Mechanical Behavior at the Nanoscale V: In-Situ Testing I
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Mechanical Behavior of Materials Committee, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Christopher Weinberger, Colorado State University; Megan Cordill, Erich Schmid Institute of Materials Science; Garritt Tucker, Colorado School of Mines; Wendy Gu, Stanford University; Scott Mao; Yu Zou, University of Toronto

Wednesday 8:30 AM
February 26, 2020
Room: Santa Rosa
Location: Marriott Marquis Hotel

Session Chair: Ben Beake, Micro Materials; Khalid Hattar, Sandia National Laboratories


8:30 AM  
Ductile-to-Brittle Transition of Micron-sized Niobium at Cryogenic Temperature: Gyuho Song1; Seok-Woo Lee1; 1University of Connecticut
    Ductile-to-brittle transition (DBT) of body-centered-cubic (bcc) metals usually occurs due to the low mobility of screw dislocation at a low temperature. The DBT behavior of micron-sized bcc metals could be different from that of their bulk counterparts because of the different plasticity mechanism, the intermittent operation of dislocation sources at the micrometer scale. In this presentation, we present our recent results on the DBT of micron-sized niobium. Micro-tensile tests were performed on [0 0 1] Nb micropillars at 298, 100 and 56K using a custom-built in-situ cryogenic micromechanical testing system. Fractography shows plasticity-driven fracture at 298K but brittle fracture at 100 and 56K. Post-mortem transmission electron microscopy suggests that the DBT at the micrometer-scale would occur via the dynamic annihilation of dislocation sources instead of the reduction of screw dislocation mobility. At a low temperature, dislocation multiplication seems to be strongly suppressed, and all dislocation sources would be quickly depleted.

8:50 AM  
Nanomechanical Properties of Pristine and Heavy Ion Irradiated Nanocrystalline Tungsten: Jonathan Gigax1; Osman El-Atwani1; Matthew Chancey1; Jon Baldwin1; Stuart Maloy1; 1Los Alamos National Laboratory
    Nanocrystalline materials are postulated as strong candidates for nuclear materials applications due to their high irradiation tolerance manifested by their high grain boundary density. Their mechanical properties before and after irradiation, however, are a concern. Nanoindentation, micropillar compression and cantilever bending tests were performed on nanocrystalline tungsten films (grain size less than 35 nm) prior to and post irradiation at high temperature (1050 K) with heavy ions. Their performance under different stress states corresponding to the different tests was evaluated. Characterizing un-irradiated and irradiated nanocrystalline materials was shown to be best performed using a combination of different micromechanical tests.

9:10 AM  Invited
What Controls Damage Tolerance in Repetitive Nano- and Micro-scale Impact Testing of Thin Films?: Ben Beake1; Sam McMaster2; Luis Isern3; Tomasz Liskiewicz4; Jose Endrino3; 1Micro Materials Ltd.; 2University of Leeds; 3Cranfield University; 4Manchester Metropolitan University
    Improving the fatigue and fracture resistance of thin films under highly mechanically loaded repetitive contact conditions is an important step to increasing their performance in demanding applications. As a screening tool to evaluate promising thin film architectures rapid nano- and micro-scale laboratory tests can be highly effective but they need to be corrected dimensioned. Nano-impact tests with sharp cube corner indenters highlight differences in resistance to contact damage. Micro-impact tests at significantly higher strain rate and energy allow study of thin film fatigue with spherical indenters enabling the thin film-substrate system behaviour to be studied as a whole. This presentation will investigate the role of thin film microstructure and mechanical properties on the damage tolerance at the different length scales probed in the nano- and micro-scale tests. Design rules for enhanced damage tolerance are proposed.

9:50 AM  
Exploring the Extremes of Mechanical Behavior Through In-situ Electron Microscopy: Khalid Hattar1; Christopher Barr1; Gowtham Jawaharram2; Nathan Heckman1; Brad Boyce1; Shen Dillon2; 1Sandia National Laboratories; 2Univeristy of Illinois
    To understand new or complex interactions of mechanisms active in extreme environments, one must be able to directly observe the materials response with adequate resolution. Sandia has developed a set of SEM- and TEM-based tools to explore mechanical behavior during a wide range of extreme conditions. This presentation will include snapshots of results during in-situ SEM and TEM fatigue, ex-situ radiation-fatigue, in-situ TEM creep above 2000 °C, as well as in-situ TEM irradiation induced creep at both room and elevated temperatures. The overlap of mechanical testing with laser and ion beam irradiation permits a wealth of complex operational environments that can be explored with nanometer resolution. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & 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-NA0003525

