ABNORMAL PHASES IN HIGH W CONTENT NICKEL BASE SUPERALLOYS AND PHASE CONTROL

In this paper, the forming conditions and harmful effects of the abnormal phases such as M6C carbides, �. (W, Mo) and µ in a series of high W content K9, K20 and 601 alloys, which are suitable for forging die, have been studied. The abnormal phases precipitated from liquid belong to primary phases. The large blocky M6C formed in as-cast alloys, as the total amount of (W+Mo) and carbon content exceeded 18 wt% and 0.15 wt% respectively, while the carbon content less than 0.06 wt% promoted the formation of primary �. and µ phases. The addition of elements such as Co or Ru and the rapid solidification exhibited a decreased tendency to precipitate M6C carbides. The primary M6C carbide is the most stable phase among the three abnormal phases and is not removed until the full melting of alloy. Contrarily, the �. and µ are unstable phases and can transform to M6C carbides. The primary M6C carbides severely deteriorate the mechanical properties of the alloys, and the stress rupture life of the alloys at 1100�� /118MPa decreased over 50%, as the volume fraction of primary M6C in alloys was higher than 1.5%. It was very difficult to avoid the formation of primary M6C for a heavy section forging die cast by this series alloys, therefore the compositional modification for these alloys was performed.