The aging of the NC 19 Fe Nb alloy (Inconel 718). previously quenched
from 990 degrees C, is characterized by a hardness peak at 650 degrees
C, then a maximum in hardness at about 750 degrees C. Over this tempe
rature. the hardness progressively decreases. In the 550-650 degrees C
temperature range. TEM observations have revealed that beta (Ni3Nb) p
recipitates are formed as long platelets parallel between them within
the same grain. as well as extremely fine gamma'[Ni-3(Ti,Al)] particle
s responsible for the observed improvement in hardness. For a temperin
g temperature higher than 650 degrees C, a first hardening occurs afte
r a 4 h treatment, which has been associated with the gamma' phase pre
cipitation, with a more or less spherical shape. Beyond this time. a s
econd hardening takes place linked to the gamma'' phase precipitation
(Ni3Nb, bct D0(22) structure), as thin platelet shaped. perfectly cohe
rent with the matrix. The misfit between the gamma and gamma'' phases
is about 3% in the (001)gamma'' direction and lower than 1% in the (10
0)gamma'' and (010)gamma'' directions. During a longer aging at 750 de
grees C, the gamma'' platelets progressively dissolve while beta preci
pitates grow.