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In this study, WC-12Co cemented carbides were prepared by laser powder bed fusion (LPBF) on a WC-20Co substrate. The effects of the scanning speed and the substrate on the microstructure, mechanical properties and wear characteristics of the WC-12Co cemented carbide were investigated. The results showed that an alternating distribution of coarse and fine WC grains is observed in the LPBF-prepared WC-12Co cemented carbides. As the laser scanning speed decreases or the laser energy density increases, the WC phase gradually decomposes, accompanied by the loss of C and the formation of ƞ phases. WC-12Co cemented carbides produced at a lower scanning speed exhibit a higher density of thermal cracks, while those produced at a higher scanning speed have a larger number of pores. As the laser scanning speed increases, the transverse rupture strength (TRS) first increases to a maximum value of 823.13 MPa and then decreases. The variation of TRS is mainly attributed to the evolution of the brittle ƞ-phase and metallurgical defects. For WC-12Co cemented carbides formed at low scanning speeds, the brittle η-phase may fracture and cut through the matrix during the friction process, leading to an increase in coefficient of friction (COF) and wear mass loss. For samples formed at high scanning speeds, the formation and detachment of the shear layer could accelerate the friction process and undermine the wear resistance of the WC-12Co cemented carbide. Therefore, the sample prepared at the scanning speed of 400 mm/s shows the best wear performance.

• A cobalt-free ODS FeCrNi MEA was fabricated by powder metallurgy method. • Nanoscale oxides are in-situ introduced into the FCC matrix. • The alloy exhibits high yield strength of 1120 MPa and 701 MPa at 25 °C and 600 °C, respectively. • The alloy shows high irradiation-induced segregation resistance. • Highly dispersed oxide nanoparticles are responsible for the high strength and outstanding irradiation resistance. A novel cobalt-free oxide dispersion strengthened (ODS) equiatomic FeCrNi medium entropy alloy (MEA) was successfully fabricated through mechanical alloying and hot extrusion (HE). The ODS FeCrNi MEA is composed of a single face-centered cubic (FCC) matrix, in which highly dispersed oxide nanoparticles, including Y 2 Ti 2 O 7 , Y 2 TiO 5 and Y 2 O 3 , are uniformly distributed. Compared with the FeCrNi MEA, the ODS FeCrNi MEA exhibits the improved yield strength (1120 MPa) and ultimate tensile strength (1274 MPa) with adequate ductility retention (12.1%). Theoretical analysis of the strengthening mechanism indicates that the high strength is mainly attributed to the grain-boundary strengthening caused by fine grains and the precipitation strengthening resulted from the oxide nanoparticles. Meanwhile, the matrix that easily activates mechanical twinning during the deformation process is the main reason to ensure moderate ductility. In addition, the introduction of high-density oxide nanoparticles can disperse the defect distribution and suppress the defect growth and irradiation-induced segregation, leading to the excellent irradiation resistance. These findings provide innovative guidance for the development of high-performance structural materials for future nuclear energy applications with balanced strength and ductility. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, graded tantalum scaffolds were manufactured by electron beam powder bed fusion (EB-PBF). Compression-compression fatigue testing was conducted on the EB-PBF built graded tantalum scaffolds. The compressive fatigue behavior and compressive fatigue strength of graded tantalum scaffolds with vertical gradient (VG) and parallel gradient (PG) states were investigated. In the fatigue testing, the failure of PG graded scaffold was concentrated on the lowest strength constituent, resulting in higher cyclic ratcheting strain and fatigue damage strain. In terms of the VG graded scaffold, due to the simultaneous uniform deformation of each structure, the strain accumulation in a single cycle was small, resulting in the low cyclic ratcheting strain. In addition, the propagation of fatigue cracks in pore structure with higher strength can be retarded by the stress redistribution, resulting in the low fatigue damage strain. Therefore, the design idea of graded pore structure under identical strain condition is beneficial to obtain porous tantalum scaffold with high compressive fatigue strength.

