Deformation and Transitions at Interfaces : Meso/Microstructural Scale Mechanical Behavior of Polycrystals I
Sponsored by: TMS Functional Materials Division (formerly EMPMD), TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Computational Materials Science and Engineering Committee, TMS: Mechanical Behavior of Materials Committee, TMS: Thin Films and Interfaces Committee
Program Organizers: Saryu Fensin, Los Alamos National Laboratory; Thomas Bieler, Michigan State University; Rozaliya Barabash, OakRidge National Lab; Shen Dillon, Universe of Illinois; Jian Luo, University of California, San Diego; Doug Spearot, University of Florida

Tuesday 8:30 AM
February 28, 2017
Room: 23B
Location: San Diego Convention Ctr

8:30 AM  Invited
New Insights into Plasticity at Grain Boundaries by Nano- and Micromechanics: Christoph Kirchlechner1; Nataliya Malyar1; Nicolas Peter1; Gerhard Dehm1; 1Max-Planck-Institut für Eisenforschung GmbH
     Nano- and micromechanical testing on focused ion beam (FIB) milled pillars is frequently used to study size effects in materials. It also allows for isolating the effect of individual interfaces, like grain- or phase-boundaries on the strength.Our current work focusses on slip transfer mechanisms and its dependence on (i) grain boundary (GB) type, (ii) loading direction and (iii) atomistic structure at the interface. In situ methods are applied to investigate the transfer from individual dislocations through the GB (via TEM), the collective storage (via µLaue diffraction) and collective transmission behavior (via SEM). In the talk, we comprehensively present first results on slip transfer in copper bi-crystals. Four different GB types, all made from copper, will be presented and discussed: (i) High Angle GBs (HAGBs) acting as strong obstacle for dislocation slip transfer; (ii) HAGBs allowing for easy slip transfer, (iii) coherent 3 twin-boundaries and (iv) coherent 5 GBs.

8:50 AM  Invited
Grain Boundary-Mediated Deformation Mechanisms Accommodating Mechanical Grain Growth in Nanocrystalline Metals: Jason Trelewicz1; 1Stony Brook University
    Stress-assisted grain growth in nanocrystalline metals transpires collectively with a number of competing deformation mechanisms. In this study, molecular dynamics simulations of surface nanoindentation were performed to quantify the plastic strain distribution among competing mechanisms during stress-assisted grain growth in nanocrystalline Ni and Ni-P. In the finest grain size nanocrystalline metals, mechanical grain growth was attributed to grain boundary-mediated deformation mechanisms involving grain boundary migration and grain rotation. The manifestation of these mechanisms in the deformation tensor was quantified by isolating specific grain coalescence events. Using these continuum metrics, a reduction in deformation temperature, increase in nanocrystalline grain size, or addition of P segregated to the grain boundaries were found to quell mechanical grain growth by suppressing grain boundary-mediated plasticity. In the absence of grain boundary deformation mechanisms, plastic strain was accommodated by dislocation plasticity, thus signifying the role of mechanistic crossovers in stress-assisted grain growth.

9:10 AM  Invited
Studying the Mechanical Response of Regions within Grains and Near Grain Boundaries Using Spherical Nanoindentation: Siddhartha Pathak1; 1University of Nevada, Reno
    We discuss the capabilities of spherical nanoindentation stress-strain curves, extracted from the measured load-displacement dataset, in characterizing the local mechanical behavior within individual grains and near grain boundaries of polycrystalline samples. Since nanoindentation length scales are smaller than the typical grain sizes in polycrystalline samples, this technique is an ideal tool for detailed characterization of the microscale heterogeneities present in these materials and their evolution during various metal shaping/working operations. Using a series of examples, we demonstrate the tremendous capabilities of our data analyses procedures in a) characterizing the local indentation yield strengths in individual grains of deformed polycrystalline metallic samples and relating them to increases in the local slip resistances, b) correlating the stored energy differences of individual grains to their Taylor factors as a function of imposed cold work, and c) understanding the role of interfaces such as grain boundaries in the deformation of a multi-phase polycrystalline sample.

