Pan American Materials Congress: Nanocrystalline and Ultra-fine Grain Materials and Bulk Metallic Glasses: SPD Processing, Mechanical Properties of Nanocrystalline Materials, BMG
Sponsored by: Third Pan American Materials Congress Organizing Committee
Program Organizers: Terence Langdon, University of Southern California; Megumi Kawasaki, Oregon State University; Roberto Figueiredo, Federal University of Minas Gerais; Jose-Maria Cabrera, Universidad Politecnica de Catalunya

Tuesday 10:20 AM
February 28, 2017
Room: Marina F
Location: Marriott Marquis Hotel

Session Chair: Terence Langdon, University of Southern California; Hans Roven, Norwegian University of Science and Technology

10:20 AM  
A Novel Method for SPD – Continuous Metal Screw Extrusion (CMSE): Hans Roven1; Kristian Skorpen1; Oddvin Reiso2; 1Norwegian University of Science and Technology; 2Hydro Aluminium
    Severe plastic deformation (SPD) methods are widely used for producing ultrafine-grained (UFG) and nanocrystalline (NC) metals and alloys. The absolute majority of the existing SPD processes and techniques are discontinuous and the work-piece is normally quite limited in size. It is therefore of great interest to develop processes that can produce UFG/NC semi-finished products at large quantities and with physical dimensions large enough for industrial applications. Our research group has worked for more than 15 years to develop a continuous process based on the screw extrusion concept. The characteristics and principal mechanisms embedded in this relative large scale technique will be presented. Preliminary analytical calculations show that the material accumulates a strain ε > 15 when passing through this process. A few examples of achievements obtained with light alloys will be presented together with a brief summary of current challenges and potentials.

10:40 AM  
Rate Sensitivity and Deformation Mechanisms of Ultrafine-Grained Single Phase and Composite Metals: Daniel Kiener1; Alexander Leitner1; Verena Maier-Kiener1; 1University of Leoben
    Nanostructured materials and composites are of high interest, as they exhibit outstanding structural properties such as high hardness and good deformability. However, knowledge about their high temperature mechanical properties and the rate-dependent deformation mechanisms governing plasticity at elevated temperatures. Here, we produced nanocrystalline bcc metals (e.g. Cr, Fe), and nanocomposites (e.g. Cu-Fe, Cu-Nb) by severe plastic deformation using high pressure torsion. Subsequently, high temperature nanoindentation strain-rate jump and nanoindentation creep experiments were performed to analyze the strength, strain-rate sensitivity, and activation volume of these nanostructured materials. We discuss the influence of the different interfaces on the related deformation mechanisms of the nanostructured bcc metals and fcc/bcc composites. In fact, the rate dependent deformation behavior of nanostructured bcc materials and composites at elevated temperatures is rationalized by the fact that above the critical temperature they behave fcc-like. Notably, new rate-dependent grain boundary interactions have to be considered.

11:00 AM  
Comparisons of Mechanical Property Development during HPT Processing and Subsequent Room Temperature Storage in High Purity Cu and a Pb-62%Sn Alloy: Yi Huang1; Shima Sabbaghianrad2; Abdulla Almazrouee3; Khaled Al-Fadhalah4; Saleh Alhajeri3; Nian Xian Zhang1; Terence Langdon1; 1University of Southampton; 2University of Southern California; 3P.A.A.E.T.; 4Kuwait University
    High purity Cu and a Pb-62% Sn alloy were both processed by high-pressure torsion (HPT) at room temperature. High purity Cu shows strain hardening behavior with refined grain structures during HPT processing. By contrast, the Pb-Sn alloy displays a strain weakening behavior where the hardness values after HPT processing are significantly lower than in the initial condition even though the grain size is reduced in HPT processing. After HPT processing, high purity Cu with lower numbers of rotations softens with the time of storage due to local recrystallization and abnormal grain growth whereas the Pb-Sn alloy hardens with the time of storage accompanied by grain growth. The low melting point of the Pb-Sn alloy plays a role in hardness increase with storage time even with a coarsened grain structure during room temperature storage.

11:20 AM  
Micro-scale Mechanical Response of Ultrafine-grained Materials Processed by High-pressure Ttorsion: Megumi Kawasaki1; Jae-il Jang1; Byungmin Ahn2; Terence Langdon3; 1Hanyang University; 2Ajou University; 3University of Southern California
    The ultrafine grains in bulk metals usually show superior mechanical and physical properties. The development of micro-mechanical response is observed after significant changes in microstructure through high-pressure torsion (HPT) processing. Accordingly, this presentation demonstrates the evolution of small-scale deformation behavior examined by the nanoindentation technique on various ultrafine-grained (UFG) materials including a ZK60 magnesium alloy, a Zn-22% Al eutectoid alloy and a metal matrix nanocomposite processed by HPT. Special emphasis is placed on demonstrating the essential microstructural changes of these materials with increased straining by HPT and the evolution of the micro-mechanical behavior in these UFG materials by estimating the strain rate sensitivity.

11:40 AM  
History-independent Fatigue Response of Polycrystalline Cu with Highly Oriented Nanosclae Twins: Qingsong Pan1; Haofei Zhou2; Qiuhong Lu1; Huajian Gao2; Lei Lu1; 1Institute of Metal Research, CAS; 2Brown University
    Nearly 90% of service failures of metallic components and structures are caused by fatigue at a cyclic stress amplitude much lower than the tensile strength of the materials involved. A long-standing obstacle to developing better materials has been that metals typically suffer from large, accumulative, irreversible damages in microstructure during fatigue, leading to history-dependent and unstable cyclic responses. Here, through both experiments and atomistic simulations, we report a history-independent and stable fatigue response in a bulk polycrystalline Cu sample containing highly oriented nanoscale twins under sequences of stepwise increasing/decreasing plastic strain amplitudes. The results demonstrate that this unusual behavior is governed by a type of highly correlated necklace dislocations formed in the nanotwinned metal under cyclic loading. This unique fatigue mechanism is fundamentally distinct from traditional strain-localizing fatigue mechanisms associated with irreversible microstructural damage.

12:00 PM  
On the Strength Effects in Hydrogenated Palladium Subjected to HPT Processing: Daria Setman1; Wolfgang Ress1; Andreas Grill1; Erhard Schafler1; Wolfgang Sprengel2; Yuzeng Chen3; Michael Zehetbauer1; 1University Vienna; 2TU Graz; 3Northwestern Polytechnical University, State Key Lab of Solidification Processing, Republic of China
    Recently it was shown that in hydrogenated HPT-processed Pd the densities of dislocations and vacancies were significantly higher than in non-hydrogenated ones [1]. These enhanced densities cause an enormous hardening through its stabilization by the hydrogen absorbed [1, 2]. Chen at al. [2] claimed that all the hardening arises from the enhanced dislocation density. Isochronal micro hardness measurements were undertaken to separate the strength effects in terms of vacancy type defects, dislocations and/or grain boundaries. Vacancy sensitive methods such as DSC and PAS were applied in parallel to dislocations sensitive ones (XPA). Surprisingly the major part of hardening was found to arise from the vacancy type defects, not from the dislocations nor from the grain boundaries. Work supported by the FWF (Austrian Science Fund) T512-N20. [1] D. Setman, et al. Thermec Conference, June 2016, Brno (Czech Rep.); [2] Y.Z. Chen, et al. Scripta Mater. 68 (2013) 743-746