Date:
Wednesday, February 26, 2020
Time:
8:30 a.m. to 4:30 p.m.
Organizers:
Michael Mills, The Ohio State University; Kevin Hemker, Johns Hopkins University
Featured Speakers
Nix Award Lecturer
Robert O. Ritchie, University of California, Berkeley, and Lawrence Berkeley National Laboratory, USA
Presentation Title: "Damage-Tolerance in Materials"
About the Presentation
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. This presentation examines 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.
Easo P. George, Oak Ridge National Laboratory and University of Tennessee, USA
Presentation Title: "Mechanical Properties of High-Entropy Alloys"
About the Presentation
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, the presenter summarizes 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.
Reinhold H. Dauskardt, Stanford University and the Stanford School of Medicine, USA
Presentation Title: "Hybrid Nanocomposites at the Extreme Limits of Molecular-Scale Confinement"
About the Presentation
This presentation reviews 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).
Marc Meyers, University of California, San Diego, USA
Presentation Title: "Amorphization: A New Dislocationless Deformation Mechanism?"
About the Presentation
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.
Brad Boyce, Sandia National Laboratories, USA
Presentation Title: "Toughening and Energy-Dissipation in Metamaterials"
About the Presentation
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. This presentation discusses 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.
George Pharr, Texas A&M University, USA
Presentation Title: "Measurement of Mechanical Properties by Nanoindentation: Recent Innovations in Testing Methodology"
About the Presentation
Great progress has been made over the past three decades in measuring mechanical properties at small scales by load- and depth-sensing indentation methods, also known as nanoindentation. Here, we outline a new innovation in nanoindentation testing motivated by a desire to make accurate hardness (H) and modulus (E) measurements in materials with extremely high E/H ratios, during very rapid testing used to map local properties, and in nanoindentation tests at very high strain rates (103 to 104 s-1). Under these conditions, the commonly used continuous stiffness method of property measurement (CSM) often breaks down because of what has been termed “plasticity error”—a measurement error that develops when loading conditions are such that a significant portion of the deformation in one oscillation cycle of the CSM measurement is plastic rather than elastic. Here, we show that by fully understanding how the phase-lock amplifier measures the contact stiffness from small oscillations in the applied force or displacement, plasticity errors can be modeled and understood, and largely corrected. The observations and analysis also suggest that there are much better ways to make CSM measurements rather than the commonly used technique of applying small oscillations at fixed displacement amplitude. The new observations and analysis procedures are documented and verified by experimental measurements in two model materials: fused silica and aluminum. This work was sponsored in part by the U.S. Department of Energy, National Nuclear Security Administration, under Award No. DE-NA0003857.
Andy Minor, University of California, Berkeley, and Lawrence Berkeley National Laboratory, USA
Presentation Title: "The Role of Solutes and Short-Range Order (SRO) in the Deformation of ɑ-Ti Alloys"
About the Presentation
This talk will center on the impact of short-range order (SRO) on the mechanical behavior of Ti alloys. Specifically, we have looked at the effect of O and Al solutes with a combination of advanced transmission electron microscopy (TEM), nanomechanical testing, and bulk testing of model alloys. In Ti-O alloys we have performed tensile deformation at low, room, and high temperatures at various strain rates and performed microstructural analysis to systematically map out the effect of solute content, temperature, and strain rate on the planar to wavy slip transition. This transition will be discussed in the context of theoretical models of O solute effects on dislocation cross-slip. In Ti-Al alloys we have used energy filtered imaging and 4D-STEM to map the local SRO and strain with nanometer precision, even during in situ nanomechanical testing. Lastly, we have investigated the phenomenon of electroplasticity with Ti-Al alloys and found interesting similarities between the effect of SRO and electrical pulsing on the defect structure.
Tresa Pollock, University of California, Santa Barbara
Presentation Title: "The Dynamics of Precipitate Shearing in fcc/L12 Alloys"
About the Presentation
Intermetallic precipitates are among the most effective strengthening agents for structural metallic alloys. Ordered intermetallic precipitates are generally resistant to shearing by dislocations, resulting in strengthening, but are also effective for damage tolerance since they will ultimately deform plastically in the presence of high local stresses. Ni-base superalloys are prototypical examples wherein L12 precipitates strengthen the solid solution nickel alloy matrix to high fractions of melting. The contribution of precipitate shearing to overall plasticity in Ni-base alloys has to-date been assessed via post-deformed transmission electron microscopy studies of mechanically deformed samples. We report here on a novel in-situ approach to studying precipitate shearing and faulting in superalloys. This approach integrates MEMs straining stages with a STEM detector in the SEM, enabling dynamic observations of defects and their interactions with precipitates, faults and boundaries, studied with diffraction contrast. New insights on the frequency of shearing and the faulting mechanisms will be reported and the implications for alloy design will be discussed.
Gunther Eggeler, Ruhr-University Bochum, Germany
Presentation Title: "Early Nanoscale Dislocation Processes and Two Creep Rate Minima in SX Ni-Base Superalloys"
About the Presentation
Creep governs the service lives of critical high temperature components like single crystal Ni-base superalloy (SX) first stage blades in gas turbines for aero engines and power plants. Creep shows a strong stress and temperature dependence and one must understand the elementary processes which govern creep in order to safely design and operate high temperature systems. Creep is generally subdivided into periods of primary, secondary, and tertiary creep, where creep rates decrease, reach one creep rate minimum and then increase towards final rupture. However, in the low temperature and high stress creep regime of SX (for SX: temperature < 800°C, stress > 600 MPa), two creep rate minima can be distinguished, a first after about 0.5% and a second after 5% strain. High resolution miniature specimen creep testing, analytical transmission electron microscopy and 2D discrete dislocation modelling are combined to identify the elementary processes which govern double minimum creep.
TMS-AIME Awards Ceremony
Robert O. Ritchie will formally receive the William D. Nix Award during the TMS-AIME Awards Ceremony, which begins at 6:30 on Wednesday evening, following the conclusion of this special symposium. Anyone is welcome to attend the awards ceremony, but tickets must be purchased in advance for the banquet following the ceremony.