At temperatures above 650 °C (1200 °F),
three precipitate phases can nucleate heterogeneously
at grain boundaries and twin
boundaries. These are μ phase, M6C carbide,
and P phase. The μ phase is described as having
a hexagonal crystal structure and an A7B6
stoichiometry. M6C has a diamond cubic
crystal structure, and P phase has a tetragonal
structure. Reference 13 indicates that μ phase
precipitates in C-276 alloy within the temperature
range 760 to 1093 °C (1400 to 2000 °F),
whereas M6C carbide precipitates at temperatures
between 650 and 1038 °C (1200 and
1900 °F). The same reference indicates that
the kinetics of carbide formation are faster
than those of μ phase