High Entropy Alloys V: Structures and Characterization
Sponsored by: TMS Structural Materials Division, TMS Functional Materials Division, TMS: Alloy Phases Committee, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Peter Liaw, University of Tennessee; Michael Gao, National Energy Technology Laboratory; Suveen Nigel Mathaudhu, University of California Riverside; Xie Xie, The University of Tennessee, Knoxville; Gongyao Wang, Alcoa Technical Center; E-Wen Huang, National Chiao Tung University

Thursday 8:30 AM
March 2, 2017
Room: 32A
Location: San Diego Convention Ctr

Session Chair: Mitra Taheri, Drexel University; E-Wen Huang, National Chiao Tung University

8:30 AM  Invited
In Situ TEM Investigation of the Thermal, Mechanical, and Corrosion Stability of High Entropy Alloys: Mitra Taheri1; Elaf Anber1; Daniel Scotto-D'Antuono1; Wayne Harlow1; Haoyan Diao2; Peter Liaw2; 1Drexel University; 2University of Tennessee
    Recently, a new class of materials, termed high entropy alloys (HEAs), has shown the ability to move beyond conventional structural materials in terms of mechanical behavior and in some cases, corrosion resistance. The stability windows under deformation, at high temperatures, and in extreme environments such oxidation remain unknown for the majority of these alloys, however. Here we describe in situ TEM-based investigations of the stability of an Al0.1CoCrFeNi, Al0.3CoCrFeNi, and Al0.5CoCrFeNi HEA under high temperature and oxidative environments. Most notably, precipitation events were monitored and characterized using precession electron diffraction and electron energy loss spectroscopy throughout various annealing cycles. Additionally, alloys treated to various temperatures were studied under tension. The work provides a step forward for developing a parameter space for operation of HEAs in extreme environments and related applications.

8:50 AM  Invited
Uncovering the Dislocation Dynamics Leading to Planar Slips in High-entropy Alloy Nanopillars: Yang Hu1; Li Shu; Peter Liaw2; Karin Dahmen3; Jian Min Zuo4; 1University of Illinois at Urbana-Champaign; 2University of Tennessee; 3 University of Illinois at Urbana Champaign; 4University of Illinois
    High-entropy alloys (HEAs), composed of five or more elements of near-equal molar percentage in random solution, have excellent thermomechanical properties. However, HEAs deform via sudden slips under certain temperature and strain-rate regimes, which are seen as “serrations” in the stress-strain curve. The cause of serrations is unknown; ex-situ experiments suggest large crystal slips. Here, with help of in-situ transmission electron microscopy (TEM), we demonstrate the spatiotemporal dislocation dynamics leading to planar slips in the Al0.1CoCrFeNi nanopillars. In situ compression tests of the HEA nanopillars were performed using a Hysitron picoindenter in a JEOL 2010 LaB6 TEM operated at 200 keV . The compression tests were performed in displacement controlled mode. Using a transducer sensor, the load and displacement were measured while the nanopillar deformation was monitored by diffraction contrast electron imaging. A video of each test was recorded using a charge­ coupled device (CCD) camera running at 10 frames per second. The experimental studies here have revealed physical details of slowly­ deformed HEAs and provided insights on the creations and multiplication of dislocations, dislocation pinning, dislocation density, dislocation resulted stress drops and slip avalanches in HEAs during deformation.

9:10 AM  Invited
Nanoscale Phase Separation in Al0.5CoCrFeNiCu High Entropy Alloys, as Studied by Atom Probe Tomography: Keith Knipling1; Joshue Tharpe2; Peter Liaw2; 1U.S. Naval Research Laboratory; 2University of Tennessee
    High-entropy alloys (HEAs) typically contain five or more principal elements in nearly equiatomic proportions. The most studied HEAs are the AlCoCrCuFeNi alloys, which solidify into dendritic and interdentritic regions with an attendant microsegration of solute species. Within these microscale heterogeneities the material microstructures further segregate into (i) nanoscale modulated structures composed of alternating Al/Ni-rich and Fe/Cr-rich phases formed by spinodal decompostion, and (ii) nanoscale Cu precipitate formation. The microstructures formed during casting and after annealing of an Al0.5CoCrFeNi HEA, with and without Cu additions, are studied by atom probe tomography. These microstructures are correlated to the observed strength, as measured by Vickers microhardness and uniaxial tensile tests.

