Materials Science Forum Vols. 747-748

Abstract: Particle surface component and valence state of nickel-based superalloy FGH96 powders fabricated by argon gas atomization have been measured by X-ray phoelectron spectrometer (XPS) to recognize the effect of powder size, surface segregation and heat treating on the oxidation behavior of powders. Type and phase structure of prior particle boundaries (PPBs) precipitated in HIPed superalloy have been studied also. The results showed that prior particle surface segregation and oxidation happened during the powder collection and storage with the C, O, Ti elements enrichment and Ti, Cr, Al, Zr elements oxidation in the surface respectively. During the heat treating of 1150/2h, normal segregation element Ti and Zr enriched more apparently in the particle surface, on the contrary, negative segregation elements Ni, Al and Co diffused into the particle inner. Furthermore, the higher oxygen level on particle surface enhanced to form much more oxides such as ZrO₂, TiO₂ and Cr₂O₃. During HIP process of 1150/2h/150MPa, the oxide ZrO₂ as the nuclei accelerated the preferential precipitation of MC-type carbides which forms the continuous harmful PPBs. Definitely, PPB precipitation depended on the prior particle surface segregation and oxidation, surface elements Ti, Cr and Nb enrichment and oxidation reaction enhancement in high temperature such as 1150. Small powders always had little segregation in the surface and thin oxidation layer, but had more oxygen content per weight for its higher specific surface. So using some powders with optimum particle size and lower oxygen content can help to reduce the PPB precipitation.

Abstract: According to the defects of powder metallurgy superalloy, especially the influence and damage of inclusions on properties of disk, the deviation between nondestructive testing and metallographic testing of inclusions in FGH96 alloy isothermal forging disk was investigated. Meanwhile, the types and deformation mechanism of inclusions were studied. The results showed that the buried depth tested by metallographic detection was less 67-180μm than nondestructive testing. The size of inclusions with metallographic detection was less about 18-50μm than nondestructive testing. The major types of inclusions in practical disk were Al₂O₃ and Al₂O₃-SiO₂, the inclusions run through several grains, no matter Al₂O₃ or Al₂O₃-SiO₂. The Al₂O₃ inclusion and matrix was purely mechanical bonding, but the Al₂O₃-SiO₂ had reaction zone. There was remarkable effect of extrusion process on crushing and dispersing Al₂O₃ inclusion, but which was unremarkable for Al₂O₃-SiO₂.

Abstract: The formation mechanism of stray grains in cross-section change region of casting clusters for single crystal superalloy DD483 were investigated experimentally during the conventional directional solidification. The results showed that the stray grains formation was different on the two sides of the platform. Indeed, stray grains formation in the region of the cross-section change on the shadow side showed a high probability, while the heater side, no stray grains appeared. Additionally, the cooling curves at the cross-section change region also showed that the distribution of temperature was different on the two side of the platform. The formation of stray grain can be attributed to the local undercooling at the cross-section change region is exceed the critical undercooling for nucleation, led to the heterogeneous nucleation, while the asymmetric temperature distribution should be responsible for asymmetrical formation of stray grains on the two sides of the platform.

Abstract: K419 superalloy turbine wheel was more susceptible to hot tearing than K418 one when they were used for auto turbocharger turbine wheel. The fracture and microstructure characteristics in the K418 and K419 turbine wheel blades were analyzed. The segregation of alloying elements was analyzed by EDS. The probable equilibrium phases in the two kinds of superalloys, the effects of aluminum, titanium and niobium contents on the precipitation of γ and γ/γ eutectic phase and the segregation of alloying elements were studied by Thermo-Calc software. The results show that the hot tearing in the K418 and K419 superalloy turbine wheel blades is caused by the fracture of dendrites structures, while the amount of γ/γ eutectic in K419 is more than that in K418, resulting in K419 being more susceptible to hot tearing. Titanium and niobium, the strong positive segregation elements promote the formation of γ/γ eutectic, which lead to severe hot tearing susceptibility of the superalloy.

