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Post-bond heat treatment (PBHT) applied to a transient liquid phase (TLP) bonding joint is an effective approach to remove the brittle borides and improve its properties. Herein, we proposed two types of PBHT strategies to obtain a TLP bonded γ′-strengthened Co-based single crystal superalloy, and the microstructural characteristics and tensile properties of the two heat treated joints were compared to identify the optimal PBHT strategy. The evolution of the brittle boride in the joint after the PBHT was studied by using in-situ microscopy. The experimental results allowed to provide a theoretic model to quantitatively evaluate the distribution of the brittle phase after the optimal PBHT and analyze the joint fractures to understand the failure mechanisms. The obtained results revealed that a post-bond solid solution treatment performed to the joint at a high temperature (over 1275 °C) could decrease the area fraction of the boride from 7.2 % to 1.4 % and increase the elongation from 1.9 % to 7.8 %. This work emphasizes the relevance of solid solution temperature when a PBHT strategy is applied.

Both surface and internal microstructures of a second-generation Ni-based single crystal (SX) superalloy were studied after creep and rejuvenation heat treatment (RHT). It is indicated that the microstructures, such as the dislocation network, the γ phase and the γ' phase, can be recovered to those after the standard heat treatment (SHT). It is found that RHT affected zone (RAZ) formed at the surface is composed of the γ'-free layer, the transition layer and the recrystallization (RX), which are less than 20 μm in depth totally. Such depth of the RAZ doesn’t affect the properties of the superalloy. The morphology of γ' phase at the RAZ is related to the composition of the elements. The average creep life after RHT is close to the average life after SHT. It is concluded that RHT could effectively repair SX parts and increase the total life of the sample after a damage by creep.

The solidification paths, microstructures and mechanical properties of two Co-Al-W base superalloys with different additions of tungsten and molybdenum were investigated. Mo addition does not influence the volume fraction of the γ′ phase; however, the morphology of the γ′ phase changes from cuboidal to near cuboidal. The μ phase and β phase were identified in both alloy systems. Although the melting point of the μ phase is decreased by Mo addition, the solidification paths remain the same: L→ L1 + γ → L2 + γ + μ → γ + μ + β → γ + μ + β + γ′. The precipitation of a γ′ phase envelop around the interdendritic regions in Co-Al-W alloy systems has never been reported. The formation of a surrounding γ′ phase was caused by the diffusion of elements between secondary phases and the substrate during the heat treatments. The microhardness of the μ phase, which can be easily influenced by Mo addition, is the highest in both alloys, followed by the microhardnesses of the β phase and the substrate. In the fracturing process, microcracks were generated in the μ phase, the number of which increased with Mo addition.

The LCF behavior of a fourth-generation Ni-based single-crystal superalloy at elevated temperature with different total strain amplitudes has been studied. Notably, the experimental alloy demonstrated a significant work hardening behavior with the total strain amplitude increasing. A great number of SFs with different configurations in the γ matrix was observed. The nature of work hardening was conclusively shown to be dependent upon the high density of SFs and Lomer-Cottrell dislocations in the γ matrix, which is rarely reported previously. This finding reported in this work provides us a new idea on high performance single-crystal superalloy design.

The influence of withdraw rate during directional solidification and elements Re and Ti on the formation of micropores during homogenization in Ni base single crystal superalloy CMSX-4 is investigated by X-ray tomography method. The results show that the number of micropores below 5 μm in sample solidified at 10 mm/min after homogenization at 1300 °C/3 h is greater than that in the case of 6 mm/min. The number of micropores below 10 μm in Re free and Ti free alloys is rather smaller than that of normal CMSX-4 alloy, while the number of micropores between 15 μm and 25 μm in Ti free alloy is relatively greater than that in Re free alloy. The variation of number of micropores is analyzed based on measurement of primary dendrite arm spacing (PDAS) and segregation ratios of alloying elements, the causes are mainly attributed to reverse segregation tendencies of Re and Ti and their remarkable diffusion coefficient difference, the increasing of PDAS also enhances the propensity of micropores formation during homogenization.

