Interface-Mediated Properties of Nanostructured Materials: Nanolaminates and Nanotwinned Materials I
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Caizhi Zhou, Missouri University of Science and Technology; Nan Li, Los Alamos National Laboratory; Peter Anderson, The Ohio State University; Michael Demkowicz, Texas A&M University

Monday 8:30 AM
February 27, 2017
Room: Pacific 23
Location: Marriott Marquis Hotel

Session Chair: Caizhi Zhou, Missouri University of Science and Technology; Nan Li, Los Alamos National Laboratory

8:30 AM  
Micro-scale Scratch Behavior of Copper-silver Nanolayers: Madhavan Radhakrishnan1; Pascal Bellon1; Robert Averback1; 1University of Illinois
    The effects of interfaces and layer spacing on micro-scale scratch properties has been investigated in a copper-silver model system. Nanolayered composite thin films having alternate layers of copper and silver were synthesized by physical vapor deposition process. Scratch tests were performed in a tribo-indenter by 2-D wear patterning and nano-scratch testing with a cono-spherical diamond indenter. The volume of displaced material during scratch, as estimated by depth profilometry, attains a quasi-steady state with increasing number of repeated wear cycles. The wear performance of multilayers improves with decreasing layer thicknesses of Cu and Ag, primarily attributed to interfacial area fraction and work hardening. Cross-section microstructural analyses show evidence of the dissolution of layers locally to form a solid solution below the worn surface. Sliding induced microstructural evolution causes local modification to the mechanical properties, which stabilize the wear rate with increasing wear cycles.

8:50 AM  Invited
Plasticity in Small-scale Metallic Composites: Amit Misra1; Jian Wang2; 1University of Michigan; 2University of Nebraska
    This presentation will review the recent progress in the understanding of plastic deformation in ultra-fine scale metal-based composites. Examples will be presented from a variety of metallic systems such as Al-Al2Cu eutectics, and vapor deposited metallic multilayers. The common aspects in interface-dominated mechanical behavior in ultra-fine scale metallic composites such as unusually high flow strengths, high strain hardening rates and plastic co-deformability will be elucidated through in situ TEM straining experiments and analyzed using atomistic modeling, dislocation theory and crystal plasticity. The strain hardening behavior of confined systems will be interpreted using a three-dimensional crystal elastic–plastic model that describes plastic deformation based on the evolution of dislocation density in the constituent phases.

9:20 AM  Invited
Intrinsic Twin Boundary Defects and Strength in Nanotwinned Ag and Ag-Cu Alloys: Frederic Sansoz1; Xing Ke1; Qiongjiali Fang1; 1The University of Vermont
    This talk will present our recent progress in understanding twin-size and grain-size effects on strength and ductility of nanotwinned Ag and Ag metals containing trace concentrations of Cu. The ability of coherent twin boundaries in strengthening and maintaining ductility has been well documented; yet most understanding of the origin of this mechanical behavior has relied on a perfect interface assumption. This presentation will focus on the important roles of intrinsic kink-like twin boundary defects on size effects in plasticity of nanotwinned face-centered cubic metals and alloys. Large-scale hybrid Monte-Carlo/Molecular Dynamics simulations are presented to discuss the influence of atom segregation on twin stability and plastic deformation mechanisms in nanotwinned Ag with microstructures stabilized by small concentrations of Cu atoms. These new materials, termed as nanocrystalline-nanotwinned Ag, show a record strength well above that of pure nanotwinned Ag or stronger metals like Cu, with excellent microstructure stability under stress and temperature.

