铸造镍基合金的稀土变质改性

本文研究了稀土变质处理对改善铸造镍基合金的组织和耐蚀性能与铸造性能的影响。试验结果表明，在这种镍基耐蚀合金中加入适量的稀土可明显地细化合金的晶粒，改变碳化物与夹杂物的形态与分布，提高合金的耐蚀性能，使其铸造性能亦有所改善。     

Challenge,response and serendipity in the design of materials

A number of condensed case-histories of successful materials innovations are presented to illustrate the author’s thesis that happy accident favouring the prepared mind (i.e. serendipity), or alternatively a response to a challenge from competing materials, are circumstances favouring effective innovation.     

Effects of processing and microstructure on the fatigue behaviour of the nickel-base superalloy René 95

Forms of the nickel-base superalloy René 95 produced by three processing methods were evaluated in tensile, low cycle fatigue and fatigue crack propagation tests at 540 and 650°C. Two powder-metallurgy (PM) forms, hot-isostatically-pressed and extruded-and-forged, and a conventionaslly cast-of-wrought form were all given the same heat treatment. The extruded-and-forged form showed superior fatigue life in low strain range tests though the two PM forms exhibited nearly identical mechanical behaviour in all other respects. Further, this life difference could not be explained by significant differences in the types, sizes or shapes of the defects initiating failure. The cast-and-wroght René 95, however, had lower strenght, ductility and fatigue life, but higher fatigue crack propagation resistance because of a larger grain size. It did not exhibit the environmentally-assisted intergranular mode of propagation which occurs in PM René 95 and other fine-grained superalloys at these test temperatures and frequencies.     

Temperature effects on fatigue thresholds and structure sensitive crack growth in a nickel-base superalloy

Fatigue thresholds and slow crack growth rates have been measured in a powder formed nickel-base superalloy from room temperature to 600°C. Two grain sizes were investigated: 5–12 μm and 50 μm. It is shown that the threshold increases with grain size, and the difference is most pronounced at room temperature. Although crack growth rates increase with temperature in both microstructures, the threshold is only temperature dependent in the material with the larger grain size. It is also only in the latter that the room temperature threshold falls when the load ratio is increased from 0.1 to 0.5. At 600°C the higher load ratio causes a 20% reduction in the threshold irrespective of grain size.The results are discussed in terms of surface roughness and oxide-induced crack closure, the former being critically related to the type of crystallographic crack growth, which is in turn shown to be both temperature and stress intensity dependent.     

Development of a Nickel-base Cast Superalloy with High Strength and Superior Creep Properties

Derived from Russian alloy CHS88U, six experimental Ni-base alloys named as A to F in the Ni-Cr-Co-W-Ti-Al-Hf system are designed, evaluated and processed. One of these alloys, F, shows excellent high temperature tensile strength and ductility with superior creep rupture properties. As predicted by using modeling tools such as PHACOM and NEW PHACOMP, there is hardly the tendency for formation of topologically close-packed phase (TCP) phase in alloy F. Furthermore, through microstructural observation, it is also found that no TCP phase is formed in alloy F after long-time exposure at high temperature. So alloy F has well balance of phase stability and mechanical properties in view of application for gas turbines. It is proved that d-electron approach can be applied for design and development of nickel-base superalloys for gas turbine application.     

Simulating the effect of aluminizing on a CoNiCrAlY-coated Ni-base superalloy

MCrAlY (M = Ni, Co) coatings are commonly used on gas-turbine components as oxidation resistant overlay coatings and bondcoats for thermal barrier systems. The present work focuses on the effect of the aluminizing process on the CoNiCrAlY coating microstructure. In the as-received condition the outer part of the coating consisted mostly of β-(Ni,Co)Al with interspersed precipitates of Cr-rich carbide and Cr-rich boride precipitates. Formation of σ-CoCr was observed at the interface between the β-layer and the inner initial CoNiCrAlY microstructure. Scanning electron microscopy (SEM) combined with energy and wavelength-dispersive X-ray spectroscopy (EDX/WDX) was employed to characterize the aluminized CoNiCrAlY coating. Phases were then identified by electron backscatter diffraction (EBSD). Detailed microstructural studies of the coating were corroborated with the help of coupled thermodynamic-kinetic calculations to model the aluminizing process. The calculations were performed with the in-house developed code employing the commercially available thermodynamic and kinetic databases (ThermoCalc). The mechanisms of the observed microstructural changes were elucidated with the help of the modelling results.     

Thermo-physical properties of Hastelloy X and Haynes 214 close to the melting range

ABSTRACT

The nickel-based superalloys Hastelloy X and Haynes 214 are widely used for honeycomb liners; however, thermo-physical properties at temperatures close to the melting range are poorly described in the literature. Based on the thermo-physical properties, endothermic effects between 550°C and 650°C have been observed that develop due to the formation of the short-range order in both alloys and in Haynes 214, in addition to the short-range order, an increase in the specific heat capacity due to the γ′-precipitation up to 950° was detected. At low and intermediate temperatures, the Hastelloy X is significantly better suited than the Haynes 214. At high temperatures the Haynes 214 is superior to the Hastelloy X due to the dissolution of the γ′-phase.     

77 to 1200 K tensile properties of several wrought superalloys after long-term 1093 K heat treatment in air and vacuum

The 77 to 1200 K tensile properties of approximately 1.3 mm thick wrought sheet Co-base Haynes alloy 188 and Ni-base Haynes alloy 230 and Inconel 617 have been measured after heat treatment in air and vacuum for periods up to 22,500 h at 1093 K. Significant changes in structure were produced by prior exposures, including precipitation of second phases and, in the case of heat treatment in air, oxide scale and surface-connected grain boundary pits/oxides, as deep as 50 to 70 μm, in all three superalloys. Due to the geometry of the experiment, the vacuum-exposed samples were protected from loss of volatile elements by evaporation; hence, such specimens were simply given 1093 K anneals in an innocuous environment, which produced very little surface attack. Compared to the properties of as-received alloys, prior exposure tended to reduce both the yield strength and ultimate tensile strength, with the greatest reductions at 77 and 298 K. The most dramatic effect of heat treatment was found in the low-temperature residual tensile elongation, where decreases from 40 to 5% at 77 K were found. Ductility is the only property that was found to have a consistent dependency on environment, with air exposure always yielding less tensile elongation than vacuum exposure.