THE INFLUENCE OF CHROMIUM CONTENT ON THE PRECIPITATION OF GAMMA' (ORDERED NI3TI) IN SOME AUSTENITIC STEELS (REPRINTED FROM METALLOGRAPHY, VOL 2, PG 309-322, 1969)

A series of four austenitic steels containing 2% Ti and 25% Ni, with c hromium contents ranging from 4% to 17%, have been solution-treated at 1150 degrees C and aged at 750 degrees C for periods up to 2200 hours . Precipitation processes were followed by electron microscopy. In eve ry case the gamma' phase precipitated initially coherently and in para llel orientation with the matrix, there being no tendency for the form ation of randomly oriented gamma', as reported by Pitier and Ansell [T rans. Am. Soc. Metals, 57 (1964) 727]. The stacking-fault energy of th ese alloys appeared to be consistently lower than that estimated by Pi tier and Ansell, and serious doubts are cast on their proposal. In all oys of lower chromium content, there was a tendency for the gamma' pha se to nucleate preferentially in the region adjacent to the grain boun daries. 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, leading to a vacancy-emitting rather than a vacancy-absorbing situation.