9:50 AM Break

10:10 AM  Invited
Atomistic Simulations at Reduced Strain Rates of Dislocation Interactions in Nanocrystalline Al: Maxime Dupraz1; Helena Van Swygenhoven2; Zhen Sun3; Christian Brandl4; 1Paul Scherrer Institut; 2Paul Scherrer Institute; Ecole Polytechnique Fèdèrale de Lausann; 3Paul Scherrer Institut; École Polytechnique Fédérale de Lausanne; 4Karlsruhe Institute of Technology
    Mechanisms involving dislocation-GB accommodation are prevalent in nanocrystalline structures and are often ascribed to atomic shuffling. Here we describe how the misfit available in a nano-dimension GBs can assist the propagation of a lattice dislocation: the impinging dislocation undergoes double cross-slip and interacts with a dislocation loop that nucleates from the misfit of the GB. The initial Burgers vector thereby spreads across three planes and a microtwin propagates along the GB. The mechanism is observed only at a reduced strain rate (106/s) and it facilitates slip of a lattice dislocation with low Schmid factor. At higher strain rates (108/s), the mechanism does not occur and instead high Schmid factor slip systems are preferred. For nanocrystalline Al subjected to a reduced strain rate (106/s), dislocation-dislocation interaction mechanisms are observed to produce point defects in grain interiors and grain boundaries displace forwards and backwards during dislocation nucleation

10:30 AM  Invited
Collective Deformation Mechanisms and their Effect on Nanoscale Interfacial Networks: Timothy Rupert1; 1University of California, Irvine
    Nanocrystalline metals experience plasticity that is dominated by collective mechanisms such as grain rotation, and grain boundary sliding. In this talk, we discuss how these mechanisms lead to a dynamic grain structure and an evolving grain boundary network. First, cyclic loading experiments and transmission Kikuchi diffraction are used to probe the response of nanocrystalline Ni and Cu films. We observe that higher applied strains and temperature lead to more evolution, yet a balance must be struck to retain a small grain size and the associated collective mechanisms. Atomistic models are also used to characterize and track mesoscale features like boundary character distribution and interfacial topology during plasticity, providing an in situ view of the collective behavior. As a whole, we find that grain structure can evolve to a lower energy state, often characterized by increased numbers of special boundaries, a disruption of random boundary percolation paths, and boundary faceting transitions.

11:00 AM  
Intrinsic Surface Stress Effects on Surface Dislocation Nucleation in Nanoscale Pristine Metals: Qingjie Li1; Bin Xu2; Evan Ma1; 1Johns Hopkins University; 2Shanghai JiaoTong University
    Recent advances in materials fabrication have created a wide range of nanoscale pristine metals. The mechanical strength and plasticity of these metals largely depend on surface dislocation nucleation (SDN). Often SDN is depicted to be at most weakly dependent on sample size. Despite of extensive studies on the extrinsic effects such as surface steps, oxidation layers and surface diffusion on SDN, the intrinsic surface stress effects on SDN has not been well understood. Using atomistic simulations, we demonstrate that surface stresses generally result in sample size dependent activation parameters of SDN. This eventually leads to obvious sample size dependence of strength: whether it is “smaller is stronger” or “smaller is weaker” depends on the loading mode. These effects can be rationalized in terms of the local maximum resolved shear stress which results from the combined effects of both the applied axial stress and the surface stresses.

11:20 AM  
Influence of Crystalline Nanoprecipitates on Shear-band Propagation in Cu-Zr-based Metallic Glasses: A Computational Study: Tobias Brink1; Karsten Albe1; Omar Adjaoud2; 1TU Darmstadt; 2Technische Universität Darmstadt
    The propagation of shear bands in metallic glasses can be affected by the presence of crystalline phases in the amorphous matrix. Here we present molecular dynamics simulations on the interaction of shear bands with crystalline nanoprecipitates in Cu-Zr-based metallic glasses. Our results reveal different interaction mechanisms: Shear bands can dissolve precipitates, can wrap around crystalline obstacles, or can be blocked depending on the size and density of the precipitates. If the crystalline phase has a low yield strength, we also observe slip transfer through the precipitate. Based on the computational results, a simple model based on an equilibrium of stresses at the precipitate is proposed which categorizes the various processes as a function of the critical stress for dislocation nucleation, precipitate size, and distance.