Mg alloys: (1) \β Mg17Al12 and (2) porosity. Alloy design and process design to optimize the morphology of the first and to minimize the second are discussed. Second phase \β can be designed to improve tensile strength, fatigue strength, toughness, texture, formability and corrosion resistance of Mg alloys. Processing is applied to refine the grain size, to array this phase in micron sizes at grain boundaries, and to further precipitate this phase in nanometer arrays within the grains. Therein, the 2nd phase \β plays the following roles: (1) retarding grain growth, (2) randomizing texture, (3) Hall\–Petch hardening, (4) Orowan hardening, and (5) moderating corrosion. Porosity is a detrimental 2nd phase common to cast Mg alloys in the form of gas or voids; its control being essential to engineering applications. Porosity can be diminished by sub-liquidus molding with high-velocity/high-pressure injection and then eliminated by subsequent hot deformation.

}, issn = {15431851}, doi = {10.1007/s11837-019-03482-z}, url = {https://doi.org/10.1007/s11837-019-03482-z}, author = {Decker, R. F. and Berman, T. D. and Miller, V. M. and Jones, J. W. and Pollock, T. M. and LeBeau, S. E.} } @article {1501, title = {In-plane and out-of-plane deformation at the sub-grain scale in polycrystalline materials assessed by confocal microscopy}, journal = {Acta Materialia}, volume = {169}, year = {2019}, pages = {260{\textendash}274}, abstract = {

High-resolution digital image correlation (HR-DIC) techniques have become essential in material mechanics to assess strain measurements at the scale of the elementary mechanisms responsible of the deformation in polycrystalline materials. The purpose of this study is to demonstrate the use of laser scanning confocal microscopy (LSCM) coupled with DIC techniques to deepen knowledge on the deformation process of a polycrystalline nickel-based superalloy at room temperature. The LSCM technique is capable of detecting both in-plane and out-of-plane strain localization within slip bands at the sub-grain level. The LSCM observations are consistent with previous in-situ scanning electron microscopy (SEM) studies: The onset of crystal plasticity occurs primarily near \Σ3 twin boundaries with macroscopic loading in the elastic domain (macroscopic stress as low as 80\% of the 0.2\% offset yield strength (Y.S. 0.2\% )). This intense irreversible strain localization occurs with either a high Schmid factor (\μ \> 0.43) or a significant elastic modulus difference between the pair of twins (\Δ\Ε \> 100 GPa). In the plastic deformation domain, transgranular slip activity following slip systems with the highest Schmid factor is mostly responsible for the deformation at the grain level, thus leading to strain percolation. The simultaneous in-plane and out-of-plane deformation assessment via the HR-LSCM-DIC technique was found to be essential for the identification of active slip systems. Finally, the HR-LSCM-DIC technique enabled the quantification of the glide amplitude involved in the three-dimensional shearing process at the grain level that solely in-plane measurements cannot provide.

}, keywords = {High Resolution-Digital Image Correlation (HR-DIC), In-situ tensile testing, Laser scanning confocal microscopy (LSCM), Nickel-based polycrystalline superalloy, Strain localization}, issn = {13596454}, doi = {10.1016/j.actamat.2019.03.001}, url = {https://doi.org/10.1016/j.actamat.2019.03.001}, author = {Liu, J. H. and Vanderesse, N. and Stinville, J. C. and Pollock, T. M. and Bocher, P. and Texier, D.} } @article {1466, title = {Designing nickel base alloys for microstructural stability through low γ-γ' interfacial energy and lattice misfit}, journal = {Materials and Design}, volume = {140}, year = {2018}, pages = {249{\textendash}256}, abstract = {

An extended stability alloy design strategy for multicomponent \γ-\γ\′ nickel-base alloys with near-zero lattice misfit and as low as possible interfacial energy was investigated by isothermal annealing of two experimental alloys at 900 \°C for times up to 256 h. The coarsening behavior of the spherical \γ\′ precipitates and the phase compositions determined by atom probe tomography were utilized to exploit a modified Lifshitz-Slyozov-Wagner relationship to estimate the interfacial energies. The estimated interfacial energies are much lower than predicted by a CALPHAD-based software as well as those typically reported for multicomponent \γ-\γ\′ nickel alloys. Despite successfully minimizing the interfacial energy and \γ-\γ\′ lattice misfit, these factors alone were not sufficient to impart high temperature extended stability through reduced coarsening kinetics.