Directed energy deposition (DED) additive manufacturing process has been attractive to the fabrications of FeCrAl alloys due to the advantage of near net shaping of complex shaped components. In order to improve the strength, TiC particles strengthened FeCrAl alloys have been fabricated by DED using blended FeCrAl powder and TiC nanoparticles. The results show that the TiC particles in the DED alloys have submicron sizes, and polygonal or dendritic shapes. Most TiC particles are distributed along the dendritic/grain boundaries. The additions of TiC particles effectively refine the grain sizes, and at the same time prompt a columnar to equiaxed transition of grains. The grain sizes are reduced from 1377 μm to 47 μm, 21 μm and 22 μm when the TiC particle contents in the DED alloys increase from 0 wt% to 0.7 wt%, 2.4 wt% and 3.5 wt%, respectively. The room temperature strength of the FeCrAl alloys increase with the increase of TiC particle content. With 3.5 wt% TiC particle content, the FeCrAl alloy has good mechanical properties with yield strength of 437 MPa, ultimate tensile strength of 688 MPa, and an elongation of 19 %. The 650 ℃ high temperature strength of the FeCrAl alloys first decrease and then increase with the increase of TiC particle content. With 3.5 wt% TiC particle content, the FeCrAl alloy has excellent mechanical properties with yield strength of 208 MPa, ultimate tensile strength of 223 MPa, and an elongation of 53 %. The present study provides an important basis for fabricating high strength particle-strengthened FeCrAl alloys using the DED process.

The nickel-rich cathode materials (LiNi0.8Co0.15Al0.05O2, NCAs) in Lithium ion batteries often suffer from oxygen loss problems under partially overcharged conditions (>4.3 V), leading to capacity fading and safety problem. Herein, we utilize a facile ball-milling method to coat the NCAs with a shell of cerium aluminum composite oxide (CeO2-Al2O3), which is commonly used as oxygen storage material. Proved by in situ X-ray diffraction results, the coating CeO2-Al2O3 layer is capable to suppress the lattice volume change and irreversible phase transition of NCAs. The CeO2-Al2O3 coating layer can adsorb released oxygen under overcharged conditions and replenish lattice oxygen of NCAs under oxygen-leaned conditions. The protective layer can also prevent dissolution of the transition metals caused by HF attack in electrolytes. As a result, the as prepared electrode presents high specific capacity of 186.4 mA h g−1 and excellent cycle stability with capacity retention of 94.1% after 300 cycles at a rate of 0.5 C under overcharged conditions (3.0–4.5 V). Coating CeO2-Al2O3 is an efficient strategy to solve capacity fading problem and safety problem caused by partially overcharge, and such strategy shows significant promise for practical applications.

Nanoindention tests and SEM microstructure observations were conducted on a single crystal nickel base superalloy after laser shock peening (LSP). Distinct surface hardening behavior was found to occur under the selected LSP technology. A large discrepancy in γʹ areas happened on laser shocked regions and the large plastic deformation embodied in γʹ phases’ deformation brought a significant hardening effect.

This investigation focused on the tensile deformation behavior of a single crystal nickel-base superalloy, both in virgin condition and after laser shock processing (LSP) with varied technology parameters. Nanoindention tests were carried out on the sectioned specimens after LSP treatment to characterize the surface strengthening effect. Stress strain curves of tensile specimens were analyzed, and microstructural observations of the fracture surface and the longitudinal cross-sections of ruptured specimens were performed via scanning electron microscope (SEM), in an effort to clarify the fracture mechanisms. The results show that a surface hardening layer with the thickness of about 0.3–0.6 mm was gained by the experimental alloys after LSP treatment, but the formation of surface hardening layer had not affected the yield strength. Furthermore, fundamental differences in the plastic responses at different temperatures due to LSP treatment had been discovered. At 700 °C, the slip deformation was held back when it extended to the surface hardening layer and the ensuing slip steps improved the plasticity; however, at 1000 °C, surface hardening layer hindered the macro necking, which resulted in the relatively lower plasticity.

In this work, the influences of alumina addition on cristobalite crystallization and properties of injection molded silica-based ceramic cores were investigated. X-ray diffraction (XRD) was used to characterize phase transformations in the samples, and the XRD result indicated that the addition of alumina promoted crystallization of fused silica during sintering at 1180–1220 °C and thus increases the amount of cristobalite. The increased amount of cristobalite as well as alumina addition led to much more thermal dilation due to their higher coefficients of thermal expansion than that of fused silica. The flexural strengths at room temperature and 1500 °C were tested, and it was shown that alumina addition could not affect room temperature strength, but decreased the flexural strength at 1500 °C. In addition, deflection resistance during heating to high temperatures was investigated, and the result indicated that alumina addition speeded up high temperature softening of the samples. XRD and scanning electron microscopy equipped with energy dispersive spectrometry (SEM/EDS) analysis suggested that this softening behavior was related with viscous flow sintering which could be accelerated by the reaction of alumina and silica with a product of mullite.

