Pan American Materials Congress: Nanocrystalline and Ultra-fine Grain Materials and Bulk Metallic Glasses: Additional Topics in SPD Processing and Mechanical Properties
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

Thursday 8:30 AM
March 2, 2017
Room: Marina F
Location: Marriott Marquis Hotel

Session Chair: María Teresa Pérez Prado, IMDEA Materials Institute; Andrea Bachmaier, Erich Schmid Institute of Materials Science

8:30 AM  
Effect of Annealing of ZK60 Magnesium Alloy after Processing by High-pressure Torsion: Seyed Alireza Torbati Sarraf1; Shima Sabbaghianrad1; Terence G. Langdon1; 1University of Southern California
    A ZK60 magnesium alloy with an initial grain size of ~10 μm was processed by high-pressure torsion (HPT) through 5 revolutions under a constant compressive pressure of 2.0 GPa and rotating speed of 1 rpm. An average grain size of ~700 nm was achieved after HPT with a high fraction of high-angle grain boundaries. Tensile experiments at room temperature showed poor ductility. However, combination of reasonable ductility and good strength may be achieved with post-HPT annealing by subjecting samples to high temperature in the range of 473 to 548 K for 10 and 20 min. The grain size and texture changes were also examined by electron back scattered diffraction (EBSD) and results compared to long-term annealing for 2500 min at 450 K. The investigation in this study suggests that a post-HPT annealing for short period of time may be effective in achieving a reasonable combination of strength and ductility.

8:50 AM  
Severe Plastic Deformation as a Tool to Tune Magnetic Properties: Carmen M. Cepeda-Jimenez1; Juan Ignacio Beltrán1; Antonio Hernando2; Miguel Angel García3; Félix Ynduráin4; Alexander Zhilyaev5; María Teresa Pérez Prado1; 1IMDEA Materials Institute; 2IInstituto de Magnetismo “Salvador Velayos”, UCM, ADIF, CSIC; 3Instituto de Cerámica y Vidrio, CSIC; 4Universidad Autónoma de Madrid; 5Fundació CTM Centre Tecnològic
    The search for magnets with high Curie temperatures is of great practical interest. However, room temperature ferromagnetism is a rare phenomenon. Only four pure elements, namely Fe (Tc=1043 K), Co (Tc=388 K), Ni (Tc=627 K), and Gd (Tc=289 K), exhibit this behavior. Since 1935, when room temperature ferromagnetism was discovered in Gd, no other pure elemental room temperature magnets have been identified. This work illustrates how severe plastic deformation may trigger a spontaneous magnetization at ambient temperature in pure paramagnetic metals such as zirconium, titanium, hafnium and magnesium. Subsequent strain relaxation by annealing leads to a decrease of the permanent magnetization. Density functional theory simulations and transmission electron microscopy are utilized to analyze the origin of the unprecedented magnetic behavior of metals processed by severe plastic deformation.

9:10 AM  
Investigation of Crystallographic Texture and Stored Energy after Cross Accumulative Roll-bonding of Fe-36Ni (Invar) Alloy: Hiba Azzeddine1; Kamel Tirsatine2; Thierry Baudin3; Marie-Hélène Mathon4; Anne-Laure Helbert3; François Brisset3; Djamel Bradai2; 1University of M'sila; 2USTHB; 3Université Paris-Saclay; 4Laboratoire Léon Brillouin
    In the present study, the crystallographic texture and stored energy evolution of a Fe-36%Ni alloy processed by cross accumulative roll-bonding (CARB) up to 6 cycles was investigated using neutron diffraction. The crystallographic texture evolution is cyclic due to the cyclic nature of the imposed deformation. Copper-type texture was observed after even cycles, whereas a new major texture component named H ({012}<221>) was formed after odd cycles.The dislocation density and the stored energy have been estimated based on the diffraction peak broadening measurement for Brass {110}<112>, S {231}<346>, Copper {112}<111> and Cube {001}<100> texture components. This analysis shows that the S and Copper components develop significantly higher stored energies than the Brass and Cube components.

9:30 AM  
Microstructure and Mechanical Behavior of UFG Mg-2Zn-2Gd: Sunkulp Goel1; Y Wang1; A Srinivasan2; R Jayaganthan3; Jing Tao Wang1; 1Nanjing University of Science and Technology; 2CSIR – National Institute for Interdisciplinary Science and Technology (NIIST); 3Indian Institute of Technology, Madras
    The microstructure and mechanical behavior of ultrafine grained Magnesium alloy processed by multiaxial forging followed by rolling has been investigated in the present work. The As cast Mg-2Zn-2Gd alloy is deformed by for multiaxial forging at 450 oC. 50 percent reduction in each pass is applied for 2 cycles (4.2 true strain) followed by 70% rolling reduction at a temperature of 450 oC. Characterization with the help of SEM/EBSD and TEM is performed to substantiate the mechanisms of grain refinement in the alloys. Multiaxial forged magnesium alloy shows better tensile strength and ductility as compared to as cast alloy. The ductility and strength after multiaxial forging has been doubled. The EBSD and Tem analysis showed the formation of recrystallised grains due to dynamic recrystallisation during forging followed by rolling.

