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• Forged HfNbTaTiZr refractory high-entropy alloy (RHEA) demonstrates significant strain hardening at elevated temperatures, showcasing a clear distinction from the strain softening observed at room temperature. • Defects introduced through the forging process act as a driving force for the initiation of slip bands in high-temperature tensile deformation, thereby promoting dynamic recrystallization. • The bimodal distribution of grain sizes, attributed to factors such as dynamic recrystallization and initial grains, impedes dislocation movement, consequently enhancing strain hardening. Refractory high-entropy alloys (RHEAs) exhibit outstanding strength at room temperature, but their high-temperature applications are hindered by severe strain-softening. Here, we report slip-band-driven dynamic recrystallization to enhance the high-temperature strain hardening of HfNbTaTiZr RHEA. By introducing partial lattice defects through hot forging, we increase the nucleation sites for dynamic recrystallization during subsequent thermomechanical deformation, thus suppressing the strain-softening behavior. We reveal that the high-temperature deformation is governed by the formation of heterogeneous bimodal grains along slip bands, which effectively constrain dislocation motion and improve strength, while microbands prevent premature failure. The fracture mode also changes from ductile to mixed to cleavage-dominated with increasing temperature. Our results demonstrate a simple and effective method for overcoming high-temperature strain-softening for BCC high entropy alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

A metastable β Ti alloy was additively manufactured by laser powder bed fusion (LPBF). Tensile testing along the build direction of the as-LPBF material (LPBF-0°) revealed significant work softening immediately following yielding with no uniform deformation. By contrast, substantial work hardening and uniform elongation well over 10% were achieved perpendicular to the build direction (LPBF-90°). Similar effects were obtained in the build direction after super transus heat treatment (LPBF-0°+HT) although the strength was slightly lowered. In addition, the yield drop phenomenon observed in both orientations of the as-LPBF materials disappeared after HT. The enhanced work hardening ability, and thus ductility, can be attributed to increased interactions of slip bands/slip bands owing to additional {112}<111> slip systems becoming operative in LPBF-0°+HT and LPBF-90° while LPBF-0° was dominated by {110}<111> only. The other variations after HT may be related to the coarsening of grain structure and removal of specific substructures in the as-LPBF microstructure. • Tensile anisotropy in an LPBF-fabricated β Ti was studied • Work hardening and ductility were much improved perpendicular to build direction • Super transus heat treatment transformed columnar β grains to equiaxed • The equiaxed β structure exhibited enhanced work hardening and ductility • Substructural features in LPBF microstructure led to strengthening and yield drop [ABSTRACT FROM AUTHOR]

• Slip bands formed in compressed Al 0.1 CoCrFeNi nanopillars are examined by TEM. • A slip band consists primary and secondary dislocations with the latter mostly behind the slip band. • The role of dislocation pileup in front of the slip band is discussed. • A slip band model is proposed based on browns ellipsoidal slip band with supporting evidence of dislocation dipoles. Remarkable diversity is observed in dislocation interactions that are responsible for intermittent and sudden crystal slips. While large crystal slips can be easily observed on the surface of deformed crystals, unraveling the underlying dislocation interaction mechanisms, however, has been a longstanding challenge in the study of single-crystal plasticity. A recent study demonstrated that the sluggish dislocation dynamics in the high entropy alloy (HEA) of Al 0.1 CoCrFeNi enables the observation of slip bands for a direct link to dislocation avalanches in a nanopillar. Here, we further examined the dislocation structure of slip bands in the HEA nanopillars oriented for single slip. Experimental evidence was provided on the dislocation organization in a slip band based on groups of primary dislocations, secondary dislocations, and dislocation pileups. The results were compared with the previously proposed slip band models. The unique aspects of the HEA that enable such observations were also investigated through an examination of the dislocation microstructure and its response to applied forces in the HEA nanopillars. [Display omitted] [ABSTRACT FROM AUTHOR]

The fatigue cracking behavior at twin boundaries (TBs) in a CoCrFeMnNi high-entropy alloy with three different grain sizes was systematically investigated under low-cycle fatigue. Irrespective of grain size, the change from slip band cracking to TB cracking occurred with increasing the difference in the Schmid factors (DSF) between matrix and twin. However, the required critical DSF for the transition of the dominant cracking mode decreases with decreasing grain size due to the reduced slip band spacing that increases the impingement sites on the TBs and facilitates the coalescence of defects and voids to initiate TB cracks. [ABSTRACT FROM AUTHOR]

• Peculiar yielding behavior during warm compression in Ti-5553 was studied in depth. • Completely different slip characteristics and dynamic precipitation of α s occurred. • Macro/micro compressive characteristics and mechanisms were systematically summarized. • SEM/TEM/crystallographic calculation/crystal plastic finite element simulation was used. The compressive yielding phenomenon of titanium alloys is not as focused and sufficiently ascertain as the tensile yielding phenomenon. In the present work, the peculiar compressive yielding behavior and the different dynamic responses of three different initial microstructures (single β, clavate α and lamellar α) were investigated in an attractive metastable β titanium alloy Ti-5553 using electron microscopes/crystallographic calculation/crystal plastic finite element simulation. Results reveal that the distinct compressive yielding behavior, steep peaks of sudden drop in the initial stage (very small true strain ˜0.03) of stress loading have appeared in the compression stress-strain curves except for the lamellar α initial microstructure. Dislocation slip is the essential mechanism of the initial yielding behavior. Interlaced multiple-slip bands formed in the single β initial microstructure during the warm deformation process. A small quantity of single slip bands was observed in the deformed clavate α initial microstructure. The abundant varied nano/ultrafine α s precipitates were nucleated dynamically and dispersedly in all the three deformed initial microstructures. The multiple-slip bands formation and substantial nanoscale α s result in the highest peak of flow stress for single β initial microstructure. The compressive slip bands are formed early in the elastic – plastic deformation stage. As the increasing strain, the sample showed a significant compressive bulge, or eventually forming a strong adiabatic shear band or crack. These results are expected to provide a reference for the study of deformation behavior and mechanical properties of metastable β titanium alloys. [ABSTRACT FROM AUTHOR]