Yanbin Zhang

This study examines the impact of supersolidus solution heat treatment (SSHT) on the suppression of topologically close‐packed (TCP) phases and the regulation of γ′ phase size in two CMSX‐4 nickel‐based single‐crystal superalloys with varying Re contents (3% and 5%). The results indicate that increasing the solution termination temperature or extending the holding time effectively suppresses TCP phase formation. In the 5% Re alloy, TCP phases that form after 35 h at 1340°C are eliminated by either extending the holding time to 50 h or raising the temperature to 1350–1360°C. The average γ′ phase size is influenced by the melting zone. In the 3% Re alloy, it shows little variation. While in the 5% Re alloy, it initially decreases, then increases at 1340°C, reaching a minimum of approximately 0.36 µm after 35 h. At 1350°C, the size refines from 0.41 to 0.34 µm between 15 and 35 h, but subsequently coarsens and spheroidizes to approximately 0.52 µm after 50 h. Higher temperatures expedite the process of spheroidization. This research emphasizes that SSHT effectively suppresses TCP phases. Nonetheless, excessive temperature or prolonged duration leads to spheroidization of the γ′ phases due to excessive melting.

The solution heat treatment (SHT) of the third generation of single crystal (SC) Ni-based superalloys required up to 45 h and was expensive. In this study, SHT based on liquid metal cooling (LMC) was optimized to greatly reduce processing time. The experimental and simulation results showed that residual segregation was evidently reduced, e.g., from 2.12 to 1.22 for the most heavily segregated Re. This led to a 16.7% increase in creep life, more uniform microstructures, and a decrease in solidification and homogenization porosity by a factor of 3.4. Structural refinement, approximately 0.32 times, served as the underlying mechanism for this optimization, which reduced diffusion distance and increased homogenization efficiency during SHT.

The roles of traveling magnetic fields (TMFs) within the transport phenomena during the directional solidification of nickel-based superalloys were simulated. The evolution of thermal field, flow field and solid-liquid interface morphology during the solidification process under both natural and forced convection conditions were also simulated and compared. The strength of TMFs window that suppresses the flow of the interfacial front in the melt was quantified. The association between flow velocity at the interface front and defect formation was discussed.

The tensile and compression fatigue deformation tests of AZ31 magnesium alloy were carried out at room temperature, and the activation behavior of {10-12}-{10-12} secondary twins in different loading directions (ND normal direction, RD rolling direction) and different strain amplitude (0.5%, 1%) was studied. The results showed that {10-12}-{10-12} secondary twins were observed on the fracture surface of the fatigue samples in the ND at the strain of 1%. In contrast, no secondary twins were observed in the ND at the strain of 0.5% nor in the RD at the strain of 1% and 0.5%, and the fatigue life in the ND was greater than that in the RD. The analysis showed that the generation of {10-12}-{10-12} secondary twins was associated with the grain size and loading direction and that the fatigue deformation at 1% strain was more likely to generate {10-12}-{10-12} secondary twins due to the large lamellar thickness of the primary twins generated at relatively large strain conditions. The fatigue deformation along the ND tension was more favorable for the activation of multiple {10-12} primary twins, and the interaction of multiple twin variants was more favorable to activate {10-12}-{10-12} secondary twins.

The porosity deteriorates the mechanical properties of nickel‐based superalloys, especially the fatigue properties. Herein, the kinetics model of homogenization porosity growth is optimized. The corresponding diffusion processes during solution heat treatment are simulated by the DICTRA software and verified by experiments. The results show that the homogenization porosity increases first and then decreases during the solution heat treatment, due to the imbalanced diffusion flux between dendritic core (DC) and interdendritic areas (IAs). The diffusion flux first flows toward DC and then toward IAs after fully homogenization of the fast diffusion elements. Moreover, the influence of directional solidification parameters on the homogenization porosity process is investigated. With the increase in the withdraw rate, the dendritic structure is refined, and the S‐porosity decreases. Meanwhile, the dendritic arm spacing decreases; thus, homogenization accelerated increases. In this case, after the solution heat treatment, the homogenization porosity decreases. The results provide two bases to propose a new method to control the homogenization porosity.