Alloy Behavior and Design Across Length-Scales: An SMD Symposium Honoring Easo George: Small-Scale Mechanical Behavior and Alloy Modeling
Sponsored by: TMS Structural Materials Division, TMS: High Temperature Alloys Committee, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Michael Mills, Ohio State University; George Pharr, Texas A&M University; Robert Ritchie, University of California, Berkeley; Muralidharan Govindarajan, Oak Ridge National Laboratory
Tuesday 8:00 AM
March 21, 2023
Room: Cobalt 502B
Location: Hilton
Session Chair: George Pharr, Texas A&M University
8:00 AM Invited
Easo George – Advancing Understanding of Mechanical Behavior through Test Material Preparation
: William Nix1; 1Stanford University
Easo George has advanced the understanding of mechanical behavior of metals through his meticulous care in preparing materials of purity and perfection for subsequent mechanical testing. After Uchic developed his micropillar compression technique for studying mechanical behavior in small dimensions, we and others thought that the strong size effects being observed could be caused by a dislocation starvation effect. A debate about the remarkable size effect ensued for several years until George reported on his development of a technique to create highly perfect micropillars of Mo by directionally solidifying NiAl-Mo eutectic alloys and etching the NiAl matrix to expose highly perfect Mo pillars for micro-compression testing. With this ingenious approach George showed how dislocation-free pillars exhibited ideal strengths and that pre-straining to introduce dislocations caused weakening. George’s work essentially closed the book on understanding the role of dislocation content on the strength and plasticity of crystals in small volumes.
8:30 AM Invited
Ambient-temperature Plasticity of Brittle Intermetallics at Micron-meter Size Scales: Haruyuki Inui1; Kyosuke Kishida1; 1Kyoto University
One of the common characteristics for intermetallics is their brittleness at ambient temperatures. Some intermetallics like those called sigma, mu and Laves phases are typical examples that are sometimes detrimental to the ductility and toughness of steels once incorporated unexpectedly. Of surprise to notice is that even, fundamentals for plasticity such as operating slip systems and their CRSS values have yet to be known for many of them. However, there is a chance for these hard materials to plastically deform in the form of micropillars of the micron-meter size even at ambient temperature, from which we can obtain the information of operating slip systems and their CRSS values. The results obtained for some brittle intermetallics incorporated in steels (FeCr, Fe2Ta and Fe3C) will be presented.
9:00 AM Invited
Theory-guided Design of High-strength, Ductile, Single-phase BCC High Entropy Alloys: William Curtin1; You Rao1; Carolina Baruffi1; Anthony De Luca2; Christian Leinenbach2; 1Epfl Sti Igm Lammm; 2EMPA
The immense compositional space of High Entropy Alloys presents possibilities of discovering compositions satisfying multiple application requirements. We introduce a theory-based parameter-free alloy selection procedure to satisfy specified mechanical and thermodynamic properties. We first make selections of equiatomic quinary BCC refractory HEAs in the family Cr-Mo-W-V-Nb-Ta-Ti-Zr-Hf to find alloys that are single-phase, high melting point, and strong and ductile at room temperature. We identify HfMoNbTaTi as the most-promising alloy. We then search within Hf-Mo-Nb-Ta-Ti with more-stringent design/performance criteria and find 10 quinaries with lower densities, higher specific strength, more-likely-ductile, and melting points above 2050C. We validate the selection procedure by fabricating and characterizing Hf15Mo25Nb20Ta5Ti35, confirming many predictions. Extensions to the quaternary Hf-Mo-Nb-Ti family identifies 15 new promising alloys for future study. Overall, our design procedure provides a theory-based way to quickly identify target alloys with a desired suite of properties, facilitating the discovery of next-generation materials.
9:30 AM Break
9:50 AM Invited
The Interplay between Phase Transformation and Mechanical Properties in High Entropy Alloys: Maryam Ghazisaeidi1; 1The Ohio State University
Phase prediction in multicomponent alloys remains one of the most fundamental challenges. Navigating the vast compositional space of these alloys requires a predictive capability to efficiently guide alloy discovery and microstructure design. Recently we have developed a Multicell Monte Carlo (MC)^2 method, based on first-principles calculations, to study phase formation in multicomponent alloys. This method is particularly powerful when applied to multicomponent systems, for which phase diagrams do not exist. I will present the application of (MC)^2 to refractory high entropy alloys and discuss the effect of emerging phases on deformation mechanisms and consequently the overall mechanical behavior.
10:20 AM Invited
Deformation Twinning in HCP Ti: The Role of Interfacial Complexions and Interstitial Solutes: Mohammad Hooshmand1; Buyu Zhang2; Enze Chen2; Timofey Frolov3; Ruopeng Zhang4; Yan Chong4; Andrew Minor2; Mark Asta2; 1University of California, Berkeley; 2University of California, Berkeley; Lawrence Berkeley National Laboratory; 3Lawrence Livermore National Laboratory; 4Lawrence Berkeley National Laboratory
At cryogenic temperatures, hexagonal-close-packed (HCP) Ti has been observed to undergo a brittle-ductile transition when interstitial oxygen solute concentration is increased beyond a certain threshold. This observation has been linked to the a twinning mode that is not commonly observed in HCP Ti alloys. The relevant twin boundaries associated with this deformation twinning mode are observed in electron microscopy and first-principles calculations to display a novel interfacial structure (complexion) linked to a metastable polymorph in Ti. We discuss the energetic origins of this complexion and how it influences the interactions of the twin with oxygen solutes. By drawing comparisons to previous related research related to oxygen-solute interactions with more common twinning modes, we discuss mechanisms by which oxygen can alter the selection of twinning modes favorable for growth. Implications of the work for alloy design in Ti and other related materials will be discussed.