9:30 AM  
Plastic Deformation Mechanisms in A3S and Cantor’s HEA Alloys Investigated by In Situ TEM Straining Experiments: Marc Legros1; Michal Mroz2; Anna Fraczkiewicz2; 1CEMES-CNRS; 2Ecole des Mines de St-Etienne
    In a non-equiatomic HEA from CoCrFeMnNi family (A3S® grade), mechanical resistance (YS) is strongly increased as compared with the equiatomic (Cantor’s) alloy of the same family. This behavior comes from a stable nanostructure, easily formed in the material after classical hot thermomechanical treatment (forging). In situ TEM straining experiments were carried out at room temperature in as-cast and annealed samples. Dislocations movements are analyzed with respect to the applied stress that is measured locally using dislocation curvature. Potential intrinsic (core structure) or extrinsic (grain boundary, forest mechanisms) strengthening mechanisms are discussed, as well as the collective motion of partial or perfect dislocations.

9:50 AM  Invited
Small Angle Neutron Scattering Study of HEA Microstructure Evolution with Temperature and Applied Magnetic Field: Louis Santodonato1; Lisa DeBeer-Schmitt1; Kenneth Littrell1; Peter Liaw2; 1Oak Ridge National Laboratory; 2The University of Tennessee
    High-entropy alloys (HEAs) have shown exceptional engineering properties, such as high strengths, which may be greatly influenced by the microstructure. The present work uses small angle neutron scattering (SANS) to investigate how the microstructures evolve with temperature and magnetic field in the AlxCoCrCuyFeNi family of alloys, which have varying amounts of Al (x) and Cu (y). Some of these alloys undergo two distinct changes between room temperature to 800 deg. C, possibly due to magnetic and structural transitions. An overview of the structural and magnetic features determined from the SANS studies will be given.

10:10 AM Break

10:30 AM  Invited
Structural Transition in High Entropy Alloy CoCrFeMnNi under High Pressure: Yi-Hung Chen1; E-Wen Huang1; Chin-Ming Lin2; Chia-En Hsu2; Jien-Wei Yeh3; Ke An4; 1National Chiao Tung University; 2National Hsinchu University of Education; 3National Tsing Hua University; 4Oak Ridge National Laboratory
    The equiatomic CoCrFeMnNi high entropy alloy is in a single Face-Centered Cubic phase. High entropy alloys have different mechanical properties other than traditional alloys due to severe lattice distortion. It leads to extremely low stacking fault energy of this HEA at about 77K. Low stacking fault will cause nano twining or hexagonal close packing. Increasing pressure also make atoms distance shorten. We apply Advanced Photon Source to get diffraction pattern of different pressure. Results shows that this HEA begins phase transformation at about 7.1 Gpa. Some face centered cubic phases transfer to hexagonal close packing phases. The mechanism of phase transformation is related to appearance of stacking fault.

10:50 AM  Invited
Complex Structural Factors Governing Unique Properties of FCC High Entropy Alloys Studied by Theory and Experiment: Hyun Seok Oh1; Eun Soo Park1; Fritz Körmann2; Gerard Leyson3; Duancheng Ma3; Sang Jun Kim1; Blazej Grabowski3; Cemal Cem Tasan4; Dierk Raabe3; 1Seoul National University; 2Delft University of Technology; 3Max-Planck Institut für Eisenforschung GmbH; 4Massachusetts Institute of Technology
    It has been mentioned in many studies that severe lattice distortion plays an important role in the broad class of multi-principal element solid solution alloy known as high entropy alloys (HEAs). Hence, it is essential to find key factors controlling the lattice distortion and resultant properties in order to create effective design strategies for HEAs. In this research, we investigated atomic level bonding characteristics of single phase FCC alloys from single elements to HEAs with combined experimental (XAFS, X-ray Absorption Fine Structure) and theoretical (DFT, Density Function Theory) study. By comparing the bonding characteristics with mechanical properties of single FCC alloy series, we showed that one should focus more on “complex behavior” of high entropy alloy from various elements instead of simple severe lattice distortion. We propose a relationship between structural factors and distinct mechanical properties of HEA, which can lead to a new design strategy for new generation HEAs.