10:10 AM Break

10:30 AM  
Extending the Range of Constant Strain Rate Nanoindentation Testing: Benoit Merle1; George Pharr2; 1University Erlangen-Nuremberg (FAU); 2Texas A&M University
     Constant strain rate nanoindentation is a popular method for accessing the local strength of complex materials. However, with currently available testing systems using continuous stiffness measurement (CSM), nanoindentation is so far limited to strain rates of ~0.1 s-1, which precludes it from ballistic applications. Here, we show that the current limitation derives primarily from a plasticity issue related to the continuous stiffness measurements. Using a “sweet spot” frequency one order of magnitude higher than the standard harmonic oscillation, we show that valid hardness measurements are possible up to ~1 s-1. In order to access even higher deformation rates, the Oliver-Pharr evaluation method was modified, so as to avoid the need for a measurement of the contact stiffness. With this improvement, the experimental upper strain rate limit is mostly determined by the time constants of the hardware components and lies around 100 s-1 with most current commercial systems.

10:50 AM  
Phase Transformation Induced Plasticity in High-strength Hexagonal Close Packed Co with Stacking Faults: Ruizhe Su1; Dajla Neffati2; Jaehun Cho1; Qiang Li1; Jie Ding1; Haiyan Wang1; Yashashree Kulkarni2; Xinghang Zhang1; 1Purdue University; 2University of Houston
    Deformation twinning and dislocation glide are two primary deformation mechanisms in hexagonal close packed (HCP) metals. Here we show, via in situ micropillar compression tests, that HCP Co pillars with high-density stacking faults exhibit a high yield strength and significant plasticity. Transmission electron microscopy studies reveal the formation of extensive Face-centered cubic (FCC) Co phase after deformation. Molecular dynamics simulations confirm the deformation induced phase transformation, and shed light on the deformation mechanisms in HCP Co with pre-existing stacking faults. This study provides new insights into achieving high strength and plasticity in HCP metals via stacking faults and phase transformation.

11:10 AM  
Improving Ductility of Magnesium Through Reversible Phase Transformation in bcc Mg/Nb Nanolaminates: Youxing Chen1; Nan Li2; Satyesh Yadav3; Xiang-yang Liu2; Jian Wang4; Nathan Mara5; 1University of North Carolina at Charlotte; 2Los Alamos National Laboratory; 3Indian Institute of Technology Madras; 4University of Nebraska, Lincoln; 5University of Minnesota, Twin Cities
    Mg alloys suffer from poor strength and ductility at room temperature, due to a lack of available slip systems in hcp structures. Our approach is to improve the strength and deformability of Mg alloys through stabilization of the bcc phase of Mg in metal laminates. Since bcc Mg can be stabilized when located between bcc Nb when the individual layer thickness is below 10 nm, the ductility is improved as bcc Mg has additional active room temperature slip systems over hcp Mg. In-situ TEM mechanical testing is a useful tool for real-time observation of deformation mechanisms at nanometer scales. Our results directly validate the hypothesis that bcc Mg can accommodate large plastic deformation and further reveal that in bcc/bcc Mg/Nb, a reversible bcc-hcp phase transformation occurs during loading and unloading. The fundamental understanding of deformation mechanisms of bcc Mg in Mg/Nb are investigated utilizing a combination of experiments and modeling.

11:30 AM  
Recent Innovation in In-situ Extreme Mechanics at the Micro and Nanoscale: Nicholas Randall1; Damian Frey1; Jean-Marc Breguet1; Rajaprakash Ramachandramoorthy2; Jakob Schwiedrzik2; Johann Michler2; 1Alemnis AG; 2EMPA
     This talk will focus on recent developments in instrumentation for in-situ extreme mechanics testing at the micro and nanoscales, focusing on a testing platform capable of strain rate testing from 0.0001/s up to 10’000/s (8 orders of magnitude) with simultaneous high speed actuation and sensing capabilities, with nanometer and micronewton resolution respectively. Other recent innovations include cryogenic and high temperature tests covering the envelope from -150 to 800 °C. The challenges in variable temperature tests and the associated technological and protocol advances will be discussed along with select case studies. The inherent advantages of using small volumes of sample material, e.g., small ion beam milled pillars, will be discussed together with the associated instrumentation, technique development, data analysis methodology and experimental protocols. Some examples will be presented where a wide range of strain rate has been combined with variable temperature to investigate rate effects as a function of temperature.