ABSTRACT
Nickel-base alloys can form a large number of secondary phases, including numerous intermetallic compounds. Many of these phases are undesirable and may be extremely detrimental. Although commercial superalloys have generally been optimized to avoid their formation, small deviations from the nominal compositions are often sufficient for them to appear. The formation of these phases defines the limits within which the compositions must be maintained and strongly restricts the scope for the development of new alloys. Table 4.1-1. lists these possible marginal phases, together with their crystallographic characteristics. Because of the quasi-absence of minor elements, the number of possible secondary phases in single crystal superalloys is somewhat more limited than in the other grades. Moreover, since the single crystal grades appeared at a relatively late stage in superalloys evolution, they have benefitted from past experience, with the application of more rigorous design principles, such as those outlined in §2.3.3.