11:10 AM  Invited
Composition, Temperature, and Crystal Size Effects on the Mechanical Response of AlCoCrFeNi High Entropy Alloy: Gi-Dong Sim1; Quan Jiao1; Peter K. Liaw2; Rajiv Mishra3; Jaafar El-Awady1; 1Johns Hopkins University; 2University of Tennessee; 3University of North Texes
    High Entropy Alloys (HEAs), which are solid solution alloys containing five or more principal elements in equal or near equal atomic percent, are typically reported to possess high hardness and compressive strength at room and elevated temperatures. In this talk we will present experimental results to quantify the effects of composition, temperature, and crystal on the mechanical response of AlCoCrFeNi HEAs. Microcrystals are FIB milled into single crystal and polycrystal HEAs with different Al content tested in situ SEM at various temperatures. The high temperature response during compression is achieved through an in situ SEM nanoindenter with independent heating of both the tip and the specimen. The compositional effects on the activation of different deformation mechanisms at room temperature is first assessed. Furthermore, the size-dependent mechanical response in the temperature range of 25-600 C will be discussed in view of thermally activated dislocation plasticity.

11:30 AM  
An In Situ TEM Observation on Thermal Stability of High Entropy Alloys: Elaf Anber1; Dan Scotto D'Antuono1; Pranav Suri1; Andrew Lang1; Haoyan Diao2; Peter Liaw2; Mitra Taheri1; 1Drexel University; 2The University of Tennessee Knoxville,
     High entropy alloys (HEAs) containing five or more principal elements are attractive because of their unusual structural properties. Here we describe these recent progress studies of the stability of an Al0.1CoCrFeNi, Al0.3CoCrFeNi, and Al0.5CoCrFeNi HEA under high temperature environments, in situ in a TEM. In this study, microstructure characterization at high annealing temperatures was observed. Specifically, precipitation was characterized throughout various annealing cycles. These studies were coupled with phase and orientation analysis using precession electron diffraction techniques. Overall, the work described provides a foundation for understanding the stability window for candidate HEAs in extreme environments. These results are discussed in the context of the growing literature comparing the ideal methods for stabilizing mechanisms in HEAs for use in high temperature environments.

11:50 AM  
Corrosion-resistant Nobility of AlxCoCrFeNi High-enthropy Alloys: Yunzhu Shi1; Liam Collins2; Rui Feng3; Bin Yang1; Peter Liaw3; 1University of Science and Technology Beijing; 2Oak Ridge National Laboratory; 3The University of Tennessee
    The multiple compositionally-equivalent high-entropy alloys (HEAs) have shown remarkable potential in structural and energy-related applications. The intrinsic chemical disorder of HEAs leads to a unique corrosion resistant property, which plays one of the key roles in the practical usages. Here we select AlxCoCrFeNi HEAs system to present a new basis for understanding the fundamental relationship between microstructures and corrosion properties through characterizing surface-electron activities, revealing that the complex phase types and distinctive chemical segregation increase the variation of electron work functions, which, therefore, weakens the corrosion-resistance ability of alloys. Moreover, the annealing process at 1,250℃ has a profound effect on enhancing the chemical disorder, decreasing the electron-activity variations among phases, and evidently improving the localized corrosion resistance. The present research provides an integrated experimental and theoretical approach that could be applied to meet the challenges of alloy design for novel corrosion-resistant HEAs.