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A novel vanadium-doped f.c.c. medium-entropy alloy (MEA) Ni 42.4 Co 24.3 Cr 24.3 Al 3 Ti 3 V 3 was designed to achieve excellent cryogenic mechanical properties. The vanadium was added in order to increase the temperature-dependent friction stress, and aging of the MEA, which had been given a homogenization anneal at 1150 °C followed by cold-rolling to 80% reduction, at 700–900 °C was used both to produce partial recrystallization and precipitate L1 2 nanoparticles. Aging produced a heterogeneous microstructure composed of fine, soft recrystallized grains surrounded by harder, relatively-large non-recrystallized grains, containing L1 2 -precipitates. Increasing the aging temperature from 700 °C to 900 °C promoted V partitioning into the f.c.c. matrix with the partitioning coefficient increasing from 0.8 to 1.9; the grain size increasing from 2.3 μm to 4.6 μm; an increase in the precipitate size from 10.5 nm to 80.0 nm; a reduction in the residual geometrically-necessary dislocation (GND) density from 9.0 × 1014 m−2 to 2.8 × 1014 m−2; and a decrease in the volume fraction of precipitates fraction from 28% to 14%. All three aged MEAs showed extraordinary cryogenic mechanical properties with an increase in the aging temperature resulting in an increase in strength but a decrease in ductility, viz, a yield strength (YS) of 1929 MPa, an ultimate tensile strength (UTS) of 2147 MPa, and an elongation to failure (ε) of 6.6% at 700 °C, YS ∼ 1735 MPa, UTS ∼ 1976 MPa, ε ∼ 9.0% at 800 °C, and YS ∼ 1274 MPa, UTS ∼ 1694 MPa, ε ∼ 24.8% at 900 °C. The dislocation back stress rose progressively with increasing strain and was invariably higher than the effective stress, accounting for 51–59% of the flow stress i.e. the back stress played a dominant role in the ultrahigh flow stress. Increasing the aging temperature also produced a greater abundance of nanoscale deformation twins, stacking faults, and dislocation networks, which are responsible for excellent strain hardening capacity and greater ductility. After deformation, the density of low-angle grain boundaries increased by 3–6 times to 0.6–1.3 μm/μm2, and the GND density increased by 4–6 times to 1.2–2.0 × 1015 m−2. • V addition enhanced the cryogenic strength by 240–290 MPa and dcreased elongation. • V partitioned to the f.c.c. matrix instead of L1 2 -precipitates and thus improved friction stress. • MEA shows exceptional YS of 1274–1929 MPa, UTS of 1694–2147 MPa, and elongation of 6.6–24.8% at 77 K. • Increasing aging temperature produced more deformation twins, stacking faults, and dislocation networks. • Back-stress account for 51–59% of flow stress and was invariably higher than effective stress. [ABSTRACT FROM AUTHOR]

A high-N austenitic stainless steel (HNASS) was fabricated using a novel approach, in which duplex stainless steel powders were firstly gas nitrided in the temperature range of 700–900 °C and then consolidated using spark plasma sintering, resulting in an FCC matrix with average grain sizes of 1.6–8.2 μm. The Cr 2 N precipitates transformed from cellular precipitates (lamellar within the grains) to intergranular precipitation (equiaxed along the grain boundaries) as the nitriding temperature rose from 700 °C to 750 °C, and the N content rose from 0.68 wt% to 1.71 wt%. The 700 °C-treated alloy exhibits a yield strength of 783 MPa, a high tensile strength of 1091 MPa, and an elongation of 41% while the 900 °C-treated alloy exhibits an ultra-high compressive strength of 3017 MPa, and hardness of 520 HV. These values dramatically exceed the published values of similar HNASSs. Quantitative calculations showed that these superior mechanical properties could be attributed to increased solid solution and grain boundary strengthening. Furthermore, it can be inferred that while the equiaxed Cr 2 N phase increased the yield strength, it greatly degraded the fracture toughness. • A novel method, nitriding and followed spark plasma sintering, was used to fabricate HNASS. • The morphology of Cr 2 N phase changed from lamellar (within the grains) to equiaxed (along the grain boundaries). • The alloy features superior mechanical properties of yield strength (783 MPa) and ultimate tensile strength (1091 MPa). • Solid solution and grain boundary strengthening are dominant strengthening mechanisms. [ABSTRACT FROM AUTHOR]

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]

Different L1 2 -Ni 3 (Ti, Al) phase contents in (NiCo) 52+2x (AlTi) 4+2x Fe 29-4x Cr 15 (x = 0-4) high-entropy alloys are designed based on CALPHAD and its effects on mechanical properties are systematically studied. Results show that excellent combination of yield strength (635-753 MPa), ultimate tensile strength (1010-1074 MPa), and fracture strain (23-31%) could be obtained for high-entropy alloys (x = 2 and 3). Almost half of the yield strength comes from nano-sized L1 2 -type precipitates strengthening. The yield strength experienced sharp increment and the ductility showed an obvious decrease when the precipitated L1 2 phase increased in the range of 0–14.2 wt%, with corresponding Al and Ti concentrations in the range of x = 0-3. Furthermore, when the precipitated L1 2 phase exceeds 14.2 wt%, merely a slight increment in yield strength was observed, but the fracture strain declined to around 13%. The present results contribute to understanding the strengthening mechanisms of L1 2 -strengthened HEAs and may provide some reference for its application. • Effects of L1 2 contents on tensile properties were studied for (NiCo) 52+2x (AlTi) 4+2x Fe 29-4x Cr 15 HEAs. • The L1 2 -strengthened HEAs show a desired combination of tensile strength (1010-1074 MPa) and fracture strain (23-31%). • Precipitation strengthening from 15-26 nm nanoparticles contributes to half of the yield strength, around 318-375 MPa. [ABSTRACT FROM AUTHOR]

Electron beam welding of the medium-entropy alloy (MEA) (NiCoCr) 94 Al 3 Ti 3 to 304 stainless steel (SS) has been undertaken. Defect-free joints were obtained with coarse columnar grains: the columnar grains on the 304 SS side of the fusion zone (FZ) were finer (94 μm, length-to-width ratio∼2.7) than the grains on the MEA side (204 μm, 2.9), a difference that probably resulted from a higher cooling rate after welding due to the higher thermal conductivity of the 304 SS compared to the MEA. Mixing of the two materials in the FZ led to lower vol.% (14%) and larger diameter (44 nm) of L1 2 nanoparticles compared to the 23 nm dia. and 37% vol.% in the aged MEA, a result consistent with CALPHAD analysis. The joint showed a good combination of yield strength (323 MPa), ultimate tensile strength (670 MPa), and elongation (24%). Failure occurred by ductile fracture in the FZ. [Display omitted] [ABSTRACT FROM AUTHOR]