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Abstract In this work, a composite coating prepared by laser cladding on a Ni-17Mo-7Cr based superalloy was thermally exposed at 1073 K for 240 h. A detailed electron microscopy examination was performed to characterize the microstructure of the aged coating. Results indicate that: the original Mo 6 Ni 6 C dendrites in the aged coating were coarsened while the coarsening layers were essentially the Mo-deficient and Cr-rich Mo 6 Ni 6 C, due to low diffusivity of Mo. Long-term thermal exposure promoted a great amount of nano-sized Mo 6 Ni 6 C and micro-sized Cr 3 C 2 particles being precipitated in the interdendritic zones. In the aged coating, three types of Cr 3 C 2 were presented: discrete Cr 3 C 2 , Cr 3 C 2 close to the graphite and Cr 3 C 2 approaching to the Mo 6 Ni 6 C. The transformation of Mo 6 Ni 6 C to Cr 3 C 2 merely occurred in the newly formed Mo 6 Ni 6 C rather than in the original Mo 6 Ni 6 C dendrites, because deficiency of Mo and enrichment of Cr in the new Mo 6 Ni 6 C depressed their stability. Lastly, the mechanical behavior of the aged coating was characterized by Vickers hardness, nano-indentation and nano-scratch tests with the aim to unveil the effect of microstructural evolution on mechanical properties. Our findings provide basic information to understand the aged-induced microstructural modification and its effect on mechanical properties for a laser-induced coating prepared on the Ni-17Mo-7Cr based superalloy. The results obtained cannot only apply to the investigated Ni-based superalloys, but also to other Hastelloy series superalloys. Highlights • The coarsening layers around original Mo 6 Ni 6 C dendrites were Mo-deficient and Cr-rich Mo 6 Ni 6 C. • Some newly precipitated Mo 6 Ni 6 C were transformed to the Cr 3 C 2 in aging. • The aged interdendritic zones had a higher elastic modulus and hardness than the non-treated ones. • An excellent local wear resistance has been achieved in the aged coating. [ABSTRACT FROM AUTHOR]

The work investigated the effect of high- and medium-temperature thermal exposure on the microstructure and mechanical performance of the graphite/Au/Hastelloy N alloy brazed joint. The microstructure in the joint was studied with the aid of scanning electron microscope coupled with transmission electron microscope analysis. The nano-indentation was used to probe the nano-scale elastic modulus and hardness of reaction phases in the joint. The joint bond strength was evaluated by a shear test. Results indicate that: three zones (zone 1, 2 and 3) were observed in the joint with the zone 1 close to graphite while the zone 3 adjacent to Hastelloy N alloy. The precipitates in each zone were characterized and their formation mechanism was proposed. High-temperature thermal exposure was performed at 1333 K for 1–90 min. A longer soaking time facilitated the carbon diffusion and then produced a hierarchical structure with a more smooth transition of coefficient of thermal expansion, being favorable for the enhancement of the joint strength. Among the soaking time under investigation, the highest joint bond strength obtained was 34.1 MPa. Medium-temperature thermal exposure was carried out at 873–1073 K for 240 h. A high-quality joint was maintained when the joint was aged below 973 K. Over 973 K, a valid bonding was not achieved at the graphite/filler interface due to the precipitation of extensive carbon segregation in the zone 1 of the joint. The work performed will provide basic design principle for the graphite/Hastelloy N alloy brazed combinations used in molten salt reactors. • The element Mo had a separating effect on the Au-Ni system. • The elastic modulus and hardness of the Mo 2 C were 200.3 GPa and 21.2 GPa. • A hierarchical structure with a smooth CTE transition was produced in the joint. • The highest joint shear strength obtained in this work was 34.1 MPa. • Lots of carbon segregation were produced in the brazing seam when aged over 973 K. [ABSTRACT FROM AUTHOR]

A Ni–17Mo–7Cr-based superalloy was laser surface-modified to improve its tribological properties. Si particles were employed as coating materials. Si melted on the surface of the alloy during the process, triggering the formation of Mo6Ni6C carbides and Ni–Si intermetallics. A defect-free coating obtained was mostly made up of primary Mo6Ni6C and γ-Ni31Si12, as well as a eutectic structure of β1-Ni3Si and α-Ni-based solid solution (α-Ni (s.s)). The volume fraction of hard reinforcements (Mo6Ni6C, γ-Ni31Si12, and β1-Ni3Si) reached up to 85% in the coating. High-temperature microstructural stability of the coating was investigated by aging the coating at 1073 K for 240–480 h, to reveal its microstructural evolution. In addition, the mechanical performance of the coating was investigated. The nanoscale elastic modulus and hardness of Mo6Ni6C, γ-Ni31Si12, and α-Ni (s.s) were characterized using the nanoindentation tests. The nanoscratch tests were performed to measure the local wear resistance of the coating. Lastly, the Vickers hardness distribution across the cross-section of the coating before and after thermal exposure was compared. The work performed provides basic information understanding the microstructural evolution and mechanical performance of laser-induced coatings on Ni-based superalloys. [ABSTRACT FROM AUTHOR]

