查看更多>>摘要：? 2022 Elsevier Inc.The components of power generations are subjected to the repeated thermal strain, owing to the temperature fluctuations associated with start-up and shut-up. Therefore, fatigue properties of the candidate materials are essential to be understood. To fully elucidate the fracture mechanism and microstructure evolution of the Nickel-based alloy under high-temperature cyclic loading, low cycle fatigue tests were carried out under the strain amplitude of 0.4%, in which temperature was in the range of 700–800 °C and strain rates from 5 × 10?4 s?1 to 5 × 10?3 s?1. The fatigued specimens were characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), and electron backscatter diffraction (EBSD) observations. Slip band deformations mainly dominated under the temperature of 700 °C and 720 °C, and paralleled dislocation transmissions around twin boundaries were dominant under 750 °C. While free dislocation lines were typical dislocation configurations in 800 °C. Due to stress concentration caused by the accommodated dislocations, some cracks preferred to propagate along the twin boundary. Molecular dynamics simulated the dynamic interaction between dislocation and twin boundary at 750 °C. Dislocation movement was constrained on the twin boundary, which was verified by molecular dynamics simulations. As the temperature increased, the strength of the grain boundary degraded, and the fracture pattern changed from totally transgranular to both transgranular and intergranular cracking.
查看更多>>摘要：? 2022 Elsevier Inc.In this study, we present a micro-mechanical investigation of cold-drawn reduced graphene oxide (RGO) reinforced aluminum (Al) composite wires from 293 K up to 523 K. Room temperature micro-tensile, high-temperature (HT) micro-pillar compression, and HT nanoindentation tests revealed the superior mechanical properties of the composite compared to the unreinforced Al sample over the whole temperature range. Complementary thermal stability measurements and associated microstructural analysis proved that RGO effectively prohibited grain growth by grain boundary pinning. The RGO-Al composite wires are shown to be much stronger, heat-resistant, and easier to fabricate than conventional HT Al alloy-based conductors with adequate conductivity.
查看更多>>摘要：? 2022 Elsevier Inc.The effect of ram speed ranging from 1 mm/s to 10 mm/s on the microstructure and mechanical properties of a Mg-11Gd-3Y-0.5Nd-Zr (wt%) alloy was investigated using optical microscopy, scanning electron microscopy, electron backscatter diffraction analysis, transmission electron microscopy, hardness testing and uniaxial tensile testing. The results indicate that a bimodal-grained structure composed of elongated deformed grains and fine recrystallized grains is formed in the 1 mm/s extruded sample. Then, the microstructure completely transformed into equiaxed grains when extruded at a speed higher than 5 mm/s, and the recrystallized grains become coarser as the ram speed increasing. Compared with the 5 mm/s and 10 mm/s extruded samples, the 1 mm/s extruded sample exhibits the best strength and ductility. After T5 treatment, the ultimate tensile strength, tensile yield strength and elongation are 456 MPa, 333 MPa and 8.9%, respectively. The excellent mechanical properties are attributed to grain refinement strengthening, texture weakening, bimodal-grained structure and age hardening. The key to form a bimodal-grained structure is to restrict the temperature rise by controlling the extrusion speed.
查看更多>>摘要：? 2022 Elsevier Inc.The tensile deformation behavior of a duplex Fe-Mn-Al-C low-density steel with a structure of banded ferrite and austenite has been studied in the present work. The steel exhibits a good strength-ductility match and a typical three-stage deformation behavior, under the joint contributions of transformation-induced plasticity (TRIP) effect, twinning-induced plasticity (TWIP) effect and dislocation slipping. Even though the steel has a stacking fault energy of 53 mJ?m?2 that is suitable for the TWIP effect, it just worked at the final deformation stage. The TRIP effect played an important role during the majority of deformation process except at an initial stage, and the austenite was dramatically consumed at the later stage. This is mainly determined by an unnormal stability of austenite. For estimating the austenite stability, the relationship between the critical size of austenite grain (Dcrit) and critical temperature for strain-induced transformation (Msσ) was established. The calculated Dcrit of 12 μm reflects that the strain-induced martensitic transformation dominates the TRIP effect in the duplex steel.
查看更多>>摘要：? 2022The cyclic deformation behavior and fatigue fracture characteristics of cast Al-Si-Cu-T6 (A319-T6) alloys solidified at two cooling rates under uniaxial and multiaxial non-proportional loading conditions were investigated. The A319-T6 alloy exhibited cyclic hardening and additional non-proportional hardening effects during cyclic loading. The cyclic hardening rate and non-proportional loading sensitivity coefficient improved by 2.8% and 5.1%, respectively, when the solidification cooling rate was increased from 0.1 to 10 °C/s. To account for the hardening effects, the dislocation evolution and interaction between dislocations and other microstructures were analyzed. Based on the asymmetry parameter, the tension-compression symmetries of A319-T6 alloys subjected to uniaxial and multiaxial non-proportional loading were discussed. The presence or absence of a steady crack propagation area was considered to be the primary difference in the cross-section caused by cooling rates, leading to varied fracture flatness. Multiaxial non-proportional loading yielded reciprocating wear marks and a significant quantity of wear debris on the fracture surface. The majority of wear debris was identified as matrix oxide. The reciprocating wear process removed fatigue striation and secondary cracks that emerged on the uniaxial fracture, further increasing fracture flatness.
