Secondary M6C precipitation in a cobalt-base superalloy

The nature and precipitation characteristics in the directionally solidified cobalt-base superalloy are studied using scanning electron microscopy, energy dispersive analysis, and transmission electron microscopy. The appearance of M6C in the cobalt superalloy is attributed to the addition of minor alloying elements. These elements, tantalum, titanium and zirconium, induces the formation of the primary MC carbide with a high tungsten content. The subsequent decomposition of the tungsten-rich MC carbide makes a local satisfaction in chemistry for the precipitation of the secondary M6C, and the surface of the neighboring M7C3 acted as an effective nucleating base, which promoted M6C precipitation.