We report the first synthesis of subcluster segregated nanoalloys formed through the joining of immiscible metallic nanoparticles (NPs) using femtosecond (fs) laser irradiation. Immiscible alloy components consisting of Ag and Ni, and Ag and Fe, all in the form of NPs, were first deposited on a carbon film in vacuum by fs laser ablation from the parent metals. These samples of randomly distributed NPs were then irradiated with multiple fs laser pulses at a fluence of 1.5 mJ/cm2. Transmission electron microscopy (TEM) observations indicate that Ag and Ni as well as Ag and Fe NPs were successfully joined under these conditions. Energy dispersive X-ray (EDX) results show that no mixing layer exists at the interface. The nanostructure in the interface reveals the assumption of a specific angle between two matching planes on either side of the interface. Calculation of the lattice mismatch indicates that the system adjusts to this angle so as to reduce surface energy. Structural ledges were also formed at the interface to further compensate for the atomic misfit.

Pulsed plasma arc deposition (PPAD), which combines pulsed plasma cladding with rapid prototyping, is a promising technology for manufacturing near net shape components due to its superiority in cost and convenience of processing. In the present research, PPAD was successfully used to fabricate the Ni-based superalloy Inconel 625 components. The microstructures and mechanical properties of deposits were investigated by scanning electron microscopy (SEM), optical microscopy (OM), transmission electron microscopy (TEM) with energy dispersive spectrometer (EDS), microhardness and tensile testers. It was found that the as-deposited structure exhibited homogenous columnar dendrite structure, which grew epitaxially along the deposition direction. Moreover, some intermetallic phases such as Laves phase, minor MC (NbC, TiC) carbides and needle-like δ-Ni3Nb were observed in γ-Ni matrix. Precipitation mechanism and distribution characteristics of these intermetallic phases in the as-deposited 625 alloy sample were analyzed. In order to evaluate the mechanical properties of the deposits, microhardness was measured at various location (including transverse plane and longitudinal plane). The results revealed hardness was in the range of 260–285 HV0.2. In particular, microhardness at the interface region between two adjacent deposited layers was slightly higher than that at other regions due to highly refined structure and the disperse distribution of Laves particles. Finally, the influence of precipitation phases and fabrication strategies on the tensile properties of the as-deposited samples was investigated. The failure modes of the tensile specimens were analyzed with fractography.

•A novel TiH2–Ni–B brazing powder was developed to braze TiAl and C/SiC.•The in situ synthesized TiB whisker-reinforced brazing seam was achieved.•The joint presented excellent high-temperature mechanical properties.•Correlation between microstructure and joint properties was studied.(TiH2–66Ni)1 − xBx (x = 3 wt.%) brazing powder was fabricated by mechanical milling of a TiH2, Ni and B mixture with the milling time ranged from 30 to 180 min. TiAl alloy and C/SiC composite were successfully brazed using this filler metal at 1180 °C for 10 min. The microstructure and mechanical properties of the brazed joints were investigated. The typical microstructure of the joint was divided into three characteristic zones, including the TiC reaction layer formed adjacent to C/SiC composite, the TiB-whisker reinforced central zone, and the β layer transformed from TiAl alloy. The in situ synthesized TiB whiskers acted as an effective reinforcement phase aid to decrease the residual stress and improve the shear strength of the joints. The joint strength reached 105 MPa at room temperature, and cracks primarily propagated in the C/SiC substrate with the pull-out of carbon fibers and partially in the TiC layer and τ3 phase. The joint strength decreased slightly with the testing temperature increased to 500 °C, and remained at 70 MPa when tested at 600 °C. The crack propagation path diverted from the TiC reaction layer to the τ3 phase when the joint was tested at 600 °C.