9:30 AM  
Influence of Dislocation Density on Plastic Deformation near Grain Boundary in Alpha-titanium Studied by Nanoindentations and Modeling: Yang Su1; Philip Eisenlohr1; Thomas Bieler1; Martin Crimp1; 1Michigan State University
    To study how pre-existing dislocations affect plastic deformation across grain boundaries, spherical nanoindentations were placed near the same grain boundaries in large-grained alpha titanium following different levels of pre-strain. Prior to nanoindentation, the dislocation distributions, Burgers vectors, and effective densities were characterized using electron channeling contrast imaging and high-resolution electron backscattered diffraction. The plastic deformation and grain boundary strain transfer was evaluated by measuring the nanoindent topography, by atomic force microscopy, as a function of pre-strain. Crystal plasticity finite element simulations of the indentation process were carried out and compared to the experimentally measured indentation topographies. Initial slip resistance and hardening rate parameters were tuned to achieve the best match between simulations and experimental indents at different pre-indentation strain levels, to correlate with the influence of pre-existing dislocations. The role of pre-existing dislocations on both sides of the boundaries was characterized. This work was supported by NSF Grant DMR-1411102.

9:50 AM  Invited
Deformation Mechanisms of Single and Polycrystalline Zirconia Nanopillars: Ning Zhang1; Mohsen Asle Zaeem1; 1Missouri University of Science and Technology
    Molecular dynamics is employed to investigate the deformation and failure mechanisms of single and polycrystalline yttria-stabilized tetragonal zirconia (YSTZ) under uniaxial compression. Results show that the nanoscale plastic deformation of single crystal YSTZ has a strong dependence on the crystallographic orientation. A direct evidence of tetragonal to monoclinic transformation is observed when compress along [101], [101 ̅ ], [011] or [011 ̅ ] directions. While dislocation nucleation and emission is found to dominate the failure process of [001]-, [110]- and [1 ̅10]-oriented nanopillars. Interestingly, when applying loading on the [100]-, [010]-, [111]- and [112]-oriented nanopillars, a combination of dislocation motion and tetragonal to monoclinic transformation is detected. The dislocation-dominated deformation leads to the lowest strength for nanopillars, while phase transformation-dominated one results in the highest strength. In polycrystalline zirconia, phase transformation rather than dislocation motion is observed to be triggered at a lower stress and consequently dominate the plastic deformation.

10:10 AM Break

10:30 AM  Invited
Mechanical Characterization of Grain Boundary Regions Using Spherical Nanoindentation: Shraddha Vachhani1; Roger Doherty2; Surya Kalidindi3; 1Hysitron, Inc; 2Drexel University; 3Georgia Institute of Technlogy
    Understanding of the evolution of microstructure during thermo-mechanical processing of metallic materials is largely hampered by lack of methods for characterizing reliably their local (anisotropic) properties at the sub-micron length scales. Indentation stress-strain curves extracted from spherical nanoindentation testing have demonstrated tremendous potential for addressing this gap. This local mechanical property data, together with the complimentary structure information measured locally using electron backscatter diffraction (EBSD) was used to investigate local changes in the mechanical behavior as a function of crystal orientation as well as with distance from the grain boundaries with an aim to gain insights into the role of these microstructural features during macroscopic deformation. Ambient temperature and high temperature spherical indentation testing of cubic metals in annealed and deformed (to various strain levels) conditions will be presented.

10:50 AM  Invited
Phases and Phase Transformations at Interfaces: Tim Frolov1; Mark Asta2; Yuri Mishin3; 1Lawrence Livermore National Laboratory; 2University of California - Berkeley; 3George Mason University
    The presentation will review the recent progress in theoretical understanding and atomistic computer simulations of phase transformations in materials interfaces, focusing on grain boundaries (GBs) in metallic systems. Recently developed simulation approaches enable the search and structural characterization of GB phases in single-component metals and binary alloys, calculation of thermodynamic properties of individual GB phases, and modeling of the effect of the GB phase transformations on GB kinetics. Atomistic simulations demonstrate that the GB transformations can be induced by varying the temperature, loading the GB with point defects, or varying the amount of solute segregation. The atomic-level understanding obtained from such simulations can provide input for further development of thermodynamics theories and continuous models of interface phase transformations while simultaneously serving as a testing ground for validation of theories and models. They can also help interpret and guide experimental work in this field.

