Interface-Mediated Properties of Nanostructured Materials: Nanolaminates and Nanotwinned Materials II
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 2:00 PM
February 27, 2017
Room: Pacific 23
Location: Marriott Marquis Hotel

Session Chair: Michal Demkowicz, Texas A&M University; Caizhi Zhou, Missouri University of Science and Technology

2:00 PM  Invited
Strength, Plasticity, and Toughness of Nanolaminated Materials: Jian Wang1; 1University of Nebraska-Lincoln
    Future energy, transportation and defense technologies demand novel materials that tolerate extremes in temperature, stress, strain rate, and radiation to an extent that far exceeds the limits of the most advanced materials to date. Nanolaminated materials have been demonstrated as promising to achieve superb combination of strength and ductility, and to potentially enhance fracture toughness. Theatrical, modeling and experimental studies revealed that mechanical properties of laminated materials can be optimized by tailoring the combination of constituent phases, the layer thickness, the crystallography of the adjacent layers, and interfaces (including structures and properties) that play triple roles: sources and barriers for plastic deformation carriers and platforms for dynamic reassembly of interface defects. Here, we review mechanisms and mechanics of strength, plasticity, and toughness of metal-metal, metal-ceramics, and metal-amorphous nanolaminated materials.

2:30 PM  Invited
Fracture Behavior of Nanostructured Heavily Cold Drawn Pearlite: Influence of the Interface: Nagamani Jaya Balila1; Christoph Kirchlechner1; Gerhard Dehm1; 1MPIE GmbH
    Micro-cantilever fracture tests were carried out on heavily cold drawn pearlite as a function of drawing strain. In the as-drawn state, pearlite exhibits a complex nanolamellar microstructure and possess ultra-high tensile strengths. It is observed that there is a significant drop in fracture toughness as the true drawing strain is increased from 3.2 to 4.2. This is accompanied by a change in the load-displacement characteristics, which shows signature of some plastic deformation before fracture for a strain of 3.2 and linear elastic brittle fracture for a strain of 4.2 and above. There occurs a second brittle to ductile transformation for a strain of 4.2 when annealed at 500oC for 30 min due to recovery, grain growth and drop in yield strength. Influence of nano-structuring, high defect density, dissolution of cementite and phase transformation in the ferritic matrix are discussed as plausible reasons for this transition in fracture behavior.

3:00 PM  
Excess Volume and Defect Annealing in Ultrafine-grained Ni Studied by Difference Dilatometry: Jaromir Kotzurek1; Anton Hohenwarter2; Macej Krystian3; Wolfgang Sprengel1; Reinhard Pippan2; Roland Würschum1; 1Graz University of Technology; 2University of Leoben; 3Austrian Institute of Technology
    Ultrafine grained metals as obtained by "severe plastic deformation" techniques do not only contain a high amount of grain boundaries but might also exhibit an anisotropic grain structure and high concentrations of remnant dislocations and vacancies. Each defect type specifically contributes to the resulting overall excess volume of such a metastable solid. Upon heating the different defect types specifically release their excess volume. Thus, by using dilatometric techniques for the characterization of the defect annealing behavior properties such as grain-boundary excess volume can, e.g., be determined from the analysis of the recrystallization. The vacancy relaxation volume is accessible if an orientation dependence in the defect annealing due to the anisotropic grain shape is utilized. A comparative dilatometric study is presented for the two different deformation techniques of high-pressure torsion (HPT) and equal-channel angular pressing (ECAP) where nickel served as a model system. Supported by Austrian Science Fund (FWF): P25628-N20

3:20 PM  
Mechanisms for Stable Nanocrystalline Materials via Nanometallic Multilayers: J. Sebastian Riano1; Andrea Hodge1; 1University of Southern California
    Nanostructured materials have interesting mechanical properties although they generally have low thermal stability due to their multiple grain boundaries. However, grain boundaries are suitable sites for grain boundary stabilization by kinetic or thermodynamic mechanisms. In this presentation, we will demonstrate how nanometallic multilayers (NMMs) can be use as models systems to study thermal stability mechanisms. Specifically, by utilizing thermodynamic simulations to guide the selection of stable NMMs combinations, one can use multilayered geometries to control of the grain size and composition to further evaluate stable nanograin structures. In this study Hf-Ti, Ta-Hf, Mo-Au, and W-Cr NMMs were synthesized by magnetron sputtering. As-sputtered samples were heat treated under vacuum during 96 h at different temperatures. Microstructures, composition, and grain sizes of as-sputtered and heat treated samples were analyzed by TEM, and EDX profiles. The loss of layered structure, grain boundary segregation and precipitation are presented.