TiAl intermetallics were joined by transient liquid phase (TLP) bonding technique using Ti/Ni filler metal. The effects of bonding parameters and composition of filler metal on isothermal solidification and interfacial microstructure of the joints were studied. It was found that a continuous α2 layer was formed at the joint interface when the bonding temperature was below 1125 °C. This α2 layer hindered the atom interdiffusion between the TiAl substrate and liquid filler metal, which resulted in a long holding time required for complete isothermal solidification. When the bonding temperature was 1150 °C, no continuous α2 layer was formed in the joint and the isothermal solidification rate was enhanced. Furthermore, with decreasing of Ni content in the filler metal, the isothermal solidification rate was reduced due to the decrease in dissolution of TiAl substrate into the liquid filler metal. The shear testing results showed that the highest shear strength at ambient temperature (281 MPa) and at high temperature (800 °C) (243 MPa) were achieved, when the joint was bonded at 1150 °C for 5 min.Highlights► A high quality joint with uniform microstructure and high strength was obtained. ► Effect of bonding parameters on joint isothermal solidification was investigated. ► Effect of Ni content in filler metal on joint isothermal solidification was studied. ► The ambient temperature/high temperature shear strength of the joint was evaluated.

Microstructural evolution and shear strength of vacuum brazed TiAl alloys to C/SiC composites using Ag–Cu filler metal were investigated. The dissolution of active elements Ti and Al from TiAl substrate has a strong influence on the microstructure and shear strength of the joint. Ag is the less active element of the filler and Cu has strong tendency to the formation of AlCu2Ti phase with the dissolved Ti and Al. Ag–Cu eutectic is gradually taken place by AlCu2Ti blocks and Ag-based solid solution with the increase of brazing temperature or time. The TiC reaction layer including a small amount of Ti5Si3 phase is formed adjacent to C/SiC composites when active element Ti diffused into C/SiC composite and chemical reaction occurred in the composite interface. The shear strength of the joint depends heavily on the thickness of the TiC reaction layer. The maximum shear strength achieved 85 MPa for the joint vacuum brazed at 900 °C for 10 min. Cracks primarily propagate along Ag-rich phase and TiC layer. The TiC layer with the thickness of 4–5 μm is formed at the boundary of SiC matrix.Research Highlights► Diffusion of Ti and Al from TiAl alloy has great influence on the joint microstructure. ► The thickness of the TiC reaction layer is the main controlling factor for the joint quality. ► A conceptual microstructural evolution for the joint has been proposed.

The Ti–Ni–Si filler metal was manufactured by mechanical milling of TiH2, Ni and Si powder mixture. The microstructure of the filler metal and TiAl brazed joint was analyzed by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The effect of milling time on the brazing powder was investigated. It was found that NiSi phase formed when the milling time exceeded 120 min. The typical microstructure of the TiAl brazed joint using Ti–Ni–Si filler metal was TiAl/Ti3Al/TiAlNi2/Ti3Al + Ti5Si3/TiAlNi2/Ti3Al/TiAl. The effect of Si on the microstructure was investigated and the result suggested that Si addition resulted in the aggregation of Ti and formation of Ti3Al phase in the middle of joint. The optimal parameters were brazing temperature of 1140 °C and holding time of 30 min. The fracture was brittle and propagated between the TiAlNi2 layer and Ti3Al + Ti5Si3 layer.Highlights► Ti–Ni–Si filler metal for brazing TiAl was produced by mechanical milling. ► The average crystallite sizes decreased with the increase of milling time. ► The typical microstructure consisted of Ti3Al + Ti5Si3, TiAlNi2 and Ti3Al layers.

The diffusion bonding of TiAl-based intermetallics using the hydrogenated Ti6Al4V interlayer was carried out. The results were investigated by SEM, EPMA, XRD and TG/DSC. According to the experimental observations, the reaction layer of diffusion bonding using the hydrogenated Ti6Al4V interlayer containing 0.5 wt% hydrogen formed at 850 °C for 15 min under a pressure of 15 MPa, and the shear strength of the joint was up to 290 MPa. Compared with the direct diffusion bonding of TiAl-based intermetallics, the bonding parameters obviously decreased. Particularly, the bonding temperature decreased by 350 °C, the holding time decreased by 45 min, and the bonding pressure decreased by 15 Mpa. The diffusion bonding of TiAl-based intermetallics at a low temperature was achieved. The dehydrogenated process and the reaction mechanism were also discussed.