Abstract:The age-hardening behaviour and microstructure development of high strength Ni-based superalloys ABD-D2, D4, and D6 with varying Nb/Ti ratios have been studied. The studied alloys have large volume fractions and multimodal size distributions of the γ precipitates, making them sensitive to cooling conditions following solution heat treatment. Differential scanning calorimetry was conducted with a thermal cycle that replicated a processing heat treatment. The hardness of these alloys was subsequently evaluated by nanoindentation. The Nb/Ti ratio was not observed to influence the size and distribution of primary and secondary γ precipitates; however, the difference in those of tertiary γ and precipitate morphology were observed. The nanoindentation hardness for all alloys reduces once they have been solution-heat-treated. The alloys exhibited specific peak hardness. The alloy with the greatest Nb content was found to have the best increase in hardness among the alloys studied due to its large tertiary γ precipitate.

The morphological evolution of γ ′ precipitates and lattice mis t with isothermal aging were closely investigated in wrought Ni-based superalloy Inconel X-750. The γ ′ morphology dramatically changes in terms of shape, distribution, coalescence and coherency at the γ/γ ′ interface. These processes and their dependence on temperature are summarized as a γ ′ morphology map together with a time-temperature-precipitation (TTP) diagram through quantifying relevant morphological parameters. The lattice mis t was measured by X-ray diffraction and is positive; it decreases from 0.6% at room temperature to 0.1% at the aging temperature. These results suggest that the morphological changes of the γ ′ precipitates are attributable to very low lattice mis t, the interaction of the elastic eld, the volume fraction of the precipitates and incoherence in γ/γ ′ interface.

We investigated low-temperature diffusion at the Ni/SiC interface with the assistance of femtosecond laser-induced modifications. Cross sections of the laser-irradiated lines of two different pulse energies -0.84 and 0.60 J/cm 2 in laser fluencewere compared before and after annealing at 673 K. At the laser fluence of 0.60 J/cm 2 , a single flat Ni-based particle was formed at the interface after annealing. The SiC crystal under the particle was defect-free. The present results suggest the potential application of femtosecond laser-induced modifications to the low-temperature fabrication of contacts at the interface without introducing crystal defects, e.g., dislocations and stacking faults, in SiC.

The objective of the present study was to investigate the suppression of grain-boundary sliding and its accommodation at triple junctions. Tricrystals of pure aluminum and pure copper having ©110ª-tilt s = 3, 3, and 9 boundaries were grown. Creep tests were carried out at temperatures above 0.8T M , where T M is the melting point on the absolute temperature scale. In all specimens, boundary sliding occurred only in the 9 boundary. In aluminum tricrystals, sharp folds formed as a result of the suppression of 9 boundary sliding at the triple junction. Slip lines parallel to the trace of the (100) plane were observed in the fold. In copper tricrystals, no fold formed but a wedge crack was initiated at the triple junction and extended along one of the 3 boundaries aligned perpendicularly to the tensile axis. These results suggest that the ease or difficulty of fold formation affects crack formation during creep. [