• Selective laser melted FeCrNi multi-principal-element alloy shows the excellent ultimate tensile strength of (σ UST = 996 MPa) and the splendid ductility (ε = 37 %). • All constituent elements are evenly distributed at micron scale, but Cr element is obviously segregated at atomic scale. • Dynamic evolution of microstructure during selective laser melting is revealed by molecular dynamic simulation. • A microstructure-based physical model is established to evaluate the contribution of microstructure to strength. Additive manufacturing is believed to open up a new era in precise microfabrication, and the dynamic microstructure evolution during the process as well as the experiment-simulation correlated study is conducted on a prototype multi-principal-element alloys FeCrNi fabricated using selective laser melting (SLM). Experimental results reveal that columnar crystals grow across the cladding layers and the dense cellular structures develop in the filled crystal. At the micron scale, all constituent elements are evenly distributed, while at the near-atomic scale, Cr element is obviously segregated. Simulation results at the atomic scale illustrate that i) the solid-liquid interface during the grain growth changes from horizontal to arc due to the radial temperature gradient; ii) the precipitates, microscale voids, and stacking faults also form dynamically as a result of the thermal gradient, leading to the residual stress in the SLMed structure. In addition, we established a microstructure-based physical model based on atomic simulation, which indicates that strong interface strengthening exists in the tensile deformation. The present work provides an atomic-scale understanding of the microstructural evolution in the SLM process through the combination of experiment and simulation. [ABSTRACT FROM AUTHOR]

In this work, the structural stability, fracture toughness, and thermodynamic properties of Co30Ni30Fe20Cr20high-entropy alloy (HEA) has been investigated by using first-principles calculations combined with special quasi-random structures. The calculated lattice constant of 3.485 Å was in good agreement with the experiments, and the obtained formation enthalpy value of −1.17 eV/atom indicated that this particular HEA was stable. The results showed that the Fracture energy of the [111] direction was much lower than the [001] and [110] directions. Therefore, the bond strength of the [111] direction was inferred to be the weakest. Finally, the entropy, internal energy, and specific heat capacity of the Co30Ni30Fe20Cr20HEA increased with increasing temperature, while the free energy and bulk modulus decreased. Thus, the results obtained in this work could be used for studying and designing high performance CoCrFeNi HEAs.

• The MEA shows high strain-hardening ability and moderate thermal-softening effect. • The MEA resists adiabatic shear band formation up to a high shear strain of ~14.5. • Multiple deformation mechanisms effectively improve shear localization resistance. Deformation behavior of a FeCrNi medium entropy alloy (MEA) prepared by powder metallurgy (P/M) method was investigated over a wide range of strain rates. The FeCrNi MEA exhibits high strain-hardening ability, which can be attributed to the multiple deformation mechanisms, including dislocation slip, deformation induced stacking fault and mechanical twinning. The shear localization behavior of the FeCrNi MEA was also analyzed by dynamically loading hat-shaped specimens, and the distinct adiabatic shear band cannot be observed until the shear strain reaches ~14.5. The microstructures within and outside the shear band exhibit different characteristics: the grains near the shear band are severely elongated and significantly refined by dislocation slip and twinning; inside the shear band, the initial coarse grains completely disappear, and transform into recrystallized ultrafine equiaxed grains by the classical rotational dynamic recrystallization mechanism. Moreover, microvoids preferentially nucleate in the central areas of the shear band where the temperature is very high and the shear stress is highly concentrated. These microvoids will coalesce into microcracks with the increase of strain, which eventually leads to the fracture of the shear band. [ABSTRACT FROM AUTHOR]

Cellular structure is a unique microstructure characteristic of the additive manufactured face-centered cubic phase metallic materials. It typically grows preferentially along the temperature gradient, leading to anisotropy in mechanical properties and restricting the practical application. Herein, we adopted a discontinuous laser scanning strategy for fabricating the face-centered cubic FeCrNi alloy. The scanning layer is divided into nine discontinuous equal-width regions along which the laser beam prints discontinuously. This strategy can increase the temperature gradient in the melt pool by weakening the thermal influence between the adjacent scanning regions, which helps to suppress cellular structure growth along the temperature gradient. Compared with the sample prepared by the conventional continuous laser scanning strategy, the sample printed using the discontinuous laser scanning strategy exhibits finer cellular structures with a significantly shorter length along the growth direction, and the anisotropy of mechanical properties is weakened. The in-plane anisotropy of ultimate tensile strength decreases from 21.6 to 6.4. This study presents a novel approach to optimize the mechanical properties of additively manufactured face-centered cubic phase metallic materials.