3:40 PM Break

3:55 PM  Invited
On the Frank-Bilby Equation and the Corresponding Relaxed Dislocation Structures: Aurélien Vattré1; 1CEA
    Many interfaces have non-uniform internal patterns comprised of complex misfit dislocation structures, which in turn govern interface properties. A novel formalism for describing the elastic relaxation of equilibrium interface dislocation arrays linking the Frank-Bilby equation and anisotropic elasticity theory under the condition of vanishing far-field stresses is proposed. The present "parametric dislocation dynamics-like" approach enables the determination of the reference state for interface misfit dislocations, within which the Burgers vectors of individual dislocations are defined and allows for the unequal partitioning of elastic fields between two dissimilar crystals. The elastic strain energies of interface dislocation networks with coplanar dislocation junction formation are computed using solutions for short-range elastic fields. Examples of applications to general twist grain boundaries in fcc and bcc metals as well as semicoherent fcc/bcc heterophase interfaces are given.

4:25 PM  Invited
Deformation Mode Transitions in Amorphous Cu45Zr55/Crystalline Cu Nanolaminates: Christian Sterwerf1; Tyler Kaub2; Chuang Deng3; Greg Thompson2; Lin Li2; 1Bielefeld University; 2University of Alabama; 3University of Manitoba
    Nanolaminates of amorphous Cu45Zr55 and crystalline Cu of varying thicknesses were grown and investigated in terms of deformation behaviors subject to nanoindentation. A transition of deformation modes from shear banding to co-deformation of the amorphous and crystalline phases was observed as the thickness of Cu layer increased from 5 nm to 150 nm. This transition was signified by a diminishing of ‘pop-in’ events in the load-displacement curves and further confirmed by deformation patterns around the indents as viewed by scanning electron microscopy images. Using molecular dynamics simulations, the spatial correlation of the shear transition zones as a function of Cu thicknesses was investigated. The simulations revealed a percolation created by the indent impression of the strain localization initiated at the crystalline-amorphous interfaces above and below the Cu layer prior to shear banding.

4:55 PM  
Dislocation Nucleation Controlled Deformation in Angstrom Scaled FCC Twins: Jiangwei Wang1; Frederic Sansoz2; Scott Mao1; 1University of Pittsburgh; 2The University of Vermont
    Although nanoscale twinning is an effective means to enhance yield strength and tensile ductility in metals, nanotwinned metals generally fail well below their theoretical strength limit due to heterogeneous dislocation nucleation from boundaries or surface imperfections. Here we show that Au nanowires containing angstrom-scaled twins (0.7 nm in thickness) exhibit tensile strengths up to 3.12 GPa, near the ideal limit, with a remarkable ductile-to-brittle transition with decreasing twin size. This is opposite to the behaviour of metallic nanowires with lower-density twins reported thus far. Ultrahigh-density twins (twin thickness < 2.8 nm) are shown to give rise to homogeneous dislocation nucleation and plastic shear localization, contrasting with the heterogeneous slip mechanism observed in single crystalline or low-density-twinned nanowires. The twin size dependent dislocation nucleation and deformation represent a new type of size effect distinct from the sample size effects described previously.

5:15 PM  
Grain Boundary Anisotropy-mediated Properties of fcc and bcc Materials: Brandon Runnels1; 1University of Colorado Colorado Springs
    Grain boundaries (GBs) are known to be key players in a variety of mesoscopic processes, such as solidification, recrystallization, grain boundary migration, and severe plastic deformation. Though grain boundary energy (per unit area) is frequently treated as having a constant value, numerous experimental and computational studies indicate that grain boundary energy is strongly dependent on the geometric character of the interface; that is, the 5-dimensional space of the crystallographic orientation relationship and orientation of the interface. The lattice-matching grain boundary model (with relaxation) provides a means for determining the influence of anisotropic grain boundary energy on material properties with interface-dense structures such as twins and nanocrystalline metals. The model is applied to phase-field modeling of polycrystals, to determining the energetic favoribility of twinning modes based on interface energy, and to the determination of the propensity of GBs to nucleate nanovoids under shock loading.

5:35 PM  
Molecular Dynamics Simulation of Face-centered Cubic Metallic Nanospheres under Uniaxial Compression: Selim Bel Haj Salah1; Celine Gerard1; Laurent Pizzagalli1; 1Institut Pprime, CNRS - ENSMA - Université de Poitiers
     Molecular Dynamic simulations have been performed to study the mechanical behavior of aluminum, copper and nickel nanospheres under uniaxial compression, and the influence of size on plasticity mechanisms and yield stress, on the size range from 3 to 20nm. Crystallographic orientation and stacking fault energy influence have been investigated. Plasticity always started by the nucleation of partial dislocations from the surface contact edges. Depending on the crystallographic orientation, the formation of pyramidal structures may be observed for large enough nanoparticles. The pyramid apex favors dislocation nucleation at high compression levels. The plasticity mechanisms and the yield stress do not depend on nanoparticle size. Instead, our results suggest that the geometry and atomic structure of the layers in contact with the indenters control the plastic deformation. At last, the “twinnability” is studied and discussed. The deformation mechanisms switched from a dislocation slip-dominant regime to a twin-dominant regime when the twinnability decreases.