An evaluation was conducted on the impact of a Pt–Al bond coat on the creep behaviours of a second-generation Ni-based single crystal superalloy at 750 °C/820 MPa, 850 °C/630 MPa, 1038 °C/137 MPa and 1100 °C/112 MPa, respectively. As revealed by the creep results, the creep behaviours of the coated superalloy were inferior to those of the bare superalloy to some extent under all testing conditions. The maximum deterioration in the strain-to-fracture and time-to-rupture was observed at 1100 °C/112 MPa (7.3%) and 750 °C/820 MPa (74.7 h), respectively. Following the creep test, deconvolution calculations were performed, which indicated that the creep stress for all core superalloys was as low as approximately 1.7–1.9% of the applied stress while the bond coat stress was 4.0–10.4% that at the minimum creep rate. Moreover, TEM analyses revealed that within the superalloy substrate adjacent to the bond coat, there were fewer stacking faults and an abundance of dislocation debris observed within γ′-Ni3Al at 750 °C/820 MPa. The misfit of the γ/γ′ phase was insignificant and the dislocation networks became irregular at 1100 °C/112 MPa. With an increase in temperature from 750 °C to 1100 °C, the fracture mechanism of both the substrate and bond-coat was found to shift from quasi-cleavage to micro-void coalescence. The quasi-cleavage cracks occurring within the bond coat increased the number of connections with creep cracks to cause spreading from the interior of the samples, thus resulting in premature failure at 750 °C/820 MPa and 850 °C/630 MPa. However, the degradation at 1038 °C/137 MPa and 1100 °C/112 MPa was ascribed to the increasing thickness of the inter-diffusion zone, as well as the volume of the blocky γ′-Ni3Al phase inside the bond coat and topologically close-packed phases in the superalloy near the bond coat.

Statistical analysis of white-light interferometry (WLI) experiments performed on Ni-based single-crystal superalloys (SX) have revealed a different height distribution between the dendritic cores (DCs) and the interdendritic regions (IRs) of the polished samples. The micromechanical property difference is largely ascribed to the uneven distribution of the alloying elements. In this context, possible reasons for this difference are discussed by comparing with different experiment results obtained by previous researchers, and a proposal forecasting the hardness ratio of IRs/DCs is put forward.

Surface topography of an upsetting wrought superalloy processed by laser shock peening was observed in microscale and a series of morphological characteristics such as surface reliefs with special orientation, destructive dimples were found. The longitudinal sizes of the micro surface structures are within 100 nm to 1000 nm generally. The mechanical response laws of metal materials under laser shock were proposed to explain the emergence of the special micro-structure characteristics. It can be concluded that the formation of surface reliefs is a release way of the internal stresses under laser shock and small dimples are deemed as an external representation of the release of impact energy in the mode of fracture damage.

Surface topography and microhardness of a Ni-based single crystal superalloy subjected to laser shock peening (LSP) were investigated. So many morphological surface reliefs were found on the shocked surface, and a microscopic formation mechanism of those surface reliefs was proposed. The formation of surface reliefs can be attributed to the plastic deformation of γʹ precipitates and γ matrix channels in the lateral or longitudinal directions. The nonuniformity of structures of the deformed surface layer was ascribed to the formation of surface reliefs. The microhardness of surface reliefs was higher than that of adjacent matrixes, so a nonuniformity of micromechanical properties coexisted on the surface of sample subjected to LSP. With the increase of LSP impact times, micromechanical properties difference between surface reliefs and adjacent matrixes widened due to a very high proportion of the longitudinal deformation of γʹ phases contained in the surface relief structures.

The microstructure and corrosion fatigue behavior under artificial seawater environment of 100-mm-thick Ti–6Al–4V alloy joint via vacuum electron beam welding (EBW) was systematically characterized. Obvious microstructural heterogeneity along the transverse direction of as-welded joint was observed. Bimodal-phases microstructure of granular α colonies and β strips compose the microstructure of the base metal (BM). While the microstructure of fusion zone (FZ) comprises single-phase acicular α/α′ martensite. Under corrosive environment of artificial seawater, FZ shows a lower growth rate of corrosion fatigue cracking (CFC) than that in the BM. Increasing content of residual gas contamination worsens the resistance against corrosion fatigue of the FZ of Ti–6Al–4V alloy joint. Results of local electrochemical characterization indicates that the FZ shows a better resistance against electrochemical corrosion than that of the BM. It is deduced that the slower growth rate of CFC observed in the FZ originates from the combined effects of the excellent corrosion resistance of martensite microstructure and the obstructions of interfaces of fine α/α′ laths against growth of CFC. Moreover, the growth mechanisms of CFC in the BM and FZ were discussed involving the microstructural heterogeneity of the EBW joint.

