Mechanical Behavior at the Nanoscale V: Deformation
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
Tuesday 2:00 PM
February 25, 2020
Room: Santa Rosa
Location: Marriott Marquis Hotel
Session Chair: Heny Ovri, Helmholtz Zentrum Geesthacht
2:00 PM
The Effects of Supersonic Impacts on the Micromechanical Properties of Al6061 Cold Spray Deposits: Tyler Flanagan1; Benjamin Bedard1; Avinash Dongare1; Harold Brody1; Aaron Nardi2; Victor Champagne2; Seok-Woo Lee1; 1University of Connecticut; 2Army Research Laboratory
The mechanical characterization of supersonic impacted metallic microparticles is of great interest as their unusual microstructure leads to unique deformation mechanisms, which relates to the mechanical properties of bulk cold sprayed coating. Here, we present micro-compression/tension results performed on cold-sprayed Al-6061 multi-deposit coatings. The individual particles which comprise the coatings contain two microstructural distinct regions; the centers of the particles which are comprised of an equiaxed grain structure and a squashed grain structure at the particle boundaries which undergoes significant deformation and recrystallization. The variance in mechanical properties of these two regions as a function of their microstructures will be discussed in terms of grain refinement and grain boundary orientation. The influence of additional impacts on the mechanical properties of particles will also be compared with that of single particle impacts. Our study will give an insight in fundamental understanding on the effects of high strain rate deformation.
2:20 PM
Small Scale Mechanical Testing of Nanoporous Tungsten: Mingyue Zhao1; Inas Issa1; Manuel Pfeifenberger2; Michael Wurmshuber1; Daniel Kiener1; 1University of Leoben; 2Erich Schmid Institute of Materials Science
Nanoporous tungsten is of great scientific interest, as it combines the beneficial properties of tungsten with the positive attributes of nanoporous foams and has a great potential to satisfy the need for high performance materials that can endure harsh environments. In this work, nanoporous tungsten, characterized by a continuous network of nanocrystalline tungsten ligaments and nanopores, was created on a bulk scale through a unique technique route involving the severe plastic deformation followed by the selective phase dissolution. The mechanical properties, which are an important consideration during many practical applications, were investigated by employing depth-sensing nanoindentation and other small-scale testing in situ in the scanning electron microscopy. Based on this, the elemental plasticity mechanisms governing the mechanical behavior were elucidated. The promising mechanical results of the created nanoporous tungsten will serve as a foundation for forthcoming related scientific studies and engineering applications.
2:40 PM
Nanotwins and Grain Boundaries: Competing Roles on the Nucleation and Propagation of Dislocations Probed via Nanoindentation: Raheleh Rahimi1; Sichuang Xue1; Siavash Ghanbari1; Xinghang Zhang1; David Bahr1; 1Purdue University
The influence of grain boundaries on plastic deformation of polycrystalline twinned Al thin films has been investigated using nanoindentation. Arrays of indentations that exhibited elastoplastic pop-ins and grain boundary excursions due to slip transfer were assessed in three textured aluminum thin films. Al (111) shows grain boundary excursions at higher loads, yet has a softer hardness (0.99 GPa), than Al (110) and Al (112) harnesses (1.16 and 1.2 GPa respectively); the (112) film has a very similar grain size as the (111) film and the added strength is attributed to the increased twin density. Molecular dynamics was used to estimate the impact of defects on the onset of plastic deformation in these films. Nanotwins lead to films which deform more uniformly than the more defect free films, and the nanotwins appear to more greatly impact the motion of dislocations rather than the onset or nucleation of dislocations in Al films.
3:00 PM
Intrinsic Deformation and Failure Response of Single Crystal MAX Phases: Zhiqiang Zhan1; Hemant Rathod1; Miladin Radovic1; Ankit Srivastava1; 1Texas A&M University
A family of ternary carbides and nitrides, referred to as MAX phases, possess unique set of properties. These are light, stiff, thermodynamically stable and refractory, like ceramics, but damage-tolerant, pseudo-ductile at high temperatures and readily machinable like metals. Prior works have shown that polycrystalline MAX phases exhibit a range of deformation and failure mechanisms, such as crystallographic slip, ripplocation, twist, delamination and kinking. Here, we aim to correlate the single crystal level mechanical response of MAX phases to the overall mechanical response of the polycrystalline aggregate. To this end, micropillars are extracted from grains of know orientations using FIB milling and are subsequently deformed under compression using a flat-punch nanoindenter. The mciropillar experiments are complemented with novel in-situ SEM indentation experiments on single crystals of MAX phases. Our results shed new lights on the activation of various competing deformation and failure mechanisms in MAX phases at the single crystal level.