• Thermomechanical processing induces phase decomposition in TiNbTa 0.5 ZrAl 0.5 alloy. • The phase decomposition behavior strongly depends on forging temperature. • Phase decomposition affects precipitation and solid solution strengthening at 25℃. • Phase decomposition decreases high temperature strength by grain boundary sliding. The mechanical properties of refractory high entropy alloys (RHEAs) strongly depend on their phase structures. In this work, the phase stability of a BCC TiNbTa 0.5 ZrAl 0.5 refractory high entropy alloy subjected to thermomechanical processing was evaluated, and the effects of phase decomposition on room/high temperature mechanical properties were quantitatively studied. It was found that, the thermomechanical processing at 800 °C and 1200 °C leads to phase decomposition in the TiNbTa 0.5 ZrAl 0.5 alloy. The phase decomposition is caused by the rapid rising of free energy of the primary BCC phase. The effect of the precipitates on room temperature strength is determined by the competition between the increasing in precipitation strengthening and the decreasing in solid solution strengthening. But at high temperatures (800−1200 °C), the phase decomposition causes significant reduction in strength, mainly due to the grain boundary sliding and the decreasing in solid solution strengthening. [ABSTRACT FROM AUTHOR]

Oxide dispersion strengthened (ODS) ferritic steel is a potential structural material for fast neutron reactor and fusion reactor application. In this study, a new Fe–13Cr–2W-0.4Ti-0.6Zr/0.35Y2O3ODS ferritic steel was designed and fabricated through hot isostatic pressing. Results show that the ODS ferritic steel consists of highly dispersive nano-oxide particles, identified as Y2(Ti0.4Zr0.6)2O7and Y4Zr3O12, with average particle size of 5.34 nm and particle distribution density of 4.2 × 1023 m−3. In addition, heterogeneous coarse/ultrafine grains (CGs/UFGs) structure is also achieved in the matrix. The Fe–13Cr–2W-0.4Ti-0.6Zr/0.35Y2O3ODS ferritic steel exhibits superior mechanical properties at both room temperature (σb = 1208 MPa, δ = 14%) and high temperature (σb, 700°C = 382 MPa, δ700°C = 32%). The high strength is mainly due to the synergistic effect of particle dispersion strengthening and back stress strengthening, while the good ductility can be attributed to the excellent deformation ability of the coarse grains and the back stress hardening effect induced by the heterogeneous CGs/UFGs structure.

Effects of Al and La elements on mechanical properties of CoNiFe0.6Cr0.6high-entropy alloys (HEAs) with face-centered cubic (FCC) structure were investigated by first-principles calculations. The variation curves of relevant physical parameters, including lattice constants, formation enthalpy, elastic constants, elastic modulus, Poisson's ratio, Pugh's ratio, Cauchy pressure, anisotropy factors, Vickers hardness, yield strength, and energy factor, as a function of Al and La concentration were discussed in detail. Results show that the resistances to bulk, elastic, and shear deformation decrease with the increase of Al and La concentration. The addition of Al and La can improve the plasticity and ductility of HEAs. Meanwhile, with the increase of Al and La concentration, the metallic characteristic of atomic bonding strengthens and the anisotropy of materials increases. Moreover, the energy factor indicates that the addition of Al and La could promote dislocation nucleation. This study provides a guideline for further research on non-equiatomic CrFeCoNi series HEAs with designed mechanical properties.

The hot deformation behavior of Ti–3Al–5Mo–4.5V alloy with an α′′martensitic microstructure was studied in the temperature range of 700–1000∘C and strain rates range of 0.001–1s−1up to a height reduction of 50%. The results show that an ultrafine equiaxed microstructure with an average grain size of 0.45μmwere successfully produced through thermomechanical processing of a martensitic starting microstructure. A processing map was successfully constructed and provides appropriate processing parameters for hot deformation which are located in the temperature range of 700–800∘C and the low strain rate range of 0.001–0.01s−1. The instability domain mainly occurs at the strain rate higher than 0.05s−1for the whole deformation temperature, which should be avoided during practice. The flow softening mechanism of the alloy is determined to be continuous dynamic recrystallization and the unstable flow is caused by the macro-fracture and flow localization.