Mechanical Behavior of Nanostructured Materials: Mechanical Behavior of Bulk Nanostructured Materials I
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Mechanical Behavior of Materials Committee, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Xinghang Zhang, Purdue University; Yuntian Zhu, North Carolina State University; Joseph Poon, University of Virginia; Suryanarayana Challapalli, University of Central Florida; Enrique Lavernia, University of California, Irvine; Haiyan Wang, Texas A&M University

Monday 8:30 AM
February 27, 2017
Room: 30D
Location: San Diego Convention Ctr

Funding support provided by: AJA International; Hysitron Inc.

Session Chair: Xinghang Zhang, Purdue University; Ron Scattergood, North Carolina State University; Kris Darling, Army Research Laboratory

8:30 AM Introductory Comments

8:40 AM  Invited
High Temperature Mechanical Properties of Ultrafine-grained and Nanocrystalline Materials : Megumi Kawasaki1; Roberto Figueiredo2; Terence Langdon3; 1Hanyang University; 2Universidade Federal de Minas Gerais; 3University of Southern California
    The processing of metals with exceptionally small grain sizes, within the submicrometer or nanometer regions, has led to the development of significant interest in the mechanical properties of these materials. When the grain size is very small there is a potential for achieving excellent superplastic properties, even in materials such as magnesium alloys where the h.c.p. crystal structure limits the number of available slip systems. This paper examines these recent developments with an emphasis on the occurrence of superplasticity at elevated temperatures.

9:05 AM  Invited
15 Years SPD-Processed Bulk Nanostructured Materials: From Mechanical to Functional Highlights: Michael Zehetbauer1; 1University of Vienna
     15 years have passed since the pioneering works of R.Z.Valiev et al. [1] introducing the top–down approach of nanostructures in bulk materials. While the first years brought highlights in mechanical properties [2], recent researches prove outstanding functional properties, like those of shape memory [3], magnetic [3], and biomedical nanomaterials [4]. Last results in hydrogen storage and thermoelectric nanomaterials prove that for functional properties low-dimensional SPD-induced lattice defects like vacancy clusters and dislocations can be more beneficial than high-dimensional ones like grain- or phase boundaries [4]. [1] R.Z. Valiev, I. Alexandrov, R. Islamgaliev, Progr.Mater.Sci. 45, 103-189 (2000) [2] M. Zehetbauer, Y.T. Zhu, Bulk nanostructured materials, Wiley (2009) [3] M. Zehetbauer, R. Groessinger, H. Krenn, M. Krystian, R. Pippan, P. Rogl, T. Waitz, R. Wuerschum, Adv.Eng.Mater. 12, 692-700 (2010) [4] R. Z. Valiev, Y. Estrin, Z. Horita, T. G. Langdon, M. J. Zehetbauer, Y. T. Zhu, Mater.Res.Lett. 4, 1-21 (2016)

9:30 AM  Cancelled
Bulk Nanocrystalline Materials: Mechanical Behavior and Deformation Mechanisms : Farghalli Mohamed1; 1University of California, Irvine
    Nanocrystalline (nc) materials are characterized by a unique substructural feature: grain sizes are less than 100 nm. With grain sizes less than 100 nm, the intragrain Frank-Read dislocation sources become inoperable since the grain size is too small to accommodate the size of the source. As a result, conventional dislocation mechanisms that produce plastic deformation in coarse-grained materials cease to be operational in nc-materials. However, recent research activities have demonstrated that a proposed deformation mechanism involving dislocation activity can account for the mechanical behavior of nc-materials. Details of this deformation mechanism will be discussed and its correlations with experimental results will be examined.

9:55 AM  Invited
Hardening by Annealing and Abnormal Hall-Petch Relationship in Nanocrystalline Elements and Alloys: T. D. Shen1; B. R. Sun1; S. W. Xin1; 1Yanshan University
    We have found a hardening effect in annealed iron- and nickel-based nanocrystalline (NC) alloys and a softening effect in annealed pure NC iron. As a result, abnormal Hall-Petch relations were observed in annealed NC alloys. These abnormal phenomena could be explained by the segregation of solutes and/or impurities in the grain boundary because of three evidence: i) hardening by annealing has been observed in many annealed NC alloys and low-purity (< 99%) NC metals, but not in annealed high-purity (> 99.9%) NC metals; ii) annealing embrittlement; and iii) cooling rate dependent microhardness. In addition, our experiment results suggest that i) the dislocations stored in the grain interiors of NC alloys do not change the microhardness, and ii) it is the grain size and the grain boundary structure, rather than the dislocations stored in the grain interiors, that determine the strength of deformed and annealed NC elements and alloys.

