Interface-Mediated Properties of Nanostructured Materials: Hierarchical Nanostructured Materials
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

Tuesday 8:30 AM
February 28, 2017
Room: Pacific 23
Location: Marriott Marquis Hotel

Session Chair: Nan LI, Los Alamos National Laboratory; Peter Anderson, The Ohio State University

8:30 AM  
Deformation Mechanisms in bcc Mg/Nb: Youxing Chen1; Satyesh Yadav1; Nan Li1; Xiang-Yang Liu1; Kevin Baldwin1; Irene Beyerlein1; Richard Hoagland1; Jian Wang2; Nathan Mara1; 1Los Alamos National Laboratory; 2University of Nebraska – Lincoln
    Mg alloys attract attention for being one of most promising lightweight structural materials for automobile and aerospace applications. However, their low strength and poor ductility, due to scarcity of easy slip systems and localized shear in the hcp structure, limit their applicability. By using a Mg/Nb nanolayered structure, Mg can be stabilized in the bcc phase, which provides more slip systems as well as significantly improved ductility. However, the mechanism of improved ductility in bcc Mg remains unclear. In-situ mechanical testing in TEM is an ideal method to determine the deformation mechanisms as it can demonstrate the structural evolution at atomic scale in real time. Coupled with first-principles density functional theory, we explore the mechanism of ductile bcc Mg in Mg/Nb nanolayers in terms of dislocation activity and phase stability during mechanical testing. The fundamental understanding of deformation mechanisms in Mg nanocomposites may guide future design of Mg alloys.

8:50 AM  Invited
Fracture Toughness of Al/SiC Nanolaminates: Experiments and Simulation: Carl Mayer1; Ling Yang2; V. Carollo2; J. Kevin Baldwin3; Nathan Mara3; Jon Molina-Aldareguia2; Nikhilesh Chawla1; 1Arizona State University; 2IMDEA; 3Los Alamos National Laboratory
    Nanolaminate Al/SiC composites exhibit extremely high strength and toughness. The fracture toughness of Al/SiC nanolaminates were characterized using microscale cantilever beams and indentation of micropillars. The nanolaminates were processed by physical vapor deposition (PVD) using magnetron sputtering. By adding a sharp notch to the top surface of the beams, the toughness of these materials was able to be determined as a function of layer thickness with the cracks propagating both parallel and perpendicular to the layers. The fracture toughness in the perpendicular crack orientation is significantly lower than the toughness measured in the parallel crack orientation. The measured toughness was shown to scale well with the plastic zone areas determined using finite element analysis for the 100 and 50 nm layer thickness samples. The transition between deflection-controlled to plastic zone-controlled fracture will be discussed.

9:20 AM  Invited
The Role of Interfaces on Plasticity in Dislocation Nucleation-mediated Nanostructures: Jungho Shin1; Lisa Chen1; Gunther Richter2; Thomas Cornelius3; Olivier Thomas3; Daniel Gianola4; 1University of Pennsylvania; 2Max-Planck-Institut für Intelligente Systeme; 3Aix-Marseille Université; 4University of California, Santa Barbara
    A straightforward strategy for reaching the ideal strength of crystalline metallic materials is to synthesize materials with a scarcity of defects. Bottom-up synthesis of nanostructures is an ideal means of approaching this limit, where nucleation of dislocations is a requirement in otherwise perfect crystals to facilitate plastic flow and mitigate brittle fracture. In this deformation mechanism regime where thermal fluctuations assist in the nucleation process, deterministic mechanical response gives way to probabilistic yielding with a strong temperature dependence and activation energies suggestive of surface self-diffusion as the rate-limiting step needed to promote displacive activity. We show mechanical deformation experiments using various in situ electron and X-ray probes on high quality single crystalline noble metal nanowhiskers and interrogate a new means of controlling mechanical response – by control of both external (free surfaces) and internal (planar defects) interfaces to mediate dislocation nucleation and ensuing plasticity.

