Materials Science Forum Vols. 546-549

Abstract: Two directionally solidified (DS) Ni-based cast superalloys without and with 3wt. % Ru were prepared. The effects of Ru addition on the microstructures and stress rupture properties of the heat treated superalloys were investigated. It is shown that the amount of eutectic in 3wt. % Ru alloy was less than that in alloy without Ru. The incipient melting structure was found after quenching followed by 1295°C and 1300°C solid solution treatments in 3wt. % Ru alloy and in the alloy without Ru, respectively. The temperature at which the eutectic pools dissolved completely was higher than the temperature at which incipient melting appears. In order to obtain the better mechanical properties, double aging treatment was carried out for both alloys to optimize the sizes, morphologies and distribution of the γ′ phase. The stress rupture lives of the alloys were 55h and 108h under the condition of 1070°C and 137MPa the alloys without Ru and with 3wt. % Ru respectively. It is suggested that 3wt. % Ru addition can prolong the stress rupture life of the alloy.

Abstract: The effects of ruthenium and chromium contents have been investigated on phase transformation temperatures and the morphology of γ’ precipitates as well as microstructural stability in high refractory Ni-base superalloys. The solidus and liquidus temperatures were determined by differential scanning calorimetry (DSC), suggesting that the addition of Cr resulted in a decrease in solidus/liquidus temperatures while the Ru addition (3.5 at.%) had the neutral effect. The morphology of γ’ precipitates in the heat-treated microstructure was changed under the influence of the Cr and Ru additions, suggesting that Ru and Cr contents affected the γ−γ’ lattice misfit through changes in the associated partitioning to the constituent phases. The microstructural instability has been investigated at 1000 oC. High levels of Cr addition (8 at.%) strongly promoted the formation of TCP phases, while Ru improved the microstructural stability to some extent.

Abstract: Smooth specimens of single crystal (SC) superalloy SRR99 with [001] orientation were subjected to high-cycle fatigue (HCF) loading at temperatures of 700°C, 760°C, 850°C and 900°C in air atmosphere. The results demonstrated that conditional fatigue strength reached the maximum at 760°C and decreases with increasing temperature. Analysis on fracture surface showed a trend for cleavage rupture at 850°C and 900°C and ductile rupture at 700°C and 760°C. Fatigue cracks initiated at the surface or subsurface were primarily responsible for the ultimate failure. The influence of testing temperature on fatigue lifetime was studied by examining evolution of the microstructure through SEM observation. With the process of cyclic loading at elevated temperatures, the primary cuboidal γ′ precipitates tended to agglomerate and spheroidized, meanwhile a larger number of secondary γ′ particles were formed in the γ matrix in specimens fatigue tested at 700°C, which would have a significant effect on the high temperature properties.

Abstract: By means of tensile and compression creep testing and SEM, TEM observation, an investigation has been made into the microstructure evolution of a single crystal nickel base superalloy during tensile / compression creep. Results show that the cubic γ′ phase in the superalloy is transformed into the N-type meshlike structure along the direction vertical to stress axis during tensile creep. The cubic γ′ phase is transformed into the P-type structure along the direction parallel to stress axis during compression creep. An obvious asymmetry strain of the alloy occurs during tensile and compression creep, the formation of the needle-like γ′ rafts during compression creep is a main reason of the alloy displaying a smaller strain. During compressive creep, the deformation feature of the alloy is the <110> and (1/3) <112> super dislocations shearing into the γ′ rafts. The deformation mechanism of the alloy, in the stage state of tensile creep, is dislocation climb over the γ′ rafts.

Abstract: Microstructural evolution of an experimental directionally solidified superalloy with local recrystallization during creep was studied. Precipitate free zones (PFZs) were formed along the recrystallization grain boundaries that are perpendicular to the applied stress. The formation mechanisms of PFZs and creep cracks associated with PFZs were discussed based on the experimental observation.

Abstract: Effect of different aging temperatures on microstructure and stress rupture properties of DZ951 alloy were investigated in this paper. The results show that the shape of carbide changed from script-like in as-cast alloy to block during different aging treatments. MC carbide degrades into M23C6 at the aging temperature of 970°C, which made alloy have a better combination of strength and ductility than that at other aging temperatures. The size of γ′ phase increases and the shape of γ′ phase changed from sphere (870°C) to quasi-cuboid (920°C) and cuboid (970°C) with the increasing aging temperature. The stress rupture life of DZ951 alloy at 1100°C/60MPa improves with the increased of aging temperature. The fractographs at different conditions showed a ductile fracture mode.

Abstract: a kind of as-cast nickel-base single crystal superalloy was TLP bonded using Ni-Cr-B amorphous foil at different temperatures. Special attention is paid to the formation of boride in diffusion zone of TLP joints at different conditions. The chemical composition and microstructure of borides were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). At different bonding temperature, M3B2 precipitates appear distinct morphologies. At 1200°C, both blocky and plate-like borides formed owing to the diffusion of boron atoms into base metal and precipitation during the cooling process. At 1230°C or above, due to the diffusion of boron atoms the constitutional liquation of original γ/γ′ eutectics in the base metal occurs and borides formed when the system was cooled to room temperature. The analysis of TEM results reveals that M3B2 has a tetragonal structure and is rich in Mo, W, and Cr elements.

Abstract: The effects of high temperature exposure simulating service conditions on stress rupture properties were studied for the second generation single crystal superalloy DD6. The specimens with [001] orientation were exposed in air at temperatures of 980°C and 1070°C for 100h to 1000h. They were then tested using conventional mechanical tests at 1070°C/140MPa to determine the effects of exposure on stress rupture properties. The analysis indicated that stress rupture life decreased with increasing exposure time. At the temperature of 980°C, the stress rupture life is more than 180h after exposure for 1000h. When the test temperature increased to 1070°C, the stress rupture life exceeds 100h after 800h exposure. The morphology of γ prime phase after exposure was observed by using scanning electron microscopy (SEM). Morphologies evaluations have shown that alloy DD6 exhibits excellent microstructure stability after exposure. TCP (Topologically Closed Packed) phases have not been observed. It has been also found that the morphology and size of γ prime affected stress rupture life of the alloy. The decrement in stress rupture life with increasing exposure is a result of γ prime rafting.