Multiscale Architectured Materials (MAM II): Tailoring Mechanical Incompatibility for Superior Properties: Heterogeneous and Gradient Materials
Sponsored by: TMS Structural Materials Division, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Yuntian Zhu, North Carolina State University; Irene Beyerlein, University of California, Santa Barbara; Yves Brechet, Grenoble Institute of Technology; Huajian Gao, Brown University; Ke Lu, Institute of Metal Research, Chinese Academy of Science; Xiaolei Wu, Institute of Mechanics, Chinese Academy of Science

Tuesday 8:30 AM
February 28, 2017
Room: 24B
Location: San Diego Convention Ctr

Session Chair: Huajian Gao, Brown University; Irene Beyerlein, University of California


8:30 AM  Invited
The Austenite/Martensite Interface: Francesco Maresca1; W Curtin1; 1EPFL
    Mesoscale architectures/microstructures for enhancing mechanical performance are already employed in steels. In next-generation high-strength/high-toughness steels, the austenite/martensite (fcc/bcc) interface is the dominant microstructural feature controlling properties, but the structure and motion of this interface remain uncertain. Here, an atomistic fcc-bcc iron interface is constructed that completely matches experimental observations. The interface reveals a defect structure differing from longstanding assumptions and violating conditions believed essential for a glissile interface. Based on the new interface structure, we revise (i) the conditions for achieving a glissile interface and (ii) the double-shear theory of lath martensites. The new parameter-free theory is in near-perfect agreement with simulations and experiments, and thus allows for guided design of materials with higher toughness. The new structure also has implications for alloying and hydrogen embrittlement. The adroit application of atomistic modeling tools thus expands the possibilities for design and control of mesoscale architecture in high-performance steels.

8:55 AM  
A Deformation Mechanism by Correlated Necklace Dislocations in Nanotwinned Materials: Haofei Zhou1; Huajian Gao1; 1Brown University
    The introduction of nanoscale twin boundaries (TBs) is an effective strategy to achieve exceptional combination of superior strength, ductility and resistance to fracture, fatigue and wear. It has been demonstrated that the properties of nanotwinned metals are dominated by a number of deformation mechanisms unique to the interactions between dislocations and TBs. Here, we reveal yet a new type of dislocation mechanism called correlated necklace dislocations (CNDs). This mechanism controls the strengthening of nanotwinned metals as the twin thickness is reduced to around 1 nm. The presence of a cracklike defect as the dominant dislocation source could allow the same mechanism to operate at larger twin spacings. More importantly, we demonstrate that CNDs could be responsible for an unusual, history-independent and stable fatigue behavior of nanotwinned Cu containning highly oriented nanoscale twins. Our findings call for further theoretical and experimental investigations of the unique deformation mechanisms in nanotwinned metals.

9:15 AM  Invited
Simultaneous High Strength and Ductility in Nickel Induced by Nanodomains with Size Effects: Fuping Yuan1; Xiaolei Wu1; Evan Ma2; 1Institute of Mechanics, Chinese Academy of Science; 2The Johns Hopkins University
    A defect engineering strategy on the nanoscale is architected to obtain nanocrystal strength with coarse-grain ductility. Spread-out domains (average 7 nm in diameter) were produced in Nickel during electrodeposition, occupying only ~2.4% of the total volume. The resulting Ni achieves a yield strength approaching 1.3 GPa, and a uniform elongation as large as ~30%. Electron microscopy observations and molecular dynamics (MD) simulations demonstrate that the nanodomains effectively block dislocations, akin to the role of precipitates for Orowan hardening. Moreover, the abundant domain boundaries provide dislocation sources and trapping sites of running dislocations for dislocation multiplication, and the ample space in the grain interior allows dislocation storage. A pronounced strain-hardening rate is therefore sustained to enable large uniform elongation. The pinning strength was also found to be closely related to the domain boundary type, the domain size and spacing. The present results should provide insights in metals for enhanced mechanical properties.

9:40 AM  
Interfacial Incompatibilities and Crystalline Deformation and Failure: Matt Bond1; Mohammed Zikry1; 1North Carolina State University
    Interrelated aspects of behavior with a specific focus on microstructural characteristics, such as dislocations, precipitates, dispersed particles, grain-boundaries (GBs), and crystallographic slip that span the nano to the micro scales, and how these characteristics affect failure modes, such as dynamic fracture in crystalline materials, will be presented. Recently developed fracture methodologies have been used for a detailed analysis of fracture nucleation and the accurate characterization of intergranular and transgranular crack growth. Criteria for dislocation-density mechanisms and immobilization are directly related to interactions with grain boundary ledges and propagating cracks in bicrystals and polycrystalline aggregates. The effects of interfaces, such as grain boundaries blocking dislocation motion and pileups, stress concentrations, and fracture nucleation will be analyzed.