Abstract: An observation was conducted on the microstructures of as-cast as well as solid solution treated the third generation single crystal superalloy DD9 using optical microscope and SEM. The effects of solid solution temperature and time on the eutectic fractions of γ/γ and size of γ of the alloy were investigated. The results showed that the microstructure of DD9 was uniform. W and Re segregated to the dendrite cores while Al, Ta and Nb were enriched in interdendritic regions during solidification. The eutectic fractions of γ/γ reduced with the solid solution temperature raising and the solid solution time prolonging. When solid solution temperature increased to 1340, the eutectic in the alloy was entirely dissolved. The size of γ in dendritic cores was consistent, however the size of γ in interdendritic regions was gradually decreased with the increase of solid solution treatment and time, eventually the sizes of γ were completely uniform at the temperature of 1340

Abstract: The creep deformation in pure aluminum was investigated using helicoid spring samples at room temperature, 298 K, and σ < 1.19 MPa. It was found that the stress exponent is n = 0, which means the creep behavior in this region is independent on applied stress but some physical properties of materials. The creep behavior was suggested to be controlled by surface diffusion based on the strongly effect of surface area on creep behavior only in this creep region (n = 0). The threshold creep rate, , called intrinsic deformation limit, decided by surface diffusion was suggested. This discovery provided a new perspective to understand the extremely slow deformation in the nature.

Abstract: Low-cycle fatigue (LCF) fracture of the spray formed GH738 alloy was investigated under total strain controlled mode at 650°C. Basic theory for evaluating fatigue lives by fatigue striations has been introduced. The crack length a and the crack propagation rate da/dN were measured and the curve of da/dN in crack propagation zone was obtained with Paris formula and tabulation trapezoidal formula, respectively. The size and symmetry of rapid fracture zone of fatigue were also studied. The results showed that the relationship of a and N was linear with tabulation trapezoidal formula, and the da/dN and a was conic with Paris formula. At last, quantitative analysis of fatigue fracture was also discussed.

Abstract: The hot deformation behaviors of spray formed superalloy GH738 were investigated by using of Gleeble-3500 simulator in the temperature range of 950~1200, with a strain rate of 0.13~6.5s^-1 and reduction of 50%. The corresponding flow curves were determined and hot deformed microstructures were observed. The results showed that the flow stress decreased with increasing deformation temperature or decreasing strain rate. A full dynamic recrystallization microstructures with fine-equiaxed grains were obtained at the temperature of 1100~1150 and strain rate of 2.6~6.5s^-1. The hot deformation activation energy Q was 580.81kJ.mol^-1, and the constitutive equation was derived by means of linear regression.

Abstract: An Al-Si coating was prepared on IC21 alloy by powder pack cementation. The cyclic oxidation tests were carried out at 1150 in air for up to 100 h. The results indicate that the oxidation resistance of IC21 alloy is significantly improved by the Al-Si coating due to the presence of Ni₂Al₃ and β-NiAl enriched outer layer, and Si can effectively supress the outward diffusion of Mo. The oxide scales mainly consist of α-Al₂O₃, which is the favorite to the oxidation resistance. Phase transformation occurred from β-NiAl to γ-Ni₃Al and γ-Ni in the coating during oxidation. The coating still remained a certain amount of β phase after oxidation for 100h, which indicate a good protection. The microstructure change evolution was characterized, and the oxidation behavior of the coating was discussed.

Abstract: The oxidation kinetic curves of three Ni₃Al-based single crystal alloys (IC21, IC21C and IC21CR) with different Cr and Re content were examined at 1100. The microstructures and element distributions of the oxide scales on these alloys were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray spectrometry (EDS). Results revealed that the oxidation resistance of these alloys was improved in the order of IC21 < IC21C < IC21CR. The oxide products of IC21 mainly consisted of NiO, α-Al₂O₃, NiAl₂O₄ and a small amount of NiMoO₄ and MoO₂. The volatilization of Mo oxides led to the oxide scale spallation from IC21C, thus deteriorated the oxidation properties. While for IC21C and IC21CR, the oxidation resistance was significantly improved. The Mo oxides in the oxide layer were greatly reduced and a continuous α-Al₂O₃ layer was formed.