}, keywords = {Atom probe tomography, Interfacial energy, Precipitate coarsening, γ-γ' nickel-base alloy}, issn = {18734197}, doi = {10.1016/j.matdes.2017.11.065}, url = {http://dx.doi.org/10.1016/j.matdes.2017.11.065}, author = {Meher, S. and Carroll, M. C. and Pollock, T. M. and Carroll, L. J.} } @article {1461, title = {High-Resolution Deformation Mapping Across Large Fields of View Using Scanning Electron Microscopy and Digital Image Correlation}, journal = {Experimental Mechanics}, volume = {58}, year = {2018}, pages = {1407{\textendash}1421}, abstract = {

This paper details the creation of experimental and computational frameworks to capture high-resolution, microscale deformation mechanisms and their relation to microstructure over large (mm-scale) fields of view. Scanning electron microscopy with custom automation and external beam control was used to capture 209 low-distortion micrographs of 360\ \μm \× 360\ \μm each, that were individually correlated using digital image correlation to obtain displacement/strain fields with a spatial resolution of 0.44\ \μm. Displacement and strain fields, as well as secondary electron images, were subsequently stitched to create a 5.7\ mm\ \×\ 3.4\ mm field of view containing 100 million (7678 \× 13,004) data points. This approach was demonstrated on Mg WE43 under uniaxial compression, where effective strain was shown to be relatively constant with respect to distance from the grain boundary, and a noticeable increase in the effective strain was found with an increase in the basal Schmid factor. The ability to obtain high-resolution deformations over statistically relevant fields of view enables large data analytics to examine interactions between microstructure, microscale strain localizations, and macroscopic properties.

}, keywords = {Alignment, Digital image correlation (DIC), Distortion, External scan, Stitching}, issn = {17412765}, doi = {10.1007/s11340-018-0419-y}, author = {Chen, Z. and Lenthe, W. and Stinville, J. C. and Echlin, M. and Pollock, T. M. and Daly, S.} } @article {1436, title = {Measurements of plastic localization by heaviside-digital image correlation}, journal = {Acta Materialia}, volume = {157}, year = {2018}, pages = {307{\textendash}325}, abstract = {

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.

}, keywords = {Discontinuities measurements, High resolution digital image correlation, Ren{\'e} 88DT polycrystalline superalloy, Scanning electron microscopy, Slip band offset, Slip band shearing, Slip system identification, Strain localization}, issn = {13596454}, doi = {10.1016/j.actamat.2018.07.013}, url = {https://doi.org/10.1016/j.actamat.2018.07.013}, author = {Bourdin, F. and Stinville, J. C. and Echlin, M. P. and Callahan, P. G. and Lenthe, W. C. and Torbet, C. J. and Texier, D. and Bridier, F. and Cormier, J. and Villechaise, P. and Pollock, T. M. and Valle, V.} } @article {1456, title = {Microstructure and property based statistically equivalent RVEs for intragranular γ-γ{\textquoteright} microstructures of Ni-based superalloys}, journal = {Acta Materialia}, volume = {157}, year = {2018}, pages = {245{\textendash}258}, abstract = {

