Nix Award and Lecture Symposium: Mechanistic Understanding of Mechanical Behavior Across Length Scales: Session II
Program Organizers: Michael Mills, Ohio State University; Kevin Hemker, Johns Hopkins University
Wednesday 2:00 PM
February 26, 2020
Room: 4
Location: San Diego Convention Ctr
Session Chair: Seung Min Han, Korea Advanced Institute of Science and Technology; Wendelin Wright, Bucknell University
2:00 PM Invited
Measurement of Mechanical Properties by Nanoindentation: Recent Innovations in Testing Methodology: P Phani1; Benoit Merle2; Warren Oliver3; George Pharr4; 1International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI); 2Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU); 3Nanomechanics, Inc.; 4Texas A&M University
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. * Work sponsored in part by the U.S. Department of Energy, National Nuclear Security Administration, under Award No. DE-NA0003857.
2:30 PM Invited
The Role of Solutes and Short Range Order (SRO) in the Deformation of α-Ti Alloys: Ruopeng Zhang1; Shiteng Zhao1; Yan Chong1; Max Poschmann2; Eric Rothchild2; Colin Ophus3; John Morris2; Daryl Chrzan1; Mark Asta2; Andrew Minor1; 1University of California Berkeley and Lawrence Berkeley National Laboratory; 2University of California Berkeley; 3Lawrence Berkeley National Laboratory
This talk will center on the impact of 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 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.
3:00 PM Break
3:30 PM Invited
The Dynamics of Precipitate Shearing in fcc/L12 Alloys: Jean-Charles Stinville1; Michael Titus1; Daniel Gianola1; Tresa Pollock1; 1University of California Santa Barbara
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.
4:00 PM Invited
Early Nanoscale Dislocation Processes and Two Creep Rate Minima in SX Ni-base Superalloys: Gunther Eggler1; 1Ruhr-University Bochum
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.