查看更多>>摘要：? 2022This work deals with the effect of different heat treatments on directed energy deposition (DED)-produced Ti-6Al-4V samples. Annealing treatments at 1050 °C followed by different cooling rates were conducted to allow a complete recrystallization of the microstructure and remove the columnar prior-β grains, thus increasing the overall isotropy of the material. An agine treatment at 540 °C was also performed for further microstructural stabilization. The microstructures, textures and mechanical properties were then assessed. Due to the heat treatments, greatly differing microstructures were achieved in an equiaxed grain morphology. However, a “grain memory” effect was detected which resulted in the grains size increasing along the height of the samples. This effect was correlated to the intrinsic prior-β grain width variation along Z on the as-printed specimens, typical of the DED technology. Electron backscatter diffraction analyses proved that the intensity of the preferential directions increased after the heat treatments, likely due to the crystallographic variant selection mechanisms taking place when the samples cool down from the annealing temperature. This effect is also influenced by the significant difference in terms of prior-β grains sizes between the heat-treated and the as-printed specimens. To sum up, a complete homogenization of the material via heat treatment above the β-transus temperature proved to be challenging. In fact, the data suggest that the intrinsic texture-related anisotropy granted by the manufacturing process is very difficult to be eliminated.
查看更多>>摘要：? 2022FeCoNiCrMn high-entropy alloy (HEA) with FCC structure was welded by continuous drive friction welding. The microstructure and mechanical properties of welding joints were investigated using OM, XRD, EBSD, TEM, microhardness, and tensile tests. The microstructure of welding joints samples consist of three zones: the weld zone (WZ), the thermal-mechanically affected zone (TMAZ) and the base metal zone (BM). The microstructure of WZ is consistent with that of BM, which is a single FCC solid solution. Discontinuous dynamic recrystallization (DDRX) and twin-induced dynamic recrystallization (TDRX) mainly occur in WZ and TMAZ. The hardness of TMAZ is significantly increased and higher than that in WZ and BM; the hardness of WZ is slightly higher than that of BM. The increase of the hardness is related to grain refinement. The tensile strength and uniform elongation of the joint samples are increased by 4.2% and 31% respectively compared with the results of the base metal. The results exhibited ductile fracture.
查看更多>>摘要：? 2022 Elsevier Inc.Compared with the phase transformation and twinning, the single-slip, cross-slip and the interaction between slip and grain boundary in metastable refractory high entropy alloys (RHEAs) still need to be explored in sufficient detail. In this work, the deformation mechanisms of two metastable RHEAs (TiZrHfNb0.4 (denoted as Nb0.4) and TiZrHfNb0.6 (denoted as Nb0.6) RHEAs) during tensile deformation were thoroughly investigated by interrupted in-situ EBSD. The results revealed that the ultimate tensile strength (UTS) of the Nb0.4 sample is higher than that of the Nb0.6 sample. This implies that the UTS can be optimized by reducing the content of Nb element in TiZrHfNb RHEA. For the Nb0.4 sample, the deformation mechanism is dominated by the deformation of grain boundaries firstly and then by the cross-slip behavior. The cross-slip system is {112}〈111ˉ〉/{123}〈111ˉ〉 slip system. In addition to the cross-slip, the kinking bands also were found to be another important deformation mechanism during the tensile. For the Nb0.6 sample, the deformation mechanism is dominated by the deformation of grain boundaries and the single-slip behavior. The slip system is {112}〈111ˉ〉 slip system. The difference in the slip behavior mainly contribute to the difference in the UTS between the two types of samples. This work is helpful to enrich the understanding on deformation and contribute to better design strategies of high strength RHEAs.
查看更多>>摘要：? 2022 Elsevier Inc.ZnO-CC and ZnO/Bi2WO6-CC with different Bi2WO6 loadings layered heterostructures photocatalysts were synthesized on flexible carbon clothes (CC) substrate by the thermal decomposition-hydrothermal method. By characterizing the degradation of methylene blue (MB) under ultraviolet light irradiation, Z3B-CC (3 wt% Bi2WO6) showed the best photocatalytic effect, and the photodegradation rate of MB was as high as 96.9% in about 100 min. This new ZnO/Bi2WO6-CC layered heterostructure exhibits prominent photocatalytic performance, which is mainly ascribed to the type II band structure formed by ZnO and Bi2WO6. The appropriate energy band position further facilities the effective transfer of carriers and realizes the photocatalytic process of the heterostructure. This study is prospective to provide new ideas for the synthesis of other high-efficiency heterostructure photocatalytic designs, and it also has reference value for the synthesis of flexible photocatalytic materials with excellent performance.
查看更多>>摘要：? 2022 Elsevier Inc.TiC reinforced Ti-based composite coatings were prepared on the Ti6Al4V surface via laser cladding with Ti6Al4V/NiCr-Cr3C2/Y2O3 mixed powders. Influence of Y2O3 on forming quality and microstructure of composite coatings was investigated systematically. The cracks were eliminated completely and the number of pores was drastically minimised when 2 wt% Y2O3 was added. In addition, the reinforcement TiC and the matrix sloid solution β-Ti were the main precipitated phases within the coating. The addition of Y2O3 inhibited the generation of Ti2Ni at the grain boundary, however, promoted the generation of Cr1.93Ti1.07 in the coating. With the addition of Y2O3, the dendrites became largely developed and a large number of tertiary dendrites were found. Besides, a more obvious regional segregation appeared in the distribution of Ni and Cr elements in the coating with 2 wt% Y2O3. Furthermore, some nano-sized β-Ti were distributed on the surface of small TiC particles in both coatings. Combined with the theoretical analysis of the two-dimensional mismatch degree, the β-Ti and TiC interface exhibited a semi-coherent relationship. Therefore, β-Ti can nucleate with TiC as the heterogeneous nucleation matrix. In the meanwhile, there was a certain dependent growth relationship between the β-Ti and TiC.
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