This paper develops statistically equivalent RVEs or SERVEs for intragranular microstructures of Ni-based superalloys, characterized by \γ-\γ\&$\#$39; phase distribution. The SERVE represents an optimal computational domain to be used for micromechanical simulations for effective properties or response variables in the microstructure. The SERVE is further classified as a microstructure-based SERVE or M-SERVE or property-based SERVE or P-SERVE, depending on whether the statistics of morphological characteristics or convergence of chosen material properties are its determinants. Starting from FIB-SEM data for the superalloy Ren\é 88DT, the paper systematically develops a host of algorithms for generating validated statistically equivalent virtual microstructures, from which the M-SERVE is estimated from convergence of selected morphological and spatial distributions. Subsequently the P-SERVEs are established for global properties like yield strength and hardening rate, and local variables including dislocation density and the maximum resolved shear stress. Spatially-averaged quantities are found to converge quicker than the local distributions for both M-SERVE and P-SERVE.

}, keywords = {M-SERVE, Ni-based superalloys, P-SERVE, SEVM, Two-point correlation function, γ-γ{\textquoteright} distribution}, issn = {13596454}, doi = {10.1016/j.actamat.2018.07.034}, url = {https://doi.org/10.1016/j.actamat.2018.07.034}, author = {Pinz, M. and Weber, G. and Lenthe, W. C. and Uchic, M. D. and Pollock, T. M. and Ghosh, S.} } @article {1171, title = {Oxidation-Assisted Crack Growth in Single-Crystal Superalloys during Fatigue with Compressive Holds}, journal = {Metallurgical and Materials Transactions A}, volume = {49}, year = {2018}, pages = {105{\textendash}116}, doi = {10.1007/s11661-017-4392-3}, author = {Lafata, M. A. and Rettberg, L. H. and He, M. Y. and Pollock, T. M.} } @article {1476, title = {Shearing of γ{\textquoteright} particles in Co-base and Co-Ni-base superalloys}, journal = {Acta Materialia}, volume = {161}, year = {2018}, pages = {99{\textendash}109}, abstract = {

Shearing mechanisms of the primary strengthening phase in cobalt-base and cobalt-nickel-base superalloys, \γ\′ (L12), are investigated at the elementary defect level by using a combination of generalized-stacking-fault energy (GSF) calculations and phase field simulations. The GSF energy surfaces of the \γ and \γ\′ phases, as determined from available experimental data and ab initio calculations, are used in the phase field simulations. Sophisticated deformation pathways leading to various planar defects including antiphase boundaries (APB), remnant superlattice intrinsic stacking faults (SISF), APB-SISF-APB ribbons and SISF islands are predicted as a function of alloy composition and particle shapes. The predicted stacking fault configurations for both alloys are consistent with recent transmission electron microscopy observations. Effects of dislocation line tension difference in \γ and \γ\′ phases and planar defect energy variation due to segregation at the planar defects are discussed. The detailed dislocation core structures, effects of dislocation line tension differences on deformation mechanisms, and unique deformation mechanisms uncovered, which include stacking fault ribbon shearing, antiphase boundary shearing, and mixed modes, could be used to improve constitutive microstructure-property relationships in advanced crystal plasticity modeling and to assist in alloy design.

}, keywords = {Ab initio calculation, Dislocation creep, Phase field simulation, Precipitation hardening, Stacking fault ribbon}, issn = {13596454}, doi = {10.1016/j.actamat.2018.09.013}, author = {Feng, L. and Lv, D. and Rhein, R. K. and Goiri, J. G. and Titus, M. S. and Van der Ven, A. and Pollock, T. M. and Wang, Y.} } @article {1221, title = {Measurement of Strain Localization Resulting from Monotonic and Cyclic Loading at 650 oC in Nickel Base Superalloys}, journal = {Experimental Mechanics}, volume = {57}, year = {2017}, pages = {1289{\textendash}1309}, doi = {10.1007/s11340-017-0286-y}, author = {Stinville, J. C. and Echlin, M. P. and Callahan, P. G. and Miller, V. M. and Texier, D. and Bridier, F. and Bocher, P. and Pollock, T. M.} }