Search

The microstructures and deformation mechanisms of the f.c.c. medium-entropy alloy (MEA) (NiCoCr) 76 (Ni 6 AlTi) 3 has been analyzed after various thermo-mechanical treatments. The solutionized, single-phase MEA, which had a grain size of 93.4 ± 31.9 μm, was cold-rolled (CR) to a 80% thickness reduction after which it showed both a high yield strength (YS) of 1539 MPa and a high ultimate tensile strength (UTS) of 1602 MPa, but an elongation to failure, ε, of only 16%. The CR MEA was then subjected to one of two heat treatments: (1) the CR MEA was recrystallized at 1100oC for 24 h, which produced a single-phase material with 85 ± 61.7 μm grain size that exhibited a much lower YS and UTS of 364 MPa and 747 MPa, respectively, but a much greater ε of 73%; and (2) the MEA was aged at 700oC for 24 h, which produced a fine-grained (1.1 ± 0.9 μm) material and a high volume fraction (0.35) of 12 nm dia. L1 2 nanoparticles that exhibited an excellent combination of strength and ductility, viz., YS∼1501 MPa, UTS∼1651 MPa, elongation ∼26%. The grain boundary strengthening and precipitation strengthening were together estimated to account for ∼58% of the YS in the latter material. After deformation, the recrystallized MEA contained numerous stacking faults and a Taylor lattice structure containing domain boundaries and microbands, while the aged MEA exhibited numerous stacking faults and sheared particles. The density of low-angle grain boundaries (LAGBs) in the recrystallized MEA increased by 100 times to 2.2 × 10−1 μm/μm2 while the density of CSL Σ3n boundaries decreased by 71% to 7.8 × 10−3 μm/μm2 after deformation. In contrast, the density of LAGBs in the aged MEA increased by 5 times to 2.6 μm/μm2 while the density of CSL Σ3n boundaries decreased by 57% to 0.6 μm/μm2 after deformation. [ABSTRACT FROM AUTHOR]

A strong, ductile medium-entropy alloy, MEA, (CoCrNi) 94 Al 3 Ti 3 with few defects was fabricated using the selective electron beam additive manufacturing technique. Due to preheating the substrate at 1173 K, the as-printed MEA showed coherent L1 2 nanoparticles (dia.∼112 nm, volume fractioñ8.9 %) in 68 μm grains with a dual {100}<011> + {100}<001> texture. The as-built alloy exhibited a yield strength (YS) of 612 MPa, an ultimate tensile strength (UTS) of 1020 MPa, and an elongation to failure (ε) of 37 % at 298K. These values increased when testing was at 77 K, i.e. YS of 811 MPa, UTS of 1339 MPa, and an ε of 39 %. After tensile failure, the LAGBs and GND densities sharply increased at both temperatures to accommodate strain gradient along with <111> fiber texture. The lack of annealing twin boundaries led to a relatively low strain hardening rate (SHR) of 2500 MPa at 298 K and 3890 MPa at 77 K (at 5 % strain) as compared to values of thermo-mechanically processed (CoCrNi) 94 Al 3 Ti 3. The SHR curves exhibited three distinct stages, where an abrupt upturn at an intermediate strain level can be attributed to the occurrence of DTs and SFs at 77 K, and the appearance of SFs at 298 K. • A strong, ductile MEA (CoCrNi) 94 Al 3 Ti 3 was fabricated using SEBM additive manufacturing. • As-printed MEA showed L1 2 nanoparticles (dia.∼112 nm, f∼8.9 %) within 68 μm grains with dual textures. • As-printed MEA shows excellent cryogenic mechanical properties with YS of 811 MPa, UTS of 1339 MPa, and ε of 39 %. • Lack of annealing twins led to a relatively low strain hardening rate of 2500 MPa at 298 K and 3890 MPa at 77 K. • Deformation twins and stacking faults caused substantial increase in strain hardening rate at 77 K. [ABSTRACT FROM AUTHOR]

The coarsening behaviors of γ′ particles in Ni 28 Co 28 Fe 21 Cr 15 Al 4 Ti 4 high-entropy alloys and their influences on mechanical properties were analyzed. The lattice constant of the matrix decreased with the increase in aging time due to the precipitation of L1 2 nanoparticles. The increase in annealing time led to a continuous growth of both face-centered cubic grains and L1 2 particles. Both yield strength and microhardness exhibited age hardening, which varied with the holding time at 800 °C. When the holding time was in the range of 0–24 h, the yield strength and hardness increased with the aging time and dislocation cutting through particle mechanism dominates during deformation. When the holding time exceeded 24 h, the strength and hardness gradually decreased with the increase in aging time and Orowan dislocation looping was the dominant strengthening mechanism. Upon the aging at 800 °C for 24 h, the age hardening reached a peak, which provided an excellent combination of yield strength (855 MPa) and ultimate tensile strength (1031 MPa). The age hardening was related to the precipitation and ripening of spherical L1 2 nanoparticles. The ripening rate of L1 2 precipitates was calculated to be 0.31 × 10−27 m3/s. Our results can guide the designs of tough high-entropy alloys for various applications and heat treatments for a good combination of mechanical properties. • The precipitation of L1 2 nanoparticles in investigated HEA decreased the lattice parameter of face-centered cubic matrix. • Both ripening of L1 2 nanoparticles and grain size were analyzed and the growth rate was determined. • Remarkable age hardening behaviors of the tensile mechanical properties with the annealing time were observed. • The age-hardened HEA exhibited the desired combination of yield strength (855 MPa) and ultimate tensile strength (1031 MPa). [ABSTRACT FROM AUTHOR]

The weldability, microstructures, and mechanical properties of two L1 2 -nanoparticle-strengthened medium-entropy alloys (MEAs) Ni 43.4 Co 25.3 Cr 25.3 Al 3 Ti 3 and Ni 42.4 Co 24.3 Cr 24.3 Al 3 Ti 3 V 3 (at.%) are explored after electron beam welding (EBW). Strong yet ductile defect-free joints were produced with coarse columnar grains (118–245 μm) in the fusion zones, which were larger than the equiaxed grains in the heat-affected zones (15.6–22.3 μm) and in the base materials (4.6–5.6 μm). Both EBWed MEAs showed high yield strengths (838–858 MPa), high ultimate tensile strengths (1416–1420 MPa), and good fracture strains of 20–21 % at 77 K, which are 66∼83 %, 84–89 %, and 57–81 %, respectively, of those of the respective thermo-mechanically treated (TMT) MEAs. The V-doping improved the cryogenic mechanical properties of the TMT MEA while not influencing those of the EBWed MEA. High back-stress hardening contributes to over 50 % of the cryogenic strength. Both EBWed MEAs exhibited abundant dislocation networks, stacking faults, and nanoscale deformation twins after fracture, producing a high strain hardening rate and good ductility. [ABSTRACT FROM AUTHOR]