Abstract
Wear-resistant cobalt-based alloys were thermally aged for 30, 300, and 1000 hours at 650°C and 850°C in vacuum sealed tubes of silica. Unidirectional solidification was used to promote coarser structures easier to investigate. The precipitates were characterized by scanning and transmission electron microscopy, X-ray diffraction, and microprobe analysis. During aging secondary M23C6 transforms into M6C. Concomitantly, the primary carbides undergo internal transformation from M7C3 to M6C, and M6C loses carbon and becomes M12C. Three main findings are reported: (1) a correlation between the nature of precipitates and the chemical segregations, (2) modification of the composition, the morphology, and the crystallographic structure of the carbides, and (3) in these alloys M23C6 is only an intermediate phase thermodynamically unstable.
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