9:50 AM  
In-situ TEM Observations of Grain Growth during High-cycle Fatigue and Notch Fatigue: Khalid Hattar1; Daniel Bufford1; William Mook1; Christopher O'Brien1; Fadi Abdeljawad1; Tim Furnish1; Brad Boyce1; Stephen Foiles1; 1Sandia National Laboratories
    Recent studies have suggested that mechanically-induced grain coarsening may drastically influence the fatigue lifetime of nanocrystalline metals exposed to high cycle loading conditions. This presentation will present the first direct real-time observations of fatigue-induced grain growth in nanocrystalline metals, as revealed by in-situ TEM fatigue loading experiments with notched and un-notched tensile specimens. In order to better delineate the roles of grain size, local texture, and grain boundary character, the in-situ fatigue experiments are performed in concert with precession electron diffraction (PED) orientation mapping. The results of the experiments will be compared to mechanisms and observations in the literature, as well as multiscale models. This work was fully supported by the Division of Materials Science and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

10:10 AM Break

10:25 AM  Invited
Competing Interfaces within Hierarchical Nanostructured Metallic Alloys: Daniel Foley1; Garritt Tucker1; 1Drexel University
    Nanostructured metallic alloys are rapidly becoming regarded as a new class of engineering materials with advantageous mechanical properties, such as high strength and increased wear resistance. As the inherent length-scales within nanostructured materials decrease to the nanoscale, interfaces and their associated properties tend to govern material response. In this research, we utilize a combination of atomistic simulations and microcontinuum metrics to quantify the competition between interfacial-driven behavior in metallic nanolaminates composed of alternating amorphous and nanocrystalline layers during mechanical deformation of Ni alloys. In particular, we resolve the contribution of grain boundaries within the nanocrystalline regions, during incipient plasticity and dislocation migration, to that of the amorphous-crystalline interface between the layers, as a function of microstructure and layer thickness. By developing such a fundamental understanding of the deformation mechanisms, nanostructured metallic alloys with tunable properties can be proposed to aid in the design of improved hierarchical nanostructured materials.

10:55 AM  
Twinning Paths and Twin Boundaries in Hexagonal Close-packed Titanium: Hao Wang1; 1Institute of Metal Research, Chinese Academy of Sciences
    Twinning is an important carrier of plastic deformation in titanium, however, there are still unresolved questions concerning the exact twinning paths and corresponding twin boundary structures. Employing ab initio calculations and molecular dynamics simulations, we systematically investigated {1012} twinning behavior in hexagonal close-packed (HCP) Ti. Several possible twinning paths, mediated by either the face-centered cubic (FCC) or the body-centered cubic (BCC) phase, were identified. By decoupling the shear and shuffle components, a complete map of twinning paths was obtained, which predicted relative energy consumption of various paths, including the previously proposed “twinning-like lattice reorientation”. Several easy-twinning paths were found with intermediate FCC-structured twin boundaries. Based on the map, typical twin boundaries were reconstructed, which well explained certain experimental observations, including curved and blurred boundaries. All identified twinning paths correspond with lattice dislocations in HCP Ti and hence are operative during plastic deformation.

11:15 AM  Invited
Role of Twinning, Dynamic Recrystallization, and Shear Banding in the Microstructural Evolution of Magnesium Alloys: Ibrahim Karaman1; Ebubekir Dogan1; Matthew Vaughan1; S.J. Wang1; 1Texas A&M University
    Poor formability and strong anisotropy of magnesium alloys at medium and low temperatures (<150°C) are the main factors limiting their widespread applications. One of the reasons is the activation of various twinning modes causing mechanical flow anisotropy, and their role in and interactions with dynamic recrystallization (DRX) and shear banding during deformation. In this talk, we will present how two different deformation types (tension vs. simple shear) and starting crystallographic texture can be utilized to control twinning modes, DRX, and shear banding, and how their interactions lead to damage nucleation. The results of a crystal plasticity model helped understand operating deformation modes and DRX, the latter of which was found to play a significant role in shear localization. Detailed electron backscatter diffraction analyses clearly indicated that the formation of compression twins causes deformation localization, followed by DRX within compression twins, local softening and large shear bands, and eventual failure.

11:45 AM  
The Twinning Genome: A Systematic Framework for Predicting Twinning in Materials: Dingyi Sun1; Mauricio Ponga2; Kaushik Bhattacharya1; Michael Ortiz1; 1California Institute of Technology; 2University of British Columbia
    Twinning - a mechanism of lattice reorientation in order to accommodate deformation - is present in many different classes of materials. In particular, twinning plays a crucial role in the deformation of hexagonal close-packed (HCP) materials such as magnesium, a promising basis for the engineering of new lightweight alloys to replace aluminum and steel. To thoroughly understand twinning, we propose a novel systematic framework to predict all twin modes in an arbitrary material. We first predict all possible twin modes given only basic lattice information. We then consider the energetics of these twin configurations via molecular statics and density functional theory simulations to identify likely twins. The result is the prediction of a set of likely twin modes which is much larger than what previous works of literature have considered. Taking advantage of these newly-predicted twin modes will thus hopefully lead the design of a new generation of alloys.