Abstract
Aged microstructures of a series of Fe-(10-25)Ni-16Cr-(0-18)Mn-2.4Ti-1.5Mo alloys have been investigated by microstructural observations and Vickers hardness measurements. The alloys were solution-treated at 1423 K for 7.2 ks and aged at 973 K for 173 ks and 346 ks. The solutionized microstructures of the alloys were; (a) single-phase austenite (γ), (b) γ+χ(+σ)+small amount of δ ferrite and (c) γ+χ+large amount of δ ferrite. The aged microstructures and hardening responses for the respective solutionized microstructures were as follows :
For (a), the uniform precipitation of γ′ phase was observed. The hardness increase by aging for 173 ks and 346 ks was about 185 Hv and 190-220 Hv, respectively. For (b), the precipitation of χ phase along grain boundaries and intragranular coarse γ′ phase were observed. The aged hardness remarkably decreased with an increase in the amount of χ(+σ) phase in the solutionized microstructure. For (c), the decomposition of δ ferrite into χ+γ phases and a little or no γ′ precipitate in γ grains were observed. Only a small increase in hardness due to the δ ferrite decomposition was obtained by aging for durations up to 346 ks.
Relations between the aged microstructures and the average electron concentration (e⁄a) or the average electron-vacancy number (\barNv) of the alloys have been also discussed.
