The Influence of Chromium Content on the Precipitation of γ′ (Ordered Ni3Ti) in Some Austenitic Steels

Abstract

A series of four austenitic steels containing 2% Ti and 25% Ni, with chromium contents ranging from 4% to 17%, have been solution-treated at 1150 °C and aged at 750 °C for periods up to 2200 hours. Precipitation processes were followed by electron microscopy.In every case the γ′ phase precipitated initially coherently and in parallel orientation with the matrix, there being no tendency for the formation of randomly oriented γ′, as reported by Pitler and Ansell [Trans. Am. Soc. Metals, 57 (1964) 727]. The stacking-fault energy of these alloys appeared to be consistently lower than that estimated by Pitler and Ansell, and serious doubts are cast on their proposal.In alloys of lower chromium content, there was a tendency for the γ′ phase to nucleate preferentially in the region adjacent to the grain boundaries. One factor contributing to this effect is considered to be the change in relative density of matrix and precipitate as the chromium content of the matrix is reduced, lending to a vacancy-emitting rather than a vacancy-absorbing situation.