Search

C276 superalloy is considered as a potential structural material for advanced nuclear reactor with good mechanical properties and corrosion resistance. High-temperature creep behaviour of C276 alloy was investigated in the temperature range of 650°C–700°C and at stresses of 140–430 MPa. A linear relationship was fitted between stress and minimum creep rate in the logarithmic coordinate system. The rupture time is analysed for life prediction in terms of isotherm extrapolation method, Monkman–Grant relation, and Larson–Miller parameter method, respectively. The isochronous stress–strain curves as a means of representing stress–strain–time relations under creep conditions were established by the parameter method. The fracture surface morphology of ruptured specimens was characterised by a scanning electron microscope to elucidate the failure mechanism. [ABSTRACT FROM AUTHOR]

High temperature oxidation of Fe-Ni-base alloy HR120 and Ni-base alloy HAYNES 282 in steam at 800-1000 °C was investigated and compared. Results show that the oxidation kinetics of HAYNES 282 and HR120 followed a parabolic law. Temperature affected the stability of formed chromia, leading to the observation of (Fe, Cr)2O3, Fe2NiO4 on the surface of Fe-Ni-base alloy, which is not the case for Ni-base alloy. The strengthening element titanium accelerated the growth of chromia formed on the Ni-base alloy. It is concluded that the resistance of high temperature oxidation of HR120 is superior to HAYNES 282. [ABSTRACT FROM AUTHOR]

Effect of heat treatment on the recovery of microstructure and mechanical properties of a service-exposed IN939 superalloy was studied. Four-stage heat treatment was performed on the service-exposed alloy. The microstructures of the service-exposed and rejuvenated alloys were examined by optical and scanning electron microscopes. The hardness, tension, and stress-rupture tests were carried out to characterise the mechanical properties. The results showed that the heat treatment could rejuvenate the microstructure of the alloy that was deteriorated during the service at high temperatures. Decomposed MC carbides were transformed to new fine carbides, continuous M23C6 carbides were dissolved and new discontinuous carbides along the grain boundaries were regenerated and, finally, the fine γ׳ particles were reformed. The microstructural recovery resulted in an increase in the hardness and ultimate tensile strength of the alloy. The results also showed that the creep behavior of the alloy at a testing temperature of 850°C could be improved by the heat treatment. [ABSTRACT FROM AUTHOR]

High-temperature creep tests of a novel Super304H steel under 650 °C/195 MPa were conducted and the evolutions of microstructure and property with creep time of the material were investigated by using optical microscope, scanning electron microscope, micro Vickers hardness tester and electrochemical workstation. The results show that M23C6 carbides precipitated along grain boundaries of austenite matrix in a chain distribution and then got coarsened with the increase of creep time. Creep cavities started to form near the surface when the steel was crept for 2500 h. Afterward creep cavities increased, developed, interconnected and finally formed micro cracks along grain boundaries till fracture at the time of 4578 h. The hardness of the steel increased dramatically at the early stage of creeping and reached a high level at 500 h, and then kept a stable state at the succedent stage till fracture. Intergranular corrosion susceptibility of the steel increased first and then declined gradually, indicating the occurrence of sensitization -- desensitization process of the steel during creeping. [ABSTRACT FROM AUTHOR]

The two-phase microstructural evolution and the remained fractions of γ′ were investigated by scanning electron microscope (SEM) for a series of γ′-richen single crystal superalloys from the IC alloys system. At alloys IC31 and IC32, high levels of the dissolution of γ′ cause the remarkable damages of two-phase microstructure, especially when the fraction drops to less than 45%, the γ′ phase becomes sphere significantly and the two-phase interface distorts fairly. And the relationship between different degrading stages and the remained fractions are discussed systemically. The dissolution temperatures, Tsol, of IC alloys determined by differential scanning calorimetry (DSC) are further considered. It is found that there is a good linear relationship between the Tsol and the remained fraction of the γ′ at 1200°C and the slide keeps fixed as about 0·5%/°C. Meanwhile, the remained fraction of the γ′ seems to play an important role on the mechanical properties, and a method with Tsol used to estimate the properties at 1200°C is proposed. [ABSTRACT FROM AUTHOR]

Nickel based single crystal superalloy DD5 was brazed with the AWS classification filler metal BNi-2 at brazing temperature of 1323-1423 K for holding 10-240 min. The effect of the brazing temperature and brazing time on the microstructure of the brazed joint was investigated by scanning electron microscope (SEM) and electron probe microanalyser (EPMA). The result indicates that the joints exhibit sound bonding without any defect and crack. Three distinct regions can be identified in the joint: a diffusion affect zone composed of γ-γ′ phase and fine M3B2 precipitates, an interfacial bonding zone solidified isothermally at the brazing temperature, and braze alloy zone formed by the solidification of surplus molten filler alloy during cooling. The braze alloy zone consists of γ+γ′ eutectic phase, chromium boride, and ternary eutectic phase of γ-nickel, nickel boride and nickel silicide. With extending brazing time, the thickness of interfacial bonding zone and diffusion affect zone increased while brittle compounds reduced. The bonding mechanism was discussed with reference to the microstructure evolution of the joint. [ABSTRACT FROM AUTHOR]

The effect of thermal exposure on the microstructure of an Ni3Al-based single crystal superalloy was investigated. The Ni3Al-based single crystal superalloy used in this experiment contained a high content of molybdenum (Mo) and a low content of chromium (Cr), which had good high-temperature strength and oxidation resistance. The alloy was exposed at 1020-1200°C for different time periods. The microstructure after thermal exposure was examined by scanning electron microscope (SEM) and field emission microscope (FEM). The results indicated that precipitation of needle-like topologically close-packed (TCP) phases was only observed at certain intermediate temperature intervals (1020-1080°C). Moreover, once the TCP phases precipitated, they would not dissolve back even after thermal exposed for 1000 h. No precipitation phases were observed after high-temperature thermal exposure at 1100-1200°C for 25 h. [ABSTRACT FROM AUTHOR]

This technical paper deals with high-temperature dry sliding wear behavior and its mechanism of Al2O3-50TiO2 (A50T) coating on Inconel 718 alloy. The sliding wear behavior of the A50T coating on Inconel 718 alloy was investigated using a pin on disc equipment at 500°C with varying parameters like normal load and sliding velocity. Scanning Electron Microscope (SEM) features of worn samples reveal that ploughing, deep grooving and splat exfoliation are the dominant wear mechanisms of A50T coating at 10 N, while at 30 N, they are crack extension, crack deflection, crack bridging and splat exfoliation. [ABSTRACT FROM AUTHOR]