10:20 AM Break

10:40 AM  Invited
Twinning in Small-scaled BCC Crystals: Jiangwei Wang1; Zhi Zeng2; Christopher Weinberger3; Ze Zhang4; Ting Zhu2; Scott Mao1; 1University of Pittsburgh; 2Georgia Institute of Technology; 3Sandia National Laboratories; 4Zhejiang University
    This talk will be based on recent publication on In situ atomic-scale observation of twinning dominated deformation in nanoscale body-centered cubic tungsten, Nature Materials by Jiangwei Wang, Ting Zhu, Scott X. Mao et al. Twinning is a fundamental deformation mode that competes against dislocation slip in crystalline solids. Deformation twinning has been well documented in FCC nanoscale crystals. Here, by using in situ high-resolution transmission electron microscopy, we report that twinning is the dominant deformation mechanism in nanoscale bi-crystals of BCC tungsten. Such deformation twinning is found to be pseudoelastic, manifested through reversible detwinning during unloading. We find that the competition between twinning and dislocation slip can be mediated by loading orientation, which is attributed to the competing nucleation mechanism of defects in nanoscale BCC bi-crystals. Our work provides direct observations of deformation twinning under cyclic loading as well as new insights into the deformation mechanism in BCC nanostructures.

11:05 AM  
Mechanical Properties of Nanotwinned Al: Xinghang Zhang1; Sichuang Xue1; Qiang Li2; Dan Bufford3; Yue Liu4; Haiyan Wang2; 1Texas A&M University; 2Purdue University; 3Sandia National Laboratories; 4Los Alamos National Laboratory
    Nanotwinned metals with low-to-intermediate stacking fault energy have been extensively studied as they typically exhibit high strength and ductility. Here we will summarize several cases that focus on highly twinned Al with abundant incoherent twin boundaries. The mechanisms that can introduce growth twins are briefly discussed. The deformation mechanisms of nanotwinned Al are investigated by ex situ and in situ nanoindentation (in transmission electron microscope) technique. These studies suggest that Al with high density growth twins has high strength and significant work hardening capability. This research is funded by DOE-OBES.

11:25 AM  
The Effects of Solutes on the Tensile Strength of Nano-twinned Ag Thin Films at Various Temperatures: Jie Geng1; M. F. Besser1; F. Q. Meng1; R. T. Ott1; 1Ames Laboratory
    Although metals with nano-twinned structure, i.e., densely spaced growth twins, exhibit excellent strength at room temperature, the materials exhibit significant softening at elevated temperatures. Alloying is a potentially promising way to stabilize nanotwins by modifying the microstructures, such as grain size, twin spacing and twin boundaries. In this study, thin films of pure Ag, Ag-Cu and Ag-Al with nano-scale growth twins have been synthesized by magnetron sputtering onto a (100) Si substrate. The free-standing films were tensile tested at different temperatures from 25 to 400°C. The results revealed that the addition of 0.3 at.% Cu stabilizes the tensile strength at 200°C compared to the pure Ag and Ag-Al thin films which softened significantly at 200°C. In-situ synchrotron X-ray diffractions have also been conducted during tensile loading at 25-250°C to investigate the dislocation activities in the nano-twinned thin films.

11:45 AM  
Correlation between Nanotwin Density and Texture Transformation in Thin Ag Films: Nathaniel Rogers1; Shelby Johnson1; Elizabeth Ellis1; Kyle Flemington2; Paul Lashomb2; Jonathon Yuly2; Brandon Hoffman2; Shefford Baker1; 1Cornell University; 2Houghton College
    FCC thin metal films often find use in optical coatings and microelectronics applications. However, a texture transformation from small (111) oriented grains to large (100) grains can drastically change the mechanical properties of these films. This transformation has been attributed to a competition between strain and interface energies, but, we have shown that nanotwins can both provide the necessary driving force and account for the selection of the (100) orientation during texture transformation in Ag films. In this work, we show that (a) nanotwins can be created in evaporated Ag films (previously only seen in sputtered films), (b) films produced at increasing deposition rates show a corresponding increase in nanotwin density, and (c) films with greater nanotwin densities transformed further and faster than films with fewer nanotwins. These results strongly suggest the (111) to (100) texture transformation in thin films is due to the presence of nanotwins.

12:05 PM  
On the Relationship between the Grain Boundary Character and the Microhardness in Nanocrystalline Ni-W: Mathieu Lagarde1; Niusha Shakibi Nia1; Julie Bourgon2; Egle Conforto1; Patrick Girault1; Stéphane Cohendoz1; Juan Creus1; Xavier Feaugas1; Catherine Savall1; 1Université de La Rochelle; 2ICMPE
    The microstructural features of nanocrystalline Ni-W 15 at. % alloys with a low content of light elements and different annealing heat treatments has been characterized by several in situ (TEM with heat-holder) and ex-situ (XRD, TEM-ASTAR and SEM-EBSD) techniques and correlated with the micro hardness behaviour. The thermal stability of Ni-W alloys was confirmed for the low annealing temperatures with a very low grain growth and Incoherent Twin Boundaries (ITB) development. This behaviour leads to a significant hardening (inverse Hall Petch relation) arising from twin formation, which hinders the structure deformation because of the increase of the interfaces. Nevertheless, once the thermal stability is overtaken (above 600 – 650 °C), the important grain growth occurs with a decrease in GBs fraction. Classical Hall Petch behaviour is then observed with a more important influence of grain size on the dislocation motion than the twin boundaries effect.