Nix Award and Lecture Symposium: Mechanistic Understanding of Mechanical Behavior Across Length Scales: Session I
Program Organizers: Michael Mills, Ohio State University; Kevin Hemker, Johns Hopkins University

Wednesday 8:30 AM
February 26, 2020
Room: 4
Location: San Diego Convention Ctr

Session Chair: Michael Mills, Ohio State University; Deborah Yaney


8:30 AM  Keynote
Nix Award Lecture: Damage Tolerance in Materials: Robert Ritchie1; 1University of California Berkeley and Lawrence Berkeley National Laboratory
    A material’s capacity for limited deformation is a critical aspect of toughness as this enables the local dissipation of stresses that would otherwise cause fracture. Such inelastic deformation mechanisms are diverse; they include dislocation motion in crystalline materials, in-situ phase-transformations in certain metals and ceramics, sliding of collagen fibrils in bone, rotation of fibrils in skin, frictional motion between mineral “platelets” in seashells, and through mechanisms that also cause fracture such as shear-banding in glasses and microcracking in rocks. Resistance to fracture is thus a compromise: either a combination of the mutually exclusive properties of strength and deformability, as in intrinsic toughness, or between intrinsic and extrinsic (shielding) mechanisms that act to induce toughness, respectively, ahead or behind, the tip. We examine the interplay between such mechanisms in biological materials, including skin and bone, high-temperature materials, such as ceramic-matrix composites and nuclear graphite, and in bulk-metallic glasses and high-entropy alloys.

9:30 AM  Invited
Mechanical Properties of High Entropy Alloys: Easo George1; 1Oak Ridge National Laboratory and University of Tennessee
    High-entropy alloys (HEAs) comprise multiple principal elements in near-equal amounts. They are scientifically interesting because theories that have been developed for dilute solid solutions cannot be directly applied to concentrated alloys lacking “solvents” and “solutes” in the traditional sense. Furthermore, a handful exhibit striking mechanical properties, for example, strength, ductility, and toughness that are simultaneously enhanced at cryogenic temperatures, unlike in conventional materials where they are traded off. In this talk, I will summarize what we have learned about the mechanical properties of this new class of alloys by focusing on a few model systems. While the basic mechanisms of plastic deformation in HEAs are broadly similar to those seen in conventional alloys, a common feature of HEAs with superior mechanical properties seems to be their ability to activate multiple strengthening mechanisms, often sequentially. As a result, the strain hardening regime is greatly extended and necking postponed. Based on these findings, further improvements in strength, without sacrificing ductility and toughness, can be envisioned in the vast compositional space occupied by HEAs.

10:00 AM Break

10:30 AM  Invited
Hybrid Nanocomposites at the Extreme Limits of Molecular-scale Confinement: Reinhold Dauskardt1; 1Stanford University and the Stanford School of Medicine
    We review the state-of-the-art in the molecular design and processing of low density organic-inorganic hybrids at the extreme limits of molecular-scale confinement. A particular focus is provided on unique mechanical and molecular behavior that can be achieved in the limit of such intimate molecular mixing and confinement. We show that molecular hybrids can have marked asymmetric elastic and thermal expansion properties that are inherently related to terminal chemical groups in confinement. We describe a new nanoscale design principle using hyperconnected molecular architectures to achieve remarkable mechanical properties controlled by designing connectivity into the intrinsic molecular structure in innovative ways. We probe the mechanical and fracture properties of hybrids in the extreme limits of molecular confinement, where a stiff inorganic matrix phase confines polymer chains to dimensions far smaller than their bulk radius of gyration. Finally, we describe a synthesis strategy which involves the infiltration of individual polyimide precursors into a nanoscale porous network where imidization reactions under such confinement increase the molecular backbone stiffness. We find that polyimide oligomers can then undergo crosslinking reactions even in such molecular-scale confinement, increasing the molecular weight of the organic phase and toughening the nanocomposite through a confinement-induced energy dissipation mechanism. This work demonstrates that the confinement-induced molecular bridging mechanism can be extended to thermoset polymers with multifunctional properties, such as excellent thermo-oxidative stability and high service temperatures (> 350 °C).

11:00 AM  Invited
Amorphization: A New Dislocationless Deformation Mechanism?: Marc Meyers1; S Zhao2; Eric Hahn1; Boya Li1; Bruce Remington3; Chris Wehrenberg3; Hye-Sook Park3; 1University of California San Diego; 2Lawrence Berkeley National Laboratory; 3Lawrence Livermore National Laboratory
    Intermetallics and covalent materials often exhibit high Peierls-Nabarro stresses and are therefore brittle: the energy to create and propagate cracks is lower than the one to generate the stacking faults, twins, and dislocations required for plastic deformation. Shock compression subjects materials to a unique regime of high hydrostatic and coupled shear stresses for durations on the order, in the case of laser-driven events, of 1-10 nanoseconds. The superposed hydrostatic pressure impedes the formation of cracks. Here we propose that shock/shear amorphization observed in Si, Ge, B4C, and SiC is a new deformation mechanism in a broad class of covalently bonded materials and some intermetallics. The crystalline structure transforms to a higher-density amorphous one along regions of maximum shear stress, forming nanoscale bands, thereby relaxing the shear component of the imposed shock stress. This process is preceded by the emission and propagation of a critical concentration of stacking faults. Molecular dynamics calculations confirm the new mechanism.

11:30 AM  Invited
Toughening Materials with Air?: Brad Boyce1; K. Conway1; Ben White1; Anthony Garland1; 1Sandia National Laboratories
     Lattice metamaterials have been shown to exhibit a number of beneficial properties, ranging from acoustic damping to negative Poisson response. Now, with the proliferation of additive manufacturing technologies, such structures are becoming more accessible and cost-effective. However, as previously observed in metal foams and nanoporous materials, the observed toughness of low-density materials tends to be far inferior to the constituent material. According to Gibson-Ashby scaling, such structures are expected to suffer a precipitous drop in fracture toughness as the relative density decreases. Moreover, manufacturing heterogeneities can cause a minority of weakest struts to trigger a localization that propagates to structural failure. In this presentation, we discuss strategies to architect toughening mechanisms that protect from localization or dissipate energy in novel ways, breaking free from Gibson-Ashby limits. In this talk, we honor Rob Ritchie’s extensive mastery and use of fracture toughening mechanisms, in systems ranging from teeth to high entropy alloys. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.