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1. Strength can be controlled by edge dislocations in refractory high-entropy alloys

   https://doi.org/10.1038/s41467-021-25807-w  

   Lee, Chanho; Maresca, Francesco; Feng, Rui; Chou, Yi; Ungar, T.; Widom, Michael; An, Ke; Poplawsky, Jonathan D.; Chou, Yi-Chia; Liaw, Peter K.; et al (December 2021, Nature Communications)

   Abstract Energy efficiency is motivating the search for new high-temperature (high-T) metals. Some new body-centered-cubic (BCC) random multicomponent “high-entropy alloys (HEAs)” based on refractory elements (Cr-Mo-Nb-Ta-V-W-Hf-Ti-Zr) possess exceptional strengths at high temperatures but the physical origins of this outstanding behavior are not known. Here we show, using integrated in-situ neutron-diffraction (ND), high-resolution transmission electron microscopy (HRTEM), and recent theory, that the high strength and strength retention of a NbTaTiV alloy and a high-strength/low-density CrMoNbV alloy are attributable to edge dislocations. This finding is surprising because plastic flows in BCC elemental metals and dilute alloys are generally controlled by screw dislocations. We use the insight and theory to perform a computationally-guided search over 10 7 BCC HEAs and identify over 10 6 possible ultra-strong high-T alloy compositions for future exploration. 

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2. Copper effects on the microstructures and deformation mechanisms of CoCrFeNi high entropy alloys

   https://doi.org/10.1063/5.0201647  

   Amalia, Lia; Li, Yongkang; Bei, Hongbin; Chen, Yan; Yu, Dunji; An, Ke; Lyu, Zongyang; Liaw, Peter K; Zhang, Yanwen; Ding, Qingqing; et al (April 2024, Applied Physics Letters)

   In situ neutron diffraction experiments have been performed to investigate the deformation mechanisms on CoCrFeNi high entropy alloys (HEAs) with various amounts of doped Cu. Lattice strain evolution and diffraction peak analysis were used to derive the stacking fault probability, stacking fault energy, and dislocation densities. Such diffraction analyses indirectly uncovered that a lower degree of Cu doping retained the twinning behavior in undoped CoCrFeNi HEAs, while increasing the Cu content increased the Cu clusterings which suppressed twinning and exhibited prominent dislocation strengthening. These results agree with direct observations by transmission electron microscopy. 

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3. Multiplicity of dislocation pathways in a refractory multiprincipal element alloy

   https://doi.org/10.1126/science.aba3722  

   Wang, Fulin; Balbus, Glenn H.; Xu, Shuozhi; Su, Yanqing; Shin, Jungho; Rottmann, Paul F.; Knipling, Keith E.; Stinville, Jean-Charles; Mills, Leah H.; Senkov, Oleg N.; et al (October 2020, Science)

   null (Ed.)

   Refractory multiprincipal element alloys (MPEAs) are promising materials to meet the demands of aggressive structural applications, yet require fundamentally different avenues for accommodating plastic deformation in the body-centered cubic (bcc) variants of these alloys. We show a desirable combination of homogeneous plastic deformability and strength in the bcc MPEA MoNbTi, enabled by the rugged atomic environment through which dislocations must navigate. Our observations of dislocation motion and atomistic calculations unveil the unexpected dominance of nonscrew character dislocations and numerous slip planes for dislocation glide. This behavior lends credence to theories that explain the exceptional high temperature strength of similar alloys. Our results advance a defect-aware perspective to alloy design strategies for materials capable of performance across the temperature spectrum. 

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4. Machine learning prediction of the mechanical properties of refractory multicomponent alloys based on a dataset of phase and first principles simulation

   https://doi.org/10.3389/ftmal.2022.1036656  

   Zhang, Congyan; Gu, Xuhang; Bhandari, Uttam; Lei, Jialin; Guo, Shengmin; Kourouma, Mathieu; Karoui, Abdennaceur; Yang, Shizhong (November 2022, Frontiers in Metals and Alloys)

   In this work, a dataset including structural and mechanical properties of refractory multicomponent alloys was developed by fusing computations of phase diagram (CALPHAD) and density functional theory (DFT). The refractory multicomponent alloys, also named refractory complex concentrated alloys (CCAs) which contain 2–5 types of refractory elements were constructed based on Special Quasi-random Structure (SQS). The phase of alloys was predicted using CALPHAD and the mechanical property of alloys with stable and single body-centered cubic (BCC) at high temperature (over 1,500°C) was investigated using DFT-based simulation. As a result, a dataset with 393 refractory alloys and 12 features, including volume, melting temperature, density, energy, elastic constants, mechanical moduli, and hardness, were produced. To test the capability of the dataset on supporting machine learning (ML) study to investigate the property of CCAs, CALPHAD, and DFT calculations were compared with principal components analysis (PCA) technique and rule of mixture (ROM), respectively. It is demonstrated that the CALPHAD and DFT results are more in line with experimental observations for the alloy phase, structural and mechanical properties. Furthermore, the data were utilized to train a verity of ML models to predict the performance of certain CCAs with advanced mechanical properties, highlighting the usefulness of the dataset for ML technique on CCA property prediction. 

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5. Kink bands promote exceptional fracture resistance in a NbTaTiHf refractory medium-entropy alloy

   https://doi.org/10.1126/science.adn2428  

   Cook, David H.; Kumar, Punit; Payne, Madelyn I.; Belcher, Calvin H.; Borges, Pedro; Wang, Wenqing; Walsh, Flynn; Li, Zehao; Devaraj, Arun; Zhang, Mingwei; et al (April 2024, Science)

   Single-phase body-centered cubic (bcc) refractory medium- or high-entropy alloys can retain compressive strength at elevated temperatures but suffer from extremely low tensile ductility and fracture toughness. We examined the strength and fracture toughness of a bcc refractory alloy, NbTaTiHf, from 77 to 1473 kelvin. This alloy’s behavior differed from that of comparable systems by having fracture toughness over 253 MPa·m^1/2, which we attribute to a dynamic competition between screw and edge dislocations in controlling the plasticity at a crack tip. Whereas the glide and intersection of screw and mixed dislocations promotes strain hardening controlling uniform deformation, the coordinated slip of <111> edge dislocations with {110} and {112} glide planes prolongs nonuniform strain through formation of kink bands. These bands suppress strain hardening by reorienting microscale bands of the crystal along directions of higher resolved shear stress and continually nucleate to accommodate localized strain and distribute damage away from a crack tip. 

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