11:10 AM  Invited
Atomistic Simulations of Transient Testing in Nanocrystalline Al: Maxime Dupraz1; Zhen Sun2; Christian Brandl3; Helena Van Swygenhoven2; 1Paul Scherrer Institut; 2Paul Scherrer Institut & EPFL; 3Karlsruhe Institute of Technology
    To get deeper insight in the interplay between dislocation slip and GB accommodation and to accompany insitu Xray experimental observations during transient testing of nc Ni[Acta Mat 91(2015)91], transient tests have been carried out using molecular dynamics. After deforming the sample with 108 /s, stress drops are carried out and the sample is allowed to creep up to 2.3ns at much lower strain rates (~ 106 /s). Important changes in the grain boundary structure occur via mechanisms such as GB dislocation climb and GB migration. After structural changes new dislocations are emitted and this at much lower applied stresses. Besides confirming the interpretation of the experimental in situ tests, our simulations at low strain rates reveal deformation mechanisms that have not been observed during constant strain simulations at 108 /s, such as dislocation-dislocation interaction mechanisms leaving point defects in grain interior and forwards and backwards motion of grain boundaries.

11:30 AM  
Stabilization of Nanocrystalline Alloys at High Temperatures via Utilizing High-entropy Grain Boundary Complexions: Naixie Zhou1; Tao Hu1; Mingde Qin1; Jiajia Huang1; Jian Luo1; 1UCSD Nanoengineering
    Multicomponent alloying can be utilized to enhance the thermal stability of nanocrystalline alloys. The grain boundary energy can be reduced significantly via both bulk and grain-boundary high-entropy effects with increasing temperature at/within the solid solubility limit, thereby reducing the thermodynamic driving force for grain growth. Moreover, grain boundary migration can be hindered by sluggish kinetics. Analytical models were developed on multicomponent ideal solutions ideal solutions to demonstrate both bulk and grain-boundary high-entropy effects on reducing grain boundary energies. To further test these new theories, numerical experiments were conducted. Subsequently, several nanoalloys were designed and fabricated to demonstrate outstanding thermal stabilities that outperform the Ni-based binary nanocrystalline alloys. Finally, the mechanical properties of sintered bulk nanoalloys will also be presented and discussed.

11:50 AM  Invited
Observation and Characterization of Grain Boundary Complexions in Hot-pressed Boron Carbide: Kristopher Behler1; Scott Walck1; Christopher Marvel2; Jerry LaSalvia3; Martin Harmer2; 1U.S. Army Research Laboratory (SURVICE Engineering); 2Lehigh University; 3U.S. Army Research Laboratory
    Multiple grain boundary complexion types, including possible liquid-like intergranular films (IGFs), were observed in boron carbide for the first time. The boron carbide samples were produced by hot-pressing high-purity boron carbide powders between 1800°C and 2000°C for 3 hours. Alumina, silica, and boron oxide additives were used to aide densification and foster the development of IGFs. Silicon-rich and silicon-aluminum-oxygen films with nominal thicknesses of <1 – 3 nm were observed in samples processed with multiple components. Based on theoretical considerations, additive compositions with multiple components may foster the development of IGFs by lowering their free energy through the likelihood of increased structural disorder. Aberration-corrected scanning transmission electron microscopy (AC-STEM) predominately showed multi-layer films at high-angle grain boundaries. X-ray energy-dispersive spectroscopy (XEDS) and energy-filtered TEM (EFTEM) were used to characterize the chemical nature of films and estimate their thicknesses. Experimental procedures, results, and analyses will be presented.

12:10 PM  Invited
Complexion Transitions in Metals: Unique Opportunities for Mechanical Behavior and Materials Processing: Timothy Rupert1; 1University of California, Irvine
    Doped interfaces can have intriguing structures and, in some cases, thermodynamically-stable interfacial complexions can form. In this talk, we explore the usage of complexions in transition metal alloys, with a focus on how these features affect processing routes and mechanical properties. Atomistic simulations are used to identify the effects of chemistry, temperature, and boundary character on grain boundary structural transitions, as well as identify how these features impact plasticity and fracture. Experimental validation is provided by high resolution transmission electron microscopy on specially-designed thin film samples that systematically explore these variables, as well as nanocrystalline alloys produced through a powder route and consolidated to bulk form. As a whole, this work lays the foundation for engineering internal interfaces to design better materials.