HEA 2023: Characterization of HEAs III
Program Organizers: Andrew Detor, DARPA/DSO; Amy Clarke, Los Alamos National Laboratory
Tuesday 9:00 AM
November 14, 2023
Room: William Penn Ballroom
Location: Omni William Penn
Session Chair: Carolina Frey, University of California Santa Barbara
9:00 AM Introductory Comments
9:05 AM Cancelled
On the Fracture Toughness of High-entropy Alloys: bcc vs. fcc Materials: Robert Ritchie1; 1University of California, Berkeley / Lawrence Berkeley National Laboratory
Face-centered-cubic (fcc) medium/high-entropy alloys (HEAs) can display exceptional strength, ductility and fracture toughness, properties that are further enhanced at cryogenic temperatures. Body-centered-cubic (bcc) refractory RHEAs, conversely, can display exceptional strength and compressive ductility at elevated temperatures, but are often compromised by poor lower-temperature behavior. We examine the damage-tolerance of these HEAs, and show that whereas some single-phase fcc HEAs exhibit the highest toughnesses on record, even at ultrahigh strain-rates and liquid-helium temperatures, bcc HEAs generally display extremely low ductility and toughness under tensile loads. However, the NbTaTiHf single-phase RHEA has remarkable toughness at cryogenic to elevated temperatures. With KIc values >250MPam at room temperature (with ~600MPa yield strengths, ~16% ductilities), this alloy exhibits ~90MPam toughnesses both at 77K and 1473K, which is highly unusual for bcc materials. The reasons for such properties are explored to investigate why most bcc RHEAs are brittle in tension, whereas some can display outstanding toughness. ___________*funded by the Department of Energy, Office of Science (Basic Energy Sciences), Materials Sciences and Engineering Division.
9:35 AM
Predicting the Strength of Multi-principal Element Alloys: A Mechanistic Data-driven Approach: Ali Rida1; Markus Sudmanns2; Yanfei Wang3; Zhuocheng Xie4; Xiaolong Ma5; Wenxin Zhou6; Yejun Gu7; Jaafar El-Awady1; Huajian Gao8; Jing Luo1; 1Johns Hopkins University; 2RWTH Aachen University; 3Peking University; 4South China University of Technology; 5City University of Hong Kong; 6University of California, San Diego; 7Agency for Science, Technology and Research; 8Nanyang Technological University
Although Multi-principal element alloys (MPEAs) are still in their early stages of development, they have demonstrated great potential for significant improvement in material properties compared to conventional alloys. It was found that some alloys exhibit an impressive combination of strength and ductility. However, the variation in microstructure contrasts with empiric models such as the Hall-Petch relation, which aim to capture the basic trends using a few parameters. One significant challenge in developing robust physics-based models for predicting the yield stress is therefore to provide quantitative measures for the statistical uncertainty originating from the microstructural variation. Here, we present a combined mechanistic data-driven and experimental approach for predicting the yield strength in NiCoCr, NiCoV, and CrMnFeCoNi and quantify the influence of different microstructural features on the yield strength and its statistical distribution. We demonstrate the applicability of this probabilistic approach to guide the design of polycrystalline MPEAs with superior mechanical properties.
9:55 AM
Phase Decomposition in CrFeNiMn Under Thermal Aging: Nina Perry1; Anshul Kamboj1; Emmanuelle Marquis1; 1University of Michigan
The understanding that high entropy alloys (HEAs) are stable solid solutions has been widely accepted since the discovery of these materials. However, recent studies have shown that this is often untrue, as they have revealed phase formation at high annealing temperatures. This suggests the possibility for extensive transformations at lower temperature and leaves the door open for further exploration. Indeed, our 9-week aging investigation of a CrFeNiMn alloy revealed complex phase evolution at a relatively lower annealing temperature through characterization with scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and atom probe tomography. More specifically, this alloy separated into nanoscale distributions of an L10 Ni-Mn phase, an FCC Fe-rich phase, and a BCC Cr-rich phase. This microstructure is believed to be a result of two competing growth mechanisms: homogeneous nucleation and discontinuous decomposition. The findings imply that the range of properties obtainable by HEAs can potentially be expanded through microstructure tailoring.
10:15 AM
Computational Thermodynamics-guided Alloy Design and Phase Stability In Non-equimolar Cocrfemnni Multi-principal Element Alloys: An Experimental-theoretical Study: David Silva1; Gustavo Bertoli2; Nelson Neto1; Michael Kaufman1; Amy Clarke1; Francisco Coury2; Claudemiro Bolfarini2; 1Colorado School of Mines; 2Federal University of Sao Carlos
The formation of sigma phase (brittle and undesirable) is a real concern when engineering face centered cubic (FCC) alloys subjected to operation at elevated temperatures (600-1000 °C). Therefore, predicting its formation is essential in alloy design. Phase equilibria were studied in a wide range of compositions by the CALPHAD method in combination with two empirical methods for predicting sigma phase formation (valence electron concentration (VEC) and paired sigma-forming element (PSFE)). Isothermal aging treatments at 900 ºC, 1000 °C and 1100 °C for 20 h were selected, due to the fact that CALPHAD and Tsai criteria predictions for sigma phase formation diverged in some cases. Both prediction methods (CALPHAD and Tsai criteria) were compared with experimental characterization by a combination of synchrotron high energy X-ray diffraction (HEXRD) and electron backscattering diffraction (EBSD). This work will guide further studies of CoCrFeMnNi alloys potentially sensitive to sigma phase formation.
