Heng Rong Guan

Effect of Temperature on Formation of Borides in TLP Joint of a Kind of Nickel-Base Single Crystal Superalloy

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.

Mechanical Behaviour of a Single Crystal Superalloy DD32 - A Comparison with the Alloy SRR99

Stress rupture and tensile properties of a single crystal superalloy DD32 are investigated comparing with the alloy SRR99. It is shown that the alloy DD32 offers an improved creep temperature capability of more than 60°C at higher stresses. The g¢ precipitates in the stress ruptured samples were rafted to P-N type directional coarsening. The fracture mechanism of the stress ruptured samples was initiated from the micropores.

Influence of Aging Temperature on the Microstructure and Stress Rupture Property of DZ951 Alloy

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.

Evolution of Oxide Scale Morphology on (Ni,Pd)Al Coating during High Temperature Oxidation in Air

The morphology evolution of oxide scale formed on the (Ni,Pd)Al coating during oxidation in air at 900 ∼1100°C was investigated systematically by scanning electron microscope and XRD. A long-term observation showed that the transformation of θto α-Al2O3 occurred on the (Ni,Pd)Al coating with increasing oxidation temperature and time. Furthermore, such transformation is more rapid on the (Ni,Pd)Al coating in comparison to a simple NiAl coating. In addition, the various morphologies for the θ-Al2O3 are also presented in this paper. Introduction Palladium-modified aluminide coating is a promising coating to substitute to the Pt-Al coating. Alperine et al. 1, 2 produced one kind of palladium-modified aluminide coating, which had higher cyclic oxidation and hot corrosion resistance than the simple aluminide coating. He et al. 3 studied the electroplating technique of Pd and Pd-Ni alloys, and further the effects of pre-deposition of PdNi alloys on the composition and microstructure of Pd-Ni-Al coatings. Monceau et al. 4 investigated the effects of oxygen partial pressure, heating rate and surface treatment on the high temperature oxidation kinetics of a Pd-modified aluminide coating used as the bond coat for partially stabilized zirconia (PSZ) thermal barrier coating (TBC). Li et al. 5, 6 found that the palladium-modified aluminide coating showed high temperature oxidation resistance superior to the simple aluminide. During the oxidation of aluminides and alumina-forming alloys, the formed oxide scales may contain various aluminum oxides. At lower temperatures and/or in the early stages of oxidation, the metastable oxides γ-Al2O3, δ-Al2O3 and θ-Al2O3 grow. Some literature 7-13 deals with the morphology of transition alumina, especially with the θ-Al2O3. In the present paper, we introduce some new results on the transformation from θto α-Al2O3 during oxidation of the (Ni,Pd)Al coating oxide in air at high temperatures Materials Science Forum Online: 2004-08-15 ISSN: 1662-9752, Vols. 461-464, pp 417-426 doi:10.4028/www.scientific.net/MSF.461-464.417

Recycled Applications of a Single Crystal Superalloy

The effects of application of recycled scraps on the composition, microstructure and mechanical properties of SRR99 alloy were studied. The compositions of recycle master alloy are similar to that of the fresh alloy. However, nitrogen content, the size and amount of microporosity increase significantly with the recycle material proportion. Granular, lamellar and strip-like MC particles mainly represent in the interdendritic regions of the revert alloy. With the addition of the recycled material proportion, the amount of the granular ones rapidly drop and the lamellar ones gently enhance. The stress rupture life decline with the addition of the recycled material proportion. The oxide and sulphide inclusions in revert superalloy can be effectively captured and removed by a foam ceramic filter so that the tensile and stress rupture property has been significantly improved.

Influence of Prolonged Exposure on the Microstructure and Stress-Rupture Property of a Single Crystal Ni-Base Superalloy

The single crystal Ni-base superalloy possesses excellent comprehensive mechanical properties, particularly at high temperature, so it is a best candidate for the blade material in the advanced turbine engine. During high temperature long term aging, the stability of the microstructure and mechanical properties of a single crystal Ni-base superalloy is considered to be very important for the safe operation of a turbine engine. Because the single crystal superalloy is free of grain boundary, there have no problem on the weakening effect of grain boundary as well as the microcrack formation on the transverse grain boundary. It is necessary to understand the microstructural evolution and deterioration of the mechanical property of the alloy under complicated service conditions both thermally and mechanistically. For this purpose, the influence of prolonged exposure on the microstructure and stress rupture property of a single crystal Ni-base superalloy at 950°C and 1050°C has been studied in detail.

Temperature Dependence of High-Cycle Fatigue Behavior of a Ni-Base Single Crystal Superalloy

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.

Precipitation Behavior during Ageing Treatment of a Single Crystal Nickel-Base Superalloy

The precipitation behavior during ageing treatment of a single crystal nickel-base superalloy was investigated by SEM and TEM. The results showed that tetragonal needle-like σ phase and blocky uf061-W phase precipitated during low temperature ageing treatment after this testing alloy was completely solution heat treated. σ and uf061-W phases robbed of solid solution strengthening alloying element W、Mo in the matrix and degraded high temperature creep rupture property severely. The creep curve of the crystal tested at 1010 °C and 248 MPa exhibited that the steady state creep rate ε was as high as 9.46 × 10-3/h. The creep-rupture life was only 25 hours. uf061-W phase was not formed by decrease of W content properly. A relatively low level of Co could inhibit σ phase precipitation and improve microstructural stability.

Tensile Deformation Behavior of M963 Cast Nickel-Base Superalloy

The tensile deformation behavior of M963 superalloy treated by various heat treatments has been studied at both room and high temperature (1173K). The result shows that the alloy treated by the standard solution treatment at 1483K for 4h followed by air-cooling has low ductility especially at 1173K. The additional aging treatment at 1123K for 16h followed by furnace cooling can recover the ductility of the alloy at 1173K, but further decrease its room temperature ductility. The TEM observation shows that the deformation mechanism varied with both the testing temperature and heat treatment. Finally, the mechanism of the aging treatment on the deformation behavior of the M963 superalloy is discussed.

Effect of Melt Treatment on the Microstructure and Creep-Rupture Behavior of M963 Cast Nickel-Base Superalloy

The effect of melt treatment on the microstructure and creep-rupture behavior of M963 superalloy at 1248K under 225MPa has been investigated. The microstructure of the as-cast superalloy without melt treatment consists of γ solid solution matrix, γ´precipitate, coarse blocky MC carbide and (γ+γ´)eutectic. The striking difference in microstructure is that the melt treatment changes the MC carbide from the coarse blocky morphology into the fine script-like morphology. After heat-treated at 1483K for 4h followed by air-cooling, both the creep life and rupture elongation of the melt-treated alloy are all doubled those of the alloy without melt treatment. The mechanism of the melt treatment on the creep-rupture behavior of the M963 superalloy is discussed.