@article {1796, title = {Grain-Scale Stress States in Microtextured Ti64: Implications for Dwell Fatigue}, journal = {Jom}, volume = {74}, year = {2022}, pages = {3709{\textendash}3719}, abstract = {

The room-temperature fatigue life of titanium alloys with a significant alpha phase fraction is substantially reduced when the peak stress is sustained for a few minutes on each stress cycle. Furthermore, these alloys often form microtextured regions during forging which are known to increase the dwell debit; however, the mechanisms for this reduction are still not well understood. In this research, an experimentally collected 3D microstructure of equiaxed Ti-6Al-4V containing microtextured regions is used as input for linear elastic finite element analysis of grain-level stress states relevant to dwell fatigue. The directional strength to stiffness ratio and a metric related to local plane strain are evaluated around the microtextured regions. Strength to stiffness ratios indicate that misoriented grains within hard microtextured regions will begin to yield at macroscopic stress levels below the yield strength, and, under certain loading conditions, they will yield at lower macroscopic stresses than any other location in the microstructure. Under uniaxial tension, boundaries between hard and soft microtextured regions experience some degree of plane strain loading, which may be linked to enhanced load shedding in these areas during dwell fatigue.

}, isbn = {1183702205}, issn = {15431851}, doi = {10.1007/s11837-022-05423-9}, url = {https://doi.org/10.1007/s11837-022-05423-9}, author = {Wendorf, Joe and Dawson, Paul R. and Pollock, Tresa M.} } @article {1696, title = {Grain refinement mechanisms in additively manufactured nano-functionalized aluminum}, journal = {Acta Materialia}, volume = {200}, year = {2020}, pages = {1022{\textendash}1037}, abstract = {

Additive manufacturing (AM) is a new and promising production methodology adept at producing complex geometries, which can be optimized for lower weight and enhanced capabilities. The material properties of these additive components are dictated by the microstructures developed during processing, with a high sensitivity to grain structure and associated anisotropy. With this new processing modality comes the added difficulty of understanding the thermodynamics and kinetic mechanisms that dictate the evolution of microstructure. This research addresses the unique thermal conditions present in AM and the pathways for grain refinement in nanofunctionalized aluminum alloys. The Al-Ta system, in which Al3Ta intermetallic compounds are demonstrated to have substantial grain refining capacity, are the focus of this study. The grain size is shown to be reduced relative to pure aluminum by 1000X when tantalum is added at 1 vol\%. The effectiveness of the Al3Ta intermetallic is dictated by the crystallography and availability of the inoculant phase under AM conditions.

}, keywords = {Additive manufacturing, Crystallography, Grain refinement, Nucleation, Solidification}, issn = {13596454}, doi = {10.1016/j.actamat.2020.09.043}, url = {https://doi.org/10.1016/j.actamat.2020.09.043}, author = {Martin, J. Hunter and Yahata, Brennan and Mayer, Justin and Mone, Robert and Stonkevitch, Ekaterina and Miller, Julie and O{\textquoteright}Masta, Mark R. and Schaedler, Tobias and Hundley, Jacob and Callahan, Patrick and Pollock, Tresa} } @article {1576, title = {Growth accidents induced by primary γ{\textquoteright} precipitates in a polycrystalline nickel-based superalloy}, journal = {Scripta Materialia}, volume = {186}, year = {2020}, pages = {1{\textendash}5}, keywords = {3D EBSD, Annealing twin boundaries, Critical recrystallization, Nickel-base superalloy, Recrystallization}, issn = {1359-6462}, doi = {10.1016/j.scriptamat.2020.05.020}, url = {https://doi.org/10.1016/j.scriptamat.2020.05.020}, author = {Charpagne, M A and Polonsky, A T and Echlin, M P and Jacomet, S and De Jaeger, J and De Graef, M and Bozzolo, N and Pollock, T M} } @article {1631, title = {Growth accidents induced by primary γ' precipitates in a polycrystalline nickel-based superalloy}, journal = {Scripta Materialia}, volume = {186}, year = {2020}, month = {09/2020}, pages = {109-113}, abstract = {

The development of overgrown grains during sub-solvus annealing of a polycrystalline\ \γ\−\γ\′\ nickel-base alloy is investigated, with special emphasis on the primary\ \γ\′ precipitates. Quantitative analysis conducted on a large three-dimensional electron back-scattered diffraction dataset reveals a high fraction of\ \γ\′ precipitates on the annealing twin boundaries in the large grains. When observed in-situ in a scanning electron microscope, the formation of annealing twins is correlated to the crossing of\ \γ\′ precipitates by moving grain boundaries. The role of precipitates in the occurrence of growth accidents is discussed.