10:00 AM  
Mechanical Behavior and Deformation Mechanism of Gradient Structured Cu Alloys with Varying Stacking Fault Energy: Xinkun Zhu1; 1Kunming University of Science and Technology
    Producing gradient nano-grained structure (GNS) is an efficient way to achieve high strength and ductility of the materials simultaneously. It is significant to study microstructure, mechanical behavior, and corresponding deformation mechanism of the gradient structure. In this paper, Cu alloys with different stacking fault energies (SFEs) were processed by surface mechanical attrition treatment (SMAT) to obtain the gradient structure (GS). Analysis based on Bauschinger effect reveals that the contribution of back stress on the yield strength of GS samples varies with different SFE. The high back stress caused by strain gradient and mechanical incompatibility of GS and CG layers can be responsible for the high strength of GS samples, while the good ductility is ascribed to both back-stress hardening and dislocation hardening.

10:20 AM Break

10:35 AM  Invited
Gradient Nanostructure and Residual Stresses Induced by Ultrasonic Nano-crystal Surface Modification for Improved Mechanical Properties: Chang Ye1; Yalin Dong1; Vijay Vasudevan2; 1University of Akron; 2University of Cincinnati
    Gradient nanostructured metallic materials attracted significant interests due to their unprecedented combination of strength, ductility and fatigue performance. Ultrasonic nanocrystal surface modification (UNSM) is a recently developed surface severe plastic deformation process that utilizes static load superimposed on dynamic ultrasonic impacts to induce plastic deformation on metal surfaces. Here we report how UNSM affects the microstructure and thus the properties and performance of 304 stainless steel and 52100 bearing steel. In 304 SS, nanograins have been observed in the top surface while highly dense deformation twins have been observed in the subsurface after UNSM. The sandwich microstructure with two strong surface layers and a compliant interior embedded with highly dense nanoscale deformation twins leads to both high strength and high ductility in SS 304. In 52100 bearing steel, the work-hardened surface layers and high magnitude of compressive residual stresses induced by UNSM lead to improved wear resistance and fatigue performance.

11:00 AM  Invited
Homogeneous Plastic Deformation in Heterogeneous Lamella Structures: Caizhi Zhou1; Rui Yuan1; Irene Beyerlein2; 1Missouri University of Science and Technology; 2University of California at Santa Barbara
    Unlike nanocrystalline (NC) phase, a heterogeneous lamella (HL) composite, consisting of NC and coarse-grain layers, exhibits greatly improved ductility. To understand the fundamental principles governing their high strength and good ductility, we employ a 3D discrete crystal plasticity finite element (CPFE) model to study the spatially resolved deformation fields within each layer and at their interface. Based on an analysis of the distributions of equivalent plastic strain and lattice reorientations within the NC lamella, we show that the heterogeneity of strain concentration, which could potentially lead to shear localizations in the NC layer, has been substantially reduced by the uniform deformation that is characteristic of the coarse-grain lamella. In addition, we find that coarse grains well oriented for multi-slip have an even greater homogenization effect, thereby further increasing the ductility of the entire HL structure. These findings can benefit the design of gradient or heterogeneous structures to achieve superior properties.

11:25 AM  
Gradient Nanostructured Silicon through High Power Pulsed Laser-driven Shock Compression: Shiteng Zhao1; Eric Hahn1; Bimal Kad1; Bruce Remington2; Christopher Wehrenberg2; Karren More3; Eduardo Bringa4; Marc Meyers1; 1University of California, San Diego; 2Lawrence Livermore National Laboratory; 3Oak Ridge National Laboratory; 4Universidad Nacional de Cuyo
    High-power-nanosecond-pulsed-laser, once deposited onto materials, will ablate their surface into plasma, driving a strong shock wave into the bulk material. A transient extreme condition, characteristic of pressures in excess of tens of GPa and temperatures in excess of thousands of K, can be established and decay rapidly thereafter. Here we report, using this technique, on achieving nanostructured silicon materials with gradient grain size and heterogeneous microstructure. The talk will be focused on the mechanisms of rapid shock-induced bulk and directional amorphization and subsequent nano-crystallization. Mechanical and optical behavior of the obtained materials will also be discussed.