Effects of Manganese Content on Microstructures of Solution-Treated Fe-Ni-Cr-Ti Austenitic Alloys

Solution-treated microstructures of a series of iron-base superalloys in which nickel is partially replaced by manganese for cryogenic applications have been investigated over the temperature range from 1223 to 1423 K. Alloy compositions of Fe-(10-25)Ni-16Cr-(0-18)Mn-2.4Ti-1.5Mo (mass%) were chosen on the basis of nickel- and chromium-equivalents in the Schaeffler diagram. The nickel- and chromium-equivalents of the alloys are 16 to 27 and 17.5, respectively.Occurrence of chi phase for 0 to 11.5 Mn alloys and chi plus sigma phases for a 18 Mn alloy was observed after solution treatment at 1223 to 1423 K for 7.2 ks. A close relation between the amount of these phases and the average electron concentration (e⁄a) or the average electron-vacancy number (\barNV) of the alloys was found at the respective solutionizing temperatures: The amount of these phases increased with a decrease in the e⁄a value from about 7.85 for 1223 to 1273 K treatments and from about 7.75 for 1323 to 1423 K treatments. Delta ferrite formation was also observed after solution treatment at 1423 K for alloys with e⁄a≤7.7, and the amount of delta ferrite increased rapidly with a decrease in nickel concentration from about 13 mass%.It was, therefore, concluded that e⁄a≥7.85 (at 1223-1273 K) and e⁄a≥7.75 (at 1323-1423 K) are required for the alloys to have a single austenitic phase by solution treatment.