Transient liquid phase bonding of γ′-strengthened Co-based superalloy was carried out using Ni-based filler alloy. The effects of post-bond heat treatment (PBHT) on the microstructure and mechanical properties of the γ′-strengthened Co-based superalloy joint were systematically investigated. Two distinct regions were observed in the isothermally solidified zone: primary γ′ phase zone (PZ) and secondary γ′ phase zone (SZ). The diffusion of Al from the substrate to the liquid interlayer proves to be the main reason for the precipitation of γ′ phases, showing a significant growth trend after the PBHT. The joint after the PBHT shows higher microhardness in most areas of the bonding zone. However, some parts of the diffusion affected zone show the contrary result. This could attribute to the competition consequence of the solid solution strengthening and second phase strengthening. Furthermore, the post-bond heat treated joint exhibited higher yield strength and longer stress rupture life. Our experimental results help to better understand the microstructural characteristic of the new type Co-based superalloy and provide a direction for further optimizing the brazing joint.

The oxidation behavior of a novel low-cost single crystal superalloy was thoroughly studied at high temperature. Three distinctive stages were observed in the oxidation kinetics curve, which corresponded to the different oxide film structures and oxidation mechanisms. The outer NiO grains, several spinels as well as inner Al2O3 layer took shape at first stage while the oxidation dynamics obeyed cubic law. However, the NiO particles began to spall off the specimen surface and the continuity of Al2O3 layer was partly destroyed in the next several hours. The alloy went through severe oxidation weight gaining at second stage which could be rationalized by the severe oxidation of W and Mo. The non-protective NiWO4 and NiMoO4 spinels were surprisingly found and the Al2O3 particles demonstrated the rod-like morphology during this period. The re-formation of dense Al2O3 layer and the volatilization of MoO3 gave rise to the reduction of mass gaining rate in the final stage, additionally, the oxidation kinetics transformed to subparabolic characteristic. Moreover, the experimental alloy exhibited good microstructural stability at elevated temperature, however, the microstructure degradation was considerably serious. The entire thickness of γ′-free layers had exceeded 0.1 mm on the alloy surface after 200 h oxidation at 1120 °C.

An investigation was conducted into the influence of applying Pt Al bond-coat on the tensile deformation and fracture behavior of second-generation single crystal superalloy substrate with the temperature ranging between room temperature RT and 1100 °C. In the meantime, the development of a two-layer Pt Al bond-coat, consisting of an outer layer of single-phase β-(Ni,Pt)Al and an inter-diffusion zone of refractory metal precipitates, has also been discussed. As revealed by the results, the application of Pt Al bond-coat contributed to a noticeable deterioration in both yield strength and ultimate tensile strength of the superalloy substrate at elevated temperatures. Nevertheless, the elongation of coated superalloy was shown to be 17.2% (at RT) and 10.4% (at 750 °C) lower and 9.6% (at 1100 °C) higher compared to uncoated samples. Following tensile test, the TEM analyses indicated that there were onsets of various slip systems, the traditional superlattice stacking faults were retarded at intermediate temperature and the dislocation debris could be increased to develop dislocation networks at higher temperature in the region of superalloy near the bond-coat. As the temperature was on the rise, the fracture mechanism of coated superalloy shifted from tearing ridges and dimples to quasi-cleavage and then to micro-void coalescence. While that of Pt Al bond-coat was transformed from brittle to ductile rupture. Within the temperature range from RT up to 750 °C, a combination of long crack lengths and greater crack penetration depths into bond-coat caused more deterioration in the tensile properties of superalloy. When the temperature rose above 750 °C, the application of Pt Al bond-coat improved the ductility of the substrate, as a result of the ductile failure in the bond-coat which hindered the development of through-thickness cracks.

The affordability has become a key element in the development of the modern aero-engines thus the design and research of low-cost single crystal superalloys are in great demand. A kind of novel Nickel-based single crystal superalloy with cost reduction was designed in this work and the temperature dependence on the microstructure modification as well as corresponding deformation mechanisms during tensile tests were systematically investigated. The experimental alloy exhibited a remarkable yield strength of 912 MPa but relatively poor ductility at 760 °C. At higher temperatures, an overt strain softening occurred before the tensile rupture and the fracture features were identified as dimples induced by the accumulated micro-pores. The stacking faults shearing mechanism prevailed at room temperature and there presented two types of stacking faults in the γ′ precipitates. Both decomposition and cross-slip of the a/2 superdislocation were observed at 760 °C while the deformation mechanism was controlled by APB-coupled dislocation pairs shearing the γ′ phase at 980 °C. With temperature increasing to 1100 °C and 1120 °C, the amount of shearing dislocation pairs decreased dramatically, besides, the interfacial dislocation networks and rafted γ/γ′ structures were formed. The degradation of mechanical properties was considerably slight from 1100 °C to 1120 °C, however, three primary microstructure modifications were emphasized.