3:20 PM Break
3:40 PM Invited
On the Estimation of Thermal Activation Parameters for Portevin-Le Chatelier Effect from Nanoindentation Data: Henry Ovri1; Erica Lilleodden1; 1Helmholtz Zentrum Geesthacht
Since Portevin-Le Chaterlier (PLC) effect is driven by thermally activated mechanisms, it is of interest to determine the associated activation enthalpy, ∆Ea. This parameter is important for understanding the underlying deformation mechanisms and identifying the relevant atomic species that diffuse to the dislocations during deformation. Currently, most of the available models for estimating ∆Ea are based on the critical strain, εc, for the onset of the phenomenon during uniaxial tests. However, a εc is not always observed and some of the models incorporate unverified dislocation density and vacancy concentration dependences. In view of this, we present a nanoindentation-based approach that obviates the need for εc and gives estimates of ∆Ea, along with the associated activation volume and attempt frequency. The estimated parameters are in good agreement with reported values for the Al-Mg alloy studied herein. The results also reveal the utility of the technique for investigations of PLC more generally.
4:20 PM
Characterizing Near-surface Plasticity in Aluminum-carbon Hybrid Materials: Christopher Shumeyko1; Andrew Palughi2; Daniel Cole1; Christopher Klingshirn3; Xiaioxiao Ge3; Lourdes Salamanca-Riba3; Madeline Morales3; 1U.S. Army Research Laboratory; 2Texas A&M University; 3University of Maryland
Covetic nanomaterials, a form of metal-matrix nanocarbon composite (MMCs) produced via a single-step electrocharge-assisted process, have been shown to exhibit increased mechanical and electrical properties over parent alloys. This process has been demonstrated with a wide variety of metals, with a recent focus on Al and Cu for aerospace and power transmission applications. However, scalability and consistency issues with the process limit its widespread adoption and drive the need for a fundamental understanding of process-structure-property relationships. Here, we couple atomistic simulations to experimental characterization to identify the role of the carbon nanophase on early stage plasticity in covetics. Nanoindentation is performed on regions of interest and studied via TEM and Raman to reveal the effect of graphitic structures on microstructure and dislocation density. Concurrently, MD and DFT simulations probe the influence of metal-carbon interfaces and bonding on mechanical behavior. Information gleaned will lead to process control for higher performing MMCs.
4:40 PM
Bending Dominated Plasticity and Hardening in Au-Ag Nanoboxes: Radhika Patil1; David Doan1; Zachary Aitken2; Chen Shuai2; Mehrdad Kiani1; Yong-Wei Zhang2; Wendy Gu1; 1Stanford University; 2Institute of High Performance Computing
Porous, nano-architectured metallic structures with characteristic dimensions of ~10 nm are valuable as strong, stiff and lightweight materials, but are difficult to fabricate and test experimentally. Here we colloidally synthesize hollow Au-Ag nanoboxes with smooth and rough surfaces. These structures are hollow cubes with overall dimensions of ~140 nm and wall thickness of ~15 nm. The smooth and rough nanoboxes yield at 130 ± 45 MPa and 96 ± 31 MPa stress, respectively. Both samples show strain hardening with rough nanoboxes having higher hardening rate than the smooth nanoboxes. We use finite element modelling to determine the influence of structural architecture on deformation and find that structural geometry is not responsible for the observed strain hardening. Using molecular dynamic simulations, we find that the nucleation and interaction of stacking faults is responsible for hardening.
5:00 PM
Fundamental Investigation of Fatigue Behavior in Microstructurally-stable Nanocrystalline Cu-Ta Alloys: Anqi Yu1; Christian Roach1; Khalid Hattar2; Kiran Solanki3; Suveen Mathaudhu1; 1University of California Riverside; 2Sandia National Laboratories; 3Arizona State University
There has been growing research interest in nanocrystalline (NC) materials because of their superior mechanical properties compared to their coarse-grained counterparts. However, there are very few studies on the fatigue behavior of NC alloys due to their observed microstructural instabilities, such as grain growth during fatigue. Fortunately, experiments and simulations have shown that Cu-Ta alloys can retain NC grains under thermo-mechanical loading conditions. The microstructural stability is attributed to Zener pinning; more specifically, Ta nanoclusters provide a pinning pressure against the driving force of grain boundary movement. In this project, NC Cu-Ta alloys (Cu-1, 3, 5 & 10Ta, at.%) with microstructures resistant to grain growth are fabricated by cryomilling and consolidated by spark plasma sintering. Cyclic mechanical testing is performed to measure the fatigue behavior. Finally, microstructural changes and their role on fatigue behavior in microstructurally-stable nanocrystalline Cu-Ta alloys are probed.