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Here, we report on fracture behaviors of Al–Mg–Si alloys (LT21 Al) decommissioned from a research reactor in service for 30 years. Microscopic characterizations show that the initial microstructures of the long-term low-dose-rate neutron-irradiated LT21 Al are dominated by a large number of micrometer-sized voids and second-phase particles, different from short-term high-dose-rate neutron-irradiated materials. Quasi-static tensile and shock spall experiments on the irradiated and unirradiated LT21 Al reveal considerable irradiation-induced softening (larger than 40%) in both the tensile and spall strength. We attribute the irradiation softening to the over-aging process promoted by irradiation enhanced diffusion during the long-term service. Postmortem characterizations on shock-recovered samples indicate that the long-term low-dose-rate neutron irradiation-induced microstructures (micrometer-sized voids and second-phase particles) facilitate nucleation and growth of the dynamic damage in irradiated materials, thus playing an important role in the change in facture behaviors. [ABSTRACT FROM AUTHOR]

The fully biodegradable polymer blends remain challenges for the application due to their undesirable comprehensive performance. Herein, remarkable combination of superior mechanical performance, bacterial resistance, and controllable degradability is realized in the biodegradable poly(L-lactide)/poly(butylene succinate) (PLLA/PBSU) blends by stabilizing the epoxide group modified titanium dioxide nanoparticles (m-TiO2) at the PLLA-PBSU interface through reactive blending. The m-TiO2 can not only act as interfacial compatibilizer but also play the role of photodegradation catalyst: on the one hand, binary grafted nanoparticles were in situ formed and stabilized at the interface to enhance the compatibility between polymer phases. As a consequence, the mechanical properties of the blend, such as the elongation at break, notched impact strength and tensile yield strength, were simultaneously improved. On the other hand, antibacterial and photocatalytic degradation performance of the composite films was synergistically improved. It was found that the m-TiO2 incorporated PLLA/PBSU films exhibit more effective antibacterial activity than the neat PLLA/PBSU films. Moreover, the analysis of photodegradable properties revealed that that m-TiO2 nanoparticles could act as a photocatalyst to accelerate the photodegradation rate of polymers. This study paves a new strategy to fabricate advanced PLLA/PBSU blend materials with excellent mechanical performance, antibacterial and photocatalytic degradation performance, which enables the potential utilization of fully degradable polymers. [ABSTRACT FROM AUTHOR]

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]

Influences of both aging temperature and vanadium addition on the microstructure and mechanical properties of a nanoparticle-strengthened, partially-recrystallized, medium-entropy alloy (MEA) Ni 43.4 Co 25.3 Cr 25.3 Al 3 Ti 3 were probed in the present work. Aging (700 °C or 900 °C) produced both a high density of nano-sized L1 2 precipitates, and a heterogeneous grain structure, which consisted of fine soft recrystallized grains surrounded by harder relatively large unrecrystallized grains. The volume fraction and size of the nanoparticles were strongly dependent on the heat treatment, as was the partitioning of the V, i.e. , almost evenly distributed between the f.c.c. matrix and L1 2 precipitates at 700 °C but partitioned more to the f.c.c. matrix with a coefficient of 1.9 at 900 °C. The yield strength (YS), ultimate tensile strength (UTS), and elongation to failure (ε) simultaneously increased with decreasing temperature from 298 K to 77 K. The V-doped MEA shows an excellent combination of strength and elongation, viz. YS of 1035∼1637 MPa, UTS of 1587∼1983 MPa, and ε of 17.6–37.7 % for the V-free MEA, and YS of 1274∼1929 MPa, UTS of 1694∼2147 MPa, and ε of 6.6–24.8 % for V-doped MEA. The excellent strength of the MEA resulted from a combination of back-stress hardening (1108–1195 MPa corresponding to 51–60 % of the flow stress), Hall-Petch strengthening, and hardening from pre-existing dislocations (199–355 MPa) at 298 K and 77 K. Increasing the aging temperature promoted a transition in the deformation behavior from heterogeneous to homogeneous, and produced a greater abundance of nanoscale deformation twins, stacking faults, and dislocation networks, leading to the exceptional monotonic decreased strain-hardening rates during tensile tests at 293 K and 77 K. • Vanadium addition could improve friction stress and overall strength of the present MEA. • Vanadium partition behavior between f.c.c. and L1 2 nanoparticles could be significantly influenced by aging temperature. • V-free MEA shows an YS of 1035∼1637 MPa, UTS of 1587∼1983 MPa, and ε of 17.6–37.7 %. • V-doped MEA shows an YS of 1274∼1929 MPa, UTS of 1694∼2147 MPa, and ε of 6.6–24.8 %. • Back-stress hardening of 1108∼1195 MPa corresponding to 51–60 % of the flow stress at 77 K [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]

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]

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]