9:50 AM Break

10:10 AM  
Microstructural Evolution of TWIP Steels during ECAP: Jessica Calvo1; Wang Lei1; José Antonio Benito1; José María Cabera1; 1Universitat Politècnica de Catalunya (UPC)
    TWIP (TWinning Induced Plasticity) steels have received a lot of attention from the car making industry due to their exceptional energy absorption capability. These steels, which deform by a combination of dislocation slip and mechanical twinning, exhibit ultimate tensile strengths above 1000MPa with ductility values higher than 50% and strain hardening coefficients around 0.8, but their yield strength is moderate. In order to increase the yield strength several alternatives can be explored, being severe plastic deformation processes a promising route, due to the grain refinement associated to these techniques. In particular, Equal Channel Angular Processing (ECAP) was applied to two different TWIP steels. The process was performed at 300°C and the samples were subjected to up to four deformation passes. The evolution of the microstructure after one, two and four ECAP passes was evaluated by means of Electron Back Scattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM).

10:30 AM  
Current-assisted-extrusion of Structural Amorphous Metals: Insight into Microstructure Formation and Mechanical Properties: Ekaterina Novitskaya1; Sebastian Diaz de la Torre2; Tzipatly Esquivel-Castro2; Guillermo Dieguez-Trejo2; Olivia Graeve1; 1University of California, San Diego; 2Instituto Politecnico Nacional
    Bulk metallic glass (BMG) and BMG-matrix composites are currently investigated for structural applications because of the potential to produce a material that is both strong and tough. However, most BMGs are inherently brittle at room temperature, especially in tension, because of shear localization. In situ crystalline composites (formed by devitrification) or ex situ crystalline composites (formed by addition of a second phase) can improve ductility. In this study, we report on a current-assisted-extrusion process for the formation of long rods of the structural amorphous metal Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4 with a 2.5 wt% addition of yttria nanoparticles as a reinforcement phase. Detailed analysis of phase formation, devitrification, and microstructural evolution, verifies that these SAM2X5 materials can be successfully extruded up to several cm in length at a 1000°C extrusion temperature. Subsequent analysis of mechanical properties via microhardness testing demonstrates that the properties of the extruded SAM2X5 materials significantly surpasses those of common steels.

10:50 AM  
Effect of Annealing on Microstructure and Magnetic Properties of Nanocrystalline Metastable Cu-Co Solid Solutions: Andrea Bachmaier1; Stefan Hartl1; Jörg Schmauch2; Hisham Aboulfadl3; Andreas Verch4; Heinz Krenn5; Reinhard Pippan1; 1Erich Schmid Institute, Austrian Academy of Sciences; 2Experimentalphysik, Saarland University; 3Chair of Functional Materials, Saarland University; 4INM-Leibniz Institute for New Materials; 5Institute of Physics, Karl-Franzens University Graz
    Nanocrystalline metastable Cu74Co26 solid solutions, which are processed by high-pressure torsion deformation of immiscible dual-phase Cu-Co composites, decompose during annealing at 400°C. A detailed study of the as-processed and annealed nanostructures by transmission electron microscopy and atom probe tomography reveals a compositionally modulated structure inside the grains. Near grain boundaries, alternating nanometer-sized pure fcc Cu and Co phases with a larger length scale are found. Both, the size as well as the composition of the Cu and Co regions can be influenced by the annealing time. The magnetic properties of the as-deformed and annealed structures are investigated by SQUID-magnetography and associated with the evolving nanostructures. Additionally, the hysteresis loop is measured on ring shaped samples. Another promising way to modify the microstructure and tune the magnetic properties is magnetic annealing. Hence, the microstructural evolution during annealing has been further studied in the presence of a strong magnetic field.

11:10 AM  
Mechanical Behavior and Adiabatic Shear Localization of Ultrafine-grained Titanium: Zezhou Li1; Bingfeng Wang2; Shiteng Zhao1; Ruslan Z. Valiev3; Kenneth S. Vecchio1; Marc A. Meyers1; 1University of California,San Diego; 2Central South University; 3Institute of Physics of Advanced Materials
    The mechanical behavior of ultrafine-grained titanium (120 nm and 500 nm) was examined in the low and high-strain-rate regimes. The ultrafine grained titanium shows low strain hardening rate and the strain rate sensitivity of ultrafine-gained titanium was found to be the same as the coarse grained commercial pure titanium. Dynamic deformation and failure mechanisms of ultrafine-gained titanium (~120 nm) were studied by using a specimen geometry in which forced shear is imposed to a narrow region. The smaller grain size and lower temperature can lead to the formation of narrower shear band under same strain and strain rate condition. The microstructure inside the adiabatic shear band consists of a mixture of elongated grains and equiaxed nanograins that are smaller than initial grains due to the dynamic recrystallization. The adiabatic shear band were then subjected to combined brittle and ductile failure.

11:30 AM  
Dynamic Tensile Failure of Nanocrystalline Tantalum: Eric Hahn1; 1University of California, San Diego
    Shock simulations allow us to investigate strength and damage under extreme conditions. It is known that material strength scales with applied strain-rate; a projection of this effect places the ultimate strength of tantalum above a strain rate of a billion per second. We show that tensile strength saturates and can be well estimated by a cohesive energy criterion. Additionally, as grain size decreases, tensile strength decreases due to an increased propensity to fail at a growing volume fraction of grain boundaries, which typically exhibit lower flow stresses. Voids nucleate at defect structures present in the microstructure, the exact make-up and distribution of damage is a complex combination of the initial microstructure, stress state, and the preceding plastic deformation during both compression and expansion. Void growth occurs by a combination of local disordering and nucleation of dislocation shear loops.