}, keywords = {3D EBSD, Annealing twin boundaries, Critical recrystallization, Nickel-base superalloy, Recrystallization}, doi = {https://doi.org/10.1016/j.scriptamat.2020.05.020}, url = {https://www.sciencedirect.com/science/article/abs/pii/S1359646220302979$\#$fig0005}, author = {M.A. Charpagne and A.T. Polonsky and M.P. Echlin and S. Jacomet and J. de Jaeger and M. De Graef and N. Bozzolo and T.M. Pollock} } @article {1451, title = {Grain size effects on NiTi shape memory alloy fatigue crack growth}, journal = {Journal of Materials Research}, volume = {33}, year = {2018}, pages = {91{\textendash}107}, abstract = {

Fatigue cracking in polycrystalline NiTi was investigated using a multiscale experimental framework for average grain sizes (GS) from 10 to 1500 nm for the first time. Macroscopic fatigue crack growth rates, measured by optical digital image correlation, were connected to microscopic crack opening and closing displacements, measured by scanning electron microscope DIC (SEM-DIC) using a high-precision external SEM scan controller. Among all grain sizes, the 1500 nm GS sample exhibited the slowest crack growth rate at the macroscale, and the largest crack opening level (stress intensity at first crack opening) and minimum crack opening displacements at the microscale. Smaller GS samples (10, 18, 42, and 80 nm) exhibited nonmonotonic trends in their fatigue performance, yet the correlation was strong between macroscale and microscale behaviors for each GS. The samples that exhibited the fastest crack growth rates (42 and 80 nm GS) showed a small crack opening level and the largest crack opening displacements. The irregular trends in fatigue performance across the nanocrystalline GS samples were consistent with nonmonotonic values in the elastic modulus reported previously, both of which may be related to the presence of residual martensite only evident in the small GS samples (10 and 18 nm).

}, keywords = {fatigue, nanostructure, scanning electron microscopy (SEM)}, issn = {20445326}, doi = {10.1557/jmr.2017.395}, author = {Lepage, William S. and Ahadi, Aslan and Lenthe, William C. and Sun, Qing Ping and Pollock, Tresa M. and Shaw, John A. and Daly, Samantha H.} } @article {rettberg2016growth, title = {Growth Stresses in Thermally Grown Oxides on Nickel-Based Single-Crystal Alloys}, journal = {Metallurgical and Materials Transactions A}, volume = {47}, number = {3}, year = {2016}, pages = {1132{\textendash}1142}, publisher = {Springer US}, author = {Rettberg, Luke H and Laux, Britta and He, Ming Y and Hovis, David and Heuer, Arthur H and Pollock, Tresa M} } @article {saddock2010grain, title = {Grain-scale creep processes in Mg{\textendash}Al{\textendash}Ca base alloys: Implications for alloy design}, journal = {Scripta Materialia}, volume = {63}, number = {7}, year = {2010}, pages = {692{\textendash}697}, publisher = {Elsevier}, author = {Saddock, ND and Suzuki, A and Jones, JW and Pollock, TM} } @article {zhou2009gamma, title = {g' rafting in single crystal blade alloys: a simulation study}, journal = {Materials Science and Technology}, volume = {25}, number = {2}, year = {2009}, pages = {205{\textendash}212}, publisher = {Taylor \& Francis}, author = {Zhou, N and Shen, C and Sarosi, PM and Mills, MJ and Pollock, T and Wang, Y} } @article {wu2006grain, title = {Grain-scale strain mapping for analysis of slip activity in polycrystalline B2 RuAl}, journal = {Philosophical Magazine}, volume = {86}, number = {25-26}, year = {2006}, pages = {3995{\textendash}4008}, publisher = {Taylor \& Francis}, author = {Wu, A and De Graef, M and Pollock, TM} } @article {pollock1995growth, title = {The growth and elevated temperature stability of high refractory nickel-base single crystals}, journal = {Materials Science and Engineering: B}, volume = {32}, number = {3}, year = {1995}, pages = {255{\textendash}266}, publisher = {Elsevier}, author = {Pollock, Tresa M} } @article {pollock1992grain, title = {Grain defect formation during directional solidification of nickel base single crystals}, journal = {Superalloys 1992}, year = {1992}, pages = {125{\textendash}134}, publisher = {The Minerals, Metals and Materials Society Warrendale, PA}, author = {Pollock, TM and Murphy, WH and Goldman, EH and Uram, DL and Tu, JS} }