Gang Chen

Novel structural materials, high-entropy alloys (HEAs), have attracted considerable interest owing to their tunable microstructural designs and adjustable mechanical properties. In the present work, the microstructural evolution and tensile deformation behavior of FeCoNiCrTi_0.2 HEA are comprehensively examined through cold rolling (with 80% thickness reduction) followed by annealing, combined with multiscale characterization techniques (EBSD/TEM) and mechanical tests. The results reveal that the as-rolled microstructure was characterized by the presence of strong Brass, Goss/Brass, and S textures, along with the formation of high-density dislocation walls (DDWs) and dislocation cells (DCs). As the annealing temperature increased, recrystallized grains preferentially nucleated at grain boundaries with higher stress concentrations and dislocation densities. The grain size decreased from 120.33 μm in the as-rolled state to 10.26 μm after annealing at 1000 °C. Low-angle grain boundaries (LAGBs) progressively transformed into high-angle grain boundaries (HAGBs), while the fraction of Σ3 twin boundaries initially decreased and subsequently increased, reaching a maximum of 43.7% after annealing at 1000 °C. At annealing temperatures exceeding 800 °C, deformed grains became equiaxed, with partial retention of primary texture components observed. After annealing at 1000 °C, the yield strength and tensile strength decreased compared to the as-rolled state, while the elongation significantly increased from 17.2% to 69.8% Simultaneously, the yield ratio decreased by 53%, and the strain-hardening capacity was enhanced. Ultimately, a constitutive model integrating the influences of dislocation mean free path and twin boundary obstruction was developed, providing microscopic explanations for the inverse relationship between strength and recrystallization fraction.

As the key materials of aircraft engines, nickel-based superalloys have excellent comprehensive properties. Mircotensile experiments were carried out based on in situ digital image correlation (DIC) and in situ synchrotron radiation (SR) technique. The effects of the δ phase on the grain orientation, surface roughening, and strain localization were investigated. The results showed that the average kernel average misorientation (KAM) value of the fractured specimens increased significantly compared with that of the heat-treated specimens. The surface roughness decreased with an increasing volume fraction of the δ phase. The strain localization of specimens increased with the increasing ageing time. The size and volume fraction of voids gradually increased with the increase in plastic strain. Some small voids expanded into large voids with a complex morphology during micro-tensile deformation. The needle-like δ phase near the fracture broke into short rods, while the minor spherical δ phase did not break. The rod-like and needle-like δ phases provided channels for the propagation of the microcrack, and the accumulation of the microcrack eventually led to the fracture of specimens.

In this study, a new model involving energy is established to characterize the size effect on flow stress. The new model treats the experimental machine and the specimen as an isolated system, and this isolated system satisfies the Energy Conservation Law. The total work performed on the specimen by the experimental machine is nearly equal to the energy consumed by the specimen plastic deformation and the energy consumed by friction (which can be ignored when working without friction). The new model predicts the energy consumption of the specimen deformation by quantifying the total energy input to the specimen by the experimental machine and then obtaining the relevant parameters of the constitutive model. Through uniaxial tensile tests of pure nickel thin sheets with various thickness/average grain sizes (t/d), the new model was used to optimize the parameters of the existing constitutive model that predicts the flow stress of specimens with different t/d. The prediction accuracy of the optimized constitutive model is improved, especially for specimens with a t/d < 1. The new model is established from the perspective of energy input to avoid the analysis of the material deformation mechanism and improve the prediction accuracy.

Nickel-based superalloys are widely used in aerospace and other fields due to their excellent properties. In this study, the aging treatment and tensile tests of a GH4169 alloy were carried out. The effects of the δ phase on the alloy’s mechanical properties and fracture behavior were studied. The results showed that the appearance of the δ phase changed from a short rod shape to a needle shape with an increase in aging time. The precipitation method changed from a single mode of precipitation along the grain boundary to two modes of precipitation along the grain boundary and direct precipitation inside the grain. The yield strength and ultimate tensile strength of the alloy first increased and then decreased with an increase in aging time and were related with the microstructure of the δ phase. The similar Widmanstatten structure was not conducive to the mechanical properties of the alloy. The distribution of the δ phase led to the generation of inhomogeneous strain and limited the surface roughening during plastic deformation. The voids initiated at the interface between the δ phase and the matrix γ phase or directly from the δ phase fracture site.

With the increasing demand for micro parts using metal laminates in modern production, the manufacturing processes of thin sheet parts have been elevated. However, it is difficult to predict the deformation behavior with miniaturization because of size effects in micro-scale. In this study, the flexible die micro V-bending behavior of Cu/Ni clad foils was investigated. The bending experiments with three different punch angles and Cu/Ni clad foils under different annealed temperatures were performed. The results show that the springback angle increases with the increase of bending angle and annealing temperature. The placement of Cu/Ni clad foils induced compressive stress results in the more obvious thinning of thickness and decreasing of springback angle. The interactive effects of the distribution of deformation zones and compressive stress induced by the interface layer result in the springback behavior of Cu/Ni clad foils during the flexible die micro V-bending process.

Nickel-based superalloys have become key materials for turbine disks and other aerospace components due to their excellent mechanical properties at high temperatures. Mechanical properties of nickel-based superalloys are closely related to their microstructures. Various heat treatment processes were conducted to obtain the desired microstructures of a nickel-based superalloy in this study. The effect of the initial microstructures on the tensile deformation and fracture behaviors was investigated via in situ digital image correlation (DIC) and synchrotron radiation X-ray tomography (SRXT). The results showed that the size and volume fraction of γ″ and γ′ phases increased with the aging time. The yield strength and the ultimate tensile strength increased due to the precipitation strengthening at the expense of ductility. The surface strain analysis showed severely inhomogeneous deformation. The local strains at the edge of specimens were corresponded to higher void densities. The fracture of carbides occurred owing to the stress concentration, which was caused by the dislocation accumulation. The fracture mode was dimple coalescence ductile fracture.

Nickel-based superalloys have been applied in many occasions due to perfect performance. The corrosion behaviors of such alloys always occur under actual service conditions, which seriously affect their normal service. In order to improve corrosion resistance of such alloys, Ce-sealed Ni-P coating was prepared. Ni-P coating and Ce-sealed Ni-P coating were compared through three-dimensional microscopic morphology and electrochemical measurements. The effects of rare earth on corrosion resistance of Ni-P coating were discussed. The results showed that Ce-sealed Ni-P coating possessed the excellent corrosion resistance. Ce-sealing in rare earth solution reduced the surface roughness of film layer, achieving a good sealing effect.