Epitaxial growth of titanium carbide (TiC) layer is vital for single-crystalline diamond (SCD) to ensure wettability when brazed with metals, however, how TiC grain size affects the mechanical behavior of SCD/TiC interfaces remains unclear. Herein, molecular dynamics simulations were performed to investigate the tensile properties of SCD/TiC combinations with different TiC in-plane grain sizes (d). The results show that d influenced both the elastic and plastic deformation, ascribing to the specific grain boundaries (GBs) network. The Young's modulus of the combinations was found to increase with the increase of d. After yielding, TiC{111} partial dislocations emitting from both the GBs and interfaces were observed to cross GBs or pile-up at GBs, among which d had a negligible effect on the planar-slip induced relaxation. Furthermore, it was found the interaction of dislocation-GB and dislocation-dislocation made a great contribution to strain-hardening. For the combinations with d < 8 nm, it was strengthened by piling-up dislocation at GBs, whereas for the larger d , it was dominated by intragranular dislocation entanglement. Overall, strain localization and micro crack took place at the interfaces due to the absence of plastic mechanism to accommodate strain gradient. This work provided an atomistic insight into the mechanical behaviors in SCD/TiC combinations, offering guidance for interfacial design to improve the mechanical performances in diamond devices. [Display omitted] • Mechanical performances in diamond/TiC combinations were studied by MD simulations. • Detailed stress-strain response and defect evolution were presented. • Grain-size-dependent dislocation mechanism was found. [ABSTRACT FROM AUTHOR]

A novel Ta 0.5 HfZrTi refractory high-entropy alloy instead of the traditional single refractory metal was chosen as an interlayer for diffusion bonding of silicon carbide (SiC) ceramics by spark plasma sintering. The microstructural evolution and mechanical properties of SiC joints processed at 1400–1700 °C for 5–20 min under a pressure of 30 MPa were investigated. The phase constitutes in the reaction layer varied with the joining conditions and mainly consisted of (TaHfZrTi)C, (TaHfZrTi) x Si y C z , and (TaHfZrTi) x Si y. The initial reaction phases exhibited metallic element segregation owing to the different Gibbs free energies of the interfacial reactions. The crystalline structure of the diffusion-formed (TaHfZrTi)C was identified as face-centered cubic sphalerite. A near-full (TaHfZrTi)C reaction layer was successfully achieved at 1700 °C for 20 min due to the fast formation of a relatively dense (TaHfZrTi)C layer and the decomposition of (TaHfZrTi) x Si y C z at elevated temperatures. The maximum joint hardness and shear strength at the same conditions were significantly improved to 2552.1 ± 357.1 HV and 326.2 ± 9.9 MPa, respectively. • A novel Ta 0.5 HfZrTi RHEA instead of the traditional single refractory metal was proposed for joining SiC ceramics. • A near-full (TaHfZrTi)C high-entropy reaction layer was successfully achieved in the joint. • The optimum joint strength reached 326.2 ± 9.9 MPa produced at 1700 °C for 30 min. [ABSTRACT FROM AUTHOR]

This paper reported a route to fabricate a high-performance high-entropy alloy (HEA) brazed joint with the fusion zone similar to the base metal, in which the brazing of a Ni-containing AlCoCrFeNi HEA using a novel Ni-based filler was investigated at 1150–1350 °C for 0–45 min. A microstructure similar to the base metal was received in the fusion zone through proper interface control, thus the joint shear strength obtained was approximately three times (687.2 ± 23.2 MPa) in contrast to that of the joint with the heterogenous interface (199.4 ± 2.8 MPa). The strengthening mechanism was attributed to the high-entropy solid solution, precipitation strengthening and grain refinement occurring in the joint. The highest joint strength of 687.2 ± 23.2 MPa was received at 1300 °C for 15 min with the fracture mainly happening in the base metal. The brazed joint exhibited desirable high-temperature stability with minor degradation of joint strength (∼3%) after annealing of 800 °C for 240 h. The excellent joint performances are potential for high temperature applications. • A high-quality AlCoCrFeNi high-entropy alloy brazed joint with the fusion zone similar to the base metal was achieved. • The brazed joint exhibited a desirable high-temperature stability with minor degradation of joint strength after high-temperature annealing. • The mechanisms of interfacial phase evolution and joint strengthening were discussed. [ABSTRACT FROM AUTHOR]