Carbides Evolution in a Ni-16Mo-7Cr Base Superalloy during Long-Term Thermal Exposure
The effect of long-term thermal exposure on the carbide evolution in a Ni-16Mo-7Cr base superalloy was investigated. The results show that M12C carbides are mainly precipitated on the grain boundaries during thermal exposure, and the primary massive M6C carbides can be completely transformed to M12C carbides in situ at temperatures above 750 °C for long-term thermal exposure. The transformation from M6C carbides to M12C carbides is attributed to the release of C atoms from M6C, which results in the morphology changes of massive carbides, and stabilization of the sizes of M12C carbides precipitated on the grain boundaries.
Phase Instability in Hastelloy N
Though Hastelloy N is basically a solid-solution alloy, various thermomechanical treatments do change its mechanical properties and micro-. structure. Since it is intended for-use up to 850°C and is now used in molten fluoride salt reactor systems, we must understand the nature of the precipitation processes and the effect on mechanical behavior. Identifying and characterizing the precipitates involved several complementary techniques: optical metallography, transmission electron microscopy, extraction replication, x-ray.diffraction, and electron probe microanalysis. In addition, chemical analysis with a microprobe attachment for the electron microscope and electron diffraction were employed to identify individual particles, agglomerates, and grain-boundary films on extraction replicas without interference from the matrix. These techniques help resolve differences in precipitates and relate the microstructure to mechanical properties. The microstructure is characterized by stringers of massive primary precipitates of the Ni3Mo3Ctype. Exposure between 500 and 1000°C results in precipitation of particles of the NisI%AC type in the grain boundaries. In air-melted heats that contain approximately 0.6s Si, the carbidetype precipitates are enriched in silicon and are not dissolved at high annealing temperatures but melt and transform to-a noncarbide phase. In vacuum-melted heats with low silicon contents, carbides go into solid solution. The only precipitates that form in air-melted alloys at temperatures as high as 1180°C are complex pseudocarbides of the Ni3(Mo,Cr)3(C,Si) and Niz(Mo,Cr)h(C,Si) types. The amount, and behavior of precipitates are highly silicon dependent; this impurity stabilizes the particles, preventing their being taken into solid solution at high annealing temperatures and causing them to transform to the high-temperature phase. This latter phase is possibly the 6-NiMo intermetallic and is probab,ly responsible for the increased embrittlement at high annealing temperatures. * Metals and Ceramics Division, Oak Ridge National Laboratory, operated by Union Carbide Corporation for the U.S. Atomic Energy Commission.
The British Museum (Natural History) — University of London Joint Palæontological Expedition to Northern Rhodesia and Tanganyika, 1963
The British Museum (Natural History) — University of London Joint Palaeontological Expedition to Northern Rhodesia and Tanganyika, 1963