Precipitation and growth of gamma '' phase in a Ni-22Cr-9Mo-5Fe-4Nb superalloy

Hardening behavior due to precipitation of a Ni3Nb(gamma ") phase during ag ing was experimentally studied for a Ni-22Cr-9Mo-5Fe-4Nb alloy. The alloy w as homogenized at 1373K for 3.6ks and then quenched in water. The homogeniz ed alloy was aged at temperatures between 894 and 1073K for up to 720 ks. T he aging was also carried out at 893 to 1033K for up to 720 ks for the allo ys cold roiled with reductions of 4, 10, 30 and 50% after the homogenizatio n. The micro Vickers (MV) hardness test was made for the aged specimens. Th e microstructures of the specimens were observed by transmission electron m icroscopy (TEM). For the specimens without cold rolling, the MV hardness monotonically incre ases with increasing aging time at 893, 943 and 1073K. On the other hand th e MV hardness increases with increasing aging time and then reaches the max imum values at 36 and 72 ks at 993 and 1033K, respectively. In the specimen s with the maximum values of the MV hardness, disk-like gamma " precipitate s with diameters of 30 to 35 nm