10:35 AM Break
10:55 AM
The Compositional Dependence of Deformation Mechanisms, Strength, and Ductility in a Pseudo-binary Cr-Co-Ni Alloy System: Joshua Cicotte1; Ying Yang2; Dunji Yu2; Ke An2; Weicheng Zhong2; Easo George1; 1University of Tennessee - Knoxville; 2Oak Ridge National Laboratory
We investigated the mechanical properties and deformation mechanisms off-equiatomic variants of the CrCoNi medium-entropy alloy, specifically the Co-rich alloys in the pseudo-binary Cr33-Co(x)-Ni(67-x) system. Based on neutron diffraction, we find that the Co33, Co40, and Co50 alloys are FCC solid solutions after quenching from elevated temperature whereas the Co57 alloy retains some martensitic HCP phase in an FCC solid solution. Tensile testing revealed that ductility decreases linearly with increasing Co content from a maximum in the equiatomic Co33 alloy. Ex-situ neutron diffraction of the fractured tensile specimens found significant amounts of deformation-induced HCP martensite formed during tensile testing in the Co40, Co50, and Co57 alloys. The elongation of these alloys shows a linear decrease with increasing HCP volume, directly tying the formation of HCP martensite to the reduction in ductility. Comparisons are also made to our Ni-rich pseudo-binary compositions where twinning rather than phase transformation occurs.
11:15 AM
Towards Design of Multifunctional High Entropy Alloy using Metastability Engineering: Akshit Dutta1; Ming-Hung Tsai2; Saurabh Nene1; 1Indian Institute of Technology Jodhpur; 2National Chung Hsing University
Conventional alloy design strategy aims to attain a specific property profile in a material and consequently gives rise to single material for single application (SMSA) analogy. Present work aims in modifying this analogy to single material for multiple application (SMMA) by designing alloys using high entropy alloy concept in synergy with metastability engineering. The main aim of the work is to attain harmonic combination of properties (at least five) in the designed alloy, giving rise to multifunctionality in it. The HEA system was formulated by choosing critical elements present in different categories of steels namely low-density steels, stainless steel, TRIP/TWIP steels and electrical steels such that the resultant alloy would show excellent strength-ductility synergy, corrosion resistance, electrical resistivity and formability. The resultant HEA out of Fe-Mn-Cr-Ni-Co-Si system demonstrated excellent combination of all proposed properties in comparison with the counter alloys thereby suggesting multifunctionality in it.
11:35 AM
Strength and Thermal Stability of Mechanically Driven Nanocrystalline High Entropy Alloys: Yuan Yao1; Luyan Li1; Mostafa Hassani1; 1Cornell University
Nanocrystalline materials are featured by ultra-high strength but have strong tendency of grain coarsening, while high entropy alloys provide stable microstructures due to sluggish diffusion. Nanocrystalline high entropy alloys (nc-HEAs) are expected to combine the benefits of these two worlds, further pushing the performance limits of structural materials. Here we use forced mechanical mixing to fabricate a nc-HEA in solid state. Nanoindentation is used to measure the strength of the alloy and evaluate its strain rate sensitivity. The thermal stability of the alloy is also studied with isochronal and isothermal annealing experiments. We find significant contributions from grain boundary and solid solution strengthening and discuss the strain rate sensitivity of the nc-HEA in light of the interplay between the two mechanisms. We also find excellent grain size retention as well as some unexpected increase in the hardness after annealing which we attribute to the second phase precipitations in the material.
11:55 AM Cancelled
Solid Solution Strengthening on CrCoNi-(Pd,V) Alloys: Pedro Oliveira1; Francisco Coury1; Claudemiro Bolfarini1; 1Federal University of Sao Carlos
This study examined the impact of adding vanadium (V) and palladium (Pd) on the mechanical properties and deformation mechanisms of CrCoNi alloys. Pd and V (10% at) were added to the CrCoNi system, resulting in a face-centered cubic (FCC) solid solution. The addition of Pd led to a significant change in atomic mismatch, while the electronegativity of the system remained relatively unchanged. On the other hand, the addition of V aimed to create a notable difference in electronegativity without greatly affecting the atomic mismatch. Mechanical properties were assessed through hardness and tensile tests at room temperature. X-ray diffraction analysis using synchrotron radiation was performed during the tensile test to examine microstructural changes and understand deformation mechanisms. Fractured samples were analyzed using scanning electron microscopy and transmission electron microscopy. Results showed that V and Pd additions enhanced the hardness and mechanical properties compared to the equiatomic Cr33Co33Ni33 alloy.