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. [

The age-hardening behavior of the FeNi-based alloy HR6W was investigated in the temperature range between 973 K and 1073 K. A twostep increase of hardness was detected for the alloy at every aging temperature; the first increase of hardness results from the precipitation of M 23 C 6 carbides, and the second increase corresponds to precipitation of the C14Fe 2 W Laves phase. The timetemperatureprecipitation diagram for the alloy was established on the basis of the results of hardness measurements and microstructure observations, where the precipitation of the C14Fe 2 W Laves phase was slower than that of the M 23 C 6 carbides by three orders of magnitude and the nose temperature of the Laves phase was greater than 1073 K. The M 23 C 6 carbides precipitated with a plate-like morphology along grain boundaries at the early stage of aging, followed by the precipitation of the C14Fe 2 W Laves phase with a granular morphology with increasing aging time. The M 23 C 6 carbides and C14Fe 2 W Laves phase are aligned under the stress condition because of their precipitation on the dislocations introduced during creep deformation. [

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The three-dimensional morphology and thickness of an Al 2 Ca Laves phase with a C15 crystal structure, which precipitated within the primary ¡-Mg grain of a Mg5Al1.5Ca alloy that had been over-aged at 523 K for 100 h, were investigated using high-resolution transmission electron microscopy. The C15Al 2 Ca precipitate exhibits a hexagonal plate-like morphology, with a planar surface parallel to the (0001) ¡ basal plane and the sides of the hexagonal plate parallel to the f11 20g ¡ second columnar plane of the ¡ matrix. A typical coffee bean contrast was clearly visible around the precipitate, which is indicative of the coherent precipitation of the C15Al 2 Ca phase with respect to the ¡-Mg matrix. The thickness of the Al 2 Ca precipitate, which corresponds to six layers of the (111) C15 plane composed of Ca atoms, was evaluated as approximately 1.5 nm.

The major objective of the present study was to investigate the effect of the constraint of grain-boundary sliding and its accommodation at triple junctions on the creep fracture of tricrystals. Tricrystals of pure aluminum and copper having ©110ª-tilt 3, 3, 9 boundaries were grown by the Bridgman method. Creep tests were carried out for tensile specimens in which the 9 boundary made an angle of 45°with the tensile axis at temperatures above 0.80 T M , where T M stands for the melting temperature on the absolute temperature scale. In both aluminum and copper tricrystals, dominant grain-boundary sliding occurred along the 9 boundary. The aluminum tricrystal fractured along a plane across grains, far separated from the triple junction. In contrast, the copper tricrystal fractured via a complete separation along the 9 boundary followed by a tear-off of the remaining portion. This difference is accounted for by the different local deformation behaviors around the triple junctions to release the stress concentration induced by the constraint of 9 sliding, e.g., grain-boundary sliding along the 3 boundary in aluminum and crack formation along the 9 boundary in copper.