The aim of the study was to investigate the effect of laser shock peening (LSP) on surface topography evolution of metallic targets. Samples manufactured by a Ni-based single crystal superalloy with polished finish were treated by LSP, and the surface topographies before and after LSP were examined by non-contact White-Light Interferometer (WLI). Results showed the following three aspects: (a) By taking advantage of WLI, the shrinkage porosities and the interdendritic structures were observed simultaneously. (b) With the increasing impact times, the round pit induced by laser shock became deeper. (c) The nano-scale surface reliefs were found on the bottom of round pit induced by LSP, and the specific plastic flow of metallic materials under the action of compressive stresses was deemed as the primary contributor to the formation of surface reliefs. It revealed a novel microscale plastic deformation phenomenon of metallic materials in surface strengthening.

The correlation between the coarsening of γ′ precipitate and creep properties during long-term aging at 1000 °C for 100–1000 h of a fourth-generation nickel-based single crystal superalloy was investigated. The average size of γ′ precipitate increased with the extension of aging time and the morphology of γ′ precipitate kept cuboidal shape after 1000 h. Experimental observation on the coarsening of γ′ phase demonstrated that the relationship between r t 3 − r 0 3 and t was in accordance with the LSW theory and the coarsening rate k was about 5954 nm3/h. The creep life of aged samples at 1140 °C/137 MPa increased firstly and then decreased with the exposure time prolonging. The casting pores and creep pores were observed on the fracture surface of samples after creep rupture. Compared with the creep pores, the microcracks formed at the edge of casting pores had greater effect on the creep life. The creep properties of samples during aging were mostly affected by the size of γ′ phase. The promotion of creep life of aged sample was rationalized by the slightly grown γ′ phase, the diffusion of the refractory elements and the accumulation of dislocations in the γ channel. The decrease of creep life was predominantly affected by the seriously coarsened γ′ phase because the significant increase of the size of γ′ phase leaded to a decrease in the resistance of dislocation movement.

The effects of Ru on microstructure and creep properties of the two alloys were investigated in detail. According to the creep curves of the two alloys at 1140 °C/137 MPa, the creep rupture life was significantly improved with the increase of Ru and the mechanism of each stage during the creep deformation was different. Thus, the evolution of the γ' phase, the dislocation configuration, and the effect of Ru on solution strengthening were discussed. The γ/γ' lattice misfit of the initial microstructure presented increasingly negative as the content of Ru increased, which resulted in smaller and more regular γ' particles in the initial state. Meanwhile, more consistent and denser dislocation networks on the γ/γ' interface during creep deformation were examined. Hence, the addition of Ru decreased the minimum creep rate and prolonged the secondary creep stage. Moreover, the so-called “reverse partitioning” behavior and enhancement of γ matrix strength appeared with the increase of Ru. When the alloy contained 2.5 wt% Ru, the rapidly increasing in creep strain rate induced by the topological inversion appeared. When the alloy included 3.5 wt% Ru, the needle-like and rod-like topologically close-packed (TCP) phases were observed occasionally. The stress and the supersaturation of refractory elements resulted in the precipitation of the TCP phase. The dramatic increase of the creep strain rate was principally related to the propagation of micro-cracks around the casting and creep voids in the necked regions.

Two novel single crystal superalloys were designed to obtain affordable third generation single crystal superalloys. Creep tests and long-term thermal exposure were utilized to evaluate the microstructure and creep properties of the experimental alloys. Moreover, the role of Re in the microstructure and creep performance was investigated. Interestingly, the results showed that the two experimental alloys with different addition of Re demonstrated similar creep properties under the condition of 1120 °C/137 MPa. The alloy with 1 wt% Re owned much better microstructural stability for the rare presence of the TCP phase. However, With higher volume fraction of the γ′ phase and denser γ/γ′ interfacial dislocation networks, the alloy with 2 wt% Re addition is believed to be provided with better precipitation strengthening and interfacial strengthening effects, which strongly guarantee the mechanical properties and hinder superdislocations shearing into the γ′ phase. Affected by the synergistic effects of their strengthening and weakening factors, the two experimental alloys exhibited few difference in creep rupture life under elevated-temperature and low stress condition. Important guidance for the further designs of single crystal alloys could be summarized from the present study.