High Entropy Alloys VIII: Structures and Characterization
Sponsored by: TMS Functional Materials Division, TMS Structural 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; E-Wen Huang, National Chiao Tung University; Srivatsan Tirumalai; Xie Xie, FCA US LLC; Gongyao Wang, Globus Medical
Thursday 2:00 PM
February 27, 2020
Room: Marina Ballroom F
Location: Marriott Marquis Hotel
Session Chair: Hyoung Seop Kim, Pohang University of Science and Technology; Ke An, Oak Ridge National Laboratory
2:00 PM Invited
Roles of Martensitic Phase Fractions in Mechanical Behavior of TRIP-HEA: Sichao Fu1; Dunji Yu1; Yan Chen1; Ke An1; 1Oak Ridge National Laboratory
The transformation-induced plasticity high entropy alloy (TRIP-HEA) pushes the envelope of the strength and ductility tradeoff. As revealed by the in-situ neutron diffraction, the FCC-based TRIP-HEA is strengthened by persisting transformation to the hard HCP phase with a potential work-hardening capacity. The initial HCP phase fraction and the successive transformation under deformation can be controlled by the annealing process. With less than 30% HCP fraction variation after thermal treatment, the TRIP-HEA could show over ~150 MPa higher stress level for a higher initial HCP fraction. The underlying complex mechanisms involve the contributions of critical stresses of transformation, dislocation slip and twinning, but the dominant contributor is the strengthening of increasing stress-induced hard HCP phase in the early deformation stage. An empirical direction combining one in-situ neutron measurement with a semi-empirical constitutive model is proposed to predict the change in mechanical response with different HCP phase fractions.
2:20 PM Invited
On the Exceptional Mechanical Properties and Effect of Local Order in CrCoN-based High-entropy Alloys: Robert Ritchie1; Andrew Minor1; Mark Asta1; Jun Ding2; Ruopeng Zhang1; Shiteng Zhou2; Qin Yu2; 1University of California, Berkeley; 2Lawrence Berkeley National Laboratory
CrCoNi-based medium/high-entropy alloys can display exceptional combinations of strength, ductility and toughness, properties which are further enhanced at cryogenic temperatures. In situ TEM observations identify a sequence of multiple deformation mechanisms, associated with their high friction stress yet low stacking-fault energy, that act synergistically to generate such damage-tolerance. We believe that many of these properties are influenced at the atomic level by the presence of local chemical ordering. Here we describe DFT-based Monte-Carlo simulations to show that variations in local chemical order have a profound effect on the stacking-fault and twin-boundary energies, the TRIP effect, and the formation energy of point defects, and ongoing experimental studies using energy-filtered TEM to confirm its existence and provide evidence of its significant effect on elastic/plastic deformation properties. These results present the possibility of “tuning order in disorder” for atomistic tailoring of multiple-principal-element alloys to achieve specifically desired macroscale mechanical performance.
2:40 PM Invited
In Situ Atom Probe Tomography of Oxidation Mechanism in High Entropy Alloys: Bharat Gwalani1; Elizabeth Kautz1; Sten Lambeets1; Libor Kovarik1; Arun Devaraj1; 1Pacific Northwest National Laboratory
High Entropy Alloys (HEAs) have shown promise for use in high temperature applications in which they are exposed to reactive environments containing oxygen gas (O2). Oxidation reactions directly modify the surface composition-structure, affecting material and component performance. However, due to the complexity of HEA composition, atomistic mechanisms responsible for surface modification are inadequately understood. Here, we demonstrate the gas-induced change in structure of near-surface regions using in situ gas reaction-cell in transmission electron microscope. Furthermore, we demonstrate the changes to the composition of surface and sub-surface regions of the same alloy and exposure conditions in an environmental reaction chamber attached with an atom probe tomography system. The approach presented here can aid in obtaining a detailed understanding the sequential oxidation of in HEAs which can be broadly applicable to a wide variety of complex alloy systems containing multiple alloy elements.
3:00 PM Invited
Interpreting APT Data Containing He-bubbles in Irradiated Single-phase Concentrated Solid-solution Alloys (SP-CSAs): Jonathan Poplawsky1; Xing Wang1; Constantinos Hatzouglou2; Ke Jin3; Hongbin Bei3; Yongqiang Wang4; William Weber5; Yanwen Zhang3; Francois Vurpillot2; Karren More1; 1The Center for Nanophase Materials Sciences, Oak Ridge National Laboratory; 2Normandie Université, UNIROUEN, INSA Rouen, CNRS, Groupe de Physique des Matériaux; 3Materials Science and Technology Division, Oak Ridge National Laboratory; 4Materials Science and Technology Division, Los Alamos National Laboratory; 5Department of Materials Science and Engineering, University of Tennessee-Knoxville
He-bubbles form in irradiated alloys leading to swelling and embrittlement. Single-phase Concentrated Solid-solution Alloys (SP-CSAs) have shown good promise for superior radiation resistance. Quantifying He-bubble shell radiation induced segregation (RIS) is important for understanding He-bubble formation mechanisms. Atom probe tomography (APT) excels at quantifying nano-scale segregation but suffers when the APT tip deviates from a hemi-spherical surface during the experiment, such as a void exposure. APT data for several He bubble-containing SP-CSAs has been evaluated and compared to simulated APT experiments and STEM measurements. The tip shape progression has been identified, which gives insight into the best method for quantifying and comparing RIS between samples, and therefore, He-bubble RIS has been evaluated for several SP-CSAs. The results suggest that fast vacancy migration via Cr lattice sites could accelerate the He-bubble growth rate. Research was funded by the EDDE EFRC, CNMS, CINT, and U.S. DOE Office of Science.
3:20 PM Break
3:40 PM Invited
Metastability Engineering of HIgh Entropy Alloys: Hyoung Seop Kim1; Jae Wung Bae1; 1Pohang University of Science and Technology
In this presentation, we utilized metastibility engineering and multiple-stage strain hardening in a ferrous Fe60Co15Ni15Cr10 (at%) MEA and achived a good combination of cryogenic tensile strength and ductility. Astonishingly, detailed microstructural observations at each stage reveal the sequential operation of deformation-induced phase transformation from parent fcc to newly formed bcc (body-centered cubic) phases. The in-situ neutron diffraction studies make it clear that the martensite formation and the concurrent load partitioning between the fcc and the bcc phases play an important role in the increase in strength. Replacing high-cost alloying elements cobalt and nickel with iron, as well as introduction of metastability-engineering at liquid nitrogen temperature, distinguishes the new ferrous MEAs from previously reported equiatomic HEAs. This result underlines insights to provide expanded opportunities for the future development of HEAs for cryogenic applications.
4:00 PM
Nanocrystalline High-entropy Alloys Made by Electrodeposition: Michel Hache1; Uwe Erb1; Yu Zou1; 1University of Toronto
This study explores the combination of the strengthening effects from grain refinement with enhanced thermal stability observed in HEAs, suppressing grain growth at medium to high temperatures. Though studies regarding nanocrystalline HEAs have been reported, they commonly rely of high-energy fabrication methods such as thermal-spraying, ball milling, or severe mechanical deformation. By contrast, the electrodeposition of HEAs has seen limited exploration, though it offers itself as a low temperature coating technology with few size and shape limitations, applicable to almost any substrate. This work explores the synthesis of nanocrystalline ternary alloys from the Ni-Fe-Cr system through means of electrodeposition, on a route towards the synthesis of electrodeposited HEAs. Focus is on the experimental parameters that allow for successful co-deposition of fully dense near-equimolar Ni-Fe-Cr from an aqueous electrolyte. This work subsequently explores the mechanical properties, crystal structure and other interesting properties of the electrodeposited alloys.
4:20 PM Cancelled
Welding Metallurgy and Weld Properties of High Entropy Alloys: Alexander Martin1; Carolin Fink1; 1Ohio State University
The ability to be joined and formed into complex shaped components is an important constraint for structural use of high entropy alloys (HEA). Welding is a critical fabrication process that is challenging due to its non-isothermal nature and complex chemical and physical reactions. Weld microstructures and properties directly affect service performance of welded components. Hence, a detailed understanding of the metallurgical and mechanical response in HEA considering both fusion based and solid-state welding processes is increasingly important to expand their future range of application. This talk highlights current research on weld microstructure evolution and weld structure-property relationships for main HEA systems. Results from advanced characterization techniques, physical simulation and thermodynamic and kinetic modeling are reviewed. Challenges such as strength reduction, formation of intermetallic compounds or changes in phase transformation and transformation temperatures are discussed. Finally, opportunities to implement welding considerations in the HEA development process are presented.
4:40 PM
Pitting Corrosion Behavior of Ni38Fe20Crx(MnCo)42-x High Entropy Alloys: Sarita Sahu1; Orion Swanson1; Tianshu Li1; Angela Gerard2; John Scully2; Pin Lu3; James Saal3; Gerald Frankel1; 1The Ohio State University; 2University of Virginia; 3Questek
Developing a fundamental understanding of corrosion behavior is crucial for designing new high entropy alloys (HEAs) with improved performance and selecting them for specific applications. Single phase Ni38Fe20Crx(MnCo)42-x HEAs were designed by an Integrated Computational Materials Engineering (ICME) approach. The pitting corrosion behavior of these alloys was characterized in 0.6 M NaCl at room temperature. Pitting resistance decreased with a decrease in the chromium content in these alloys. Interestingly, even the HEAs with low chromium content exhibited passivity. Crystallographic pit morphology was observed in Ni38Fe20Cr6Mn18Co18, Ni38Fe20Cr10Mn16Co16 and Ni38Fe20Cr14Mn14Co14, indicating that pit growth was under charge-transfer/ohmic control. Metastable pitting was studied by chronoamperometry experiments at potentials below the pitting potentials of HEAs. Current density was observed to be almost constant with time and was potential dependent as the current density increased with increase in applied potential in Ni38Fe20Cr10Mn16Co16, Ni38Fe20Cr14Mn14Co14, and Ni38Fe20Cr22Mn10Co10, implying that metastable pits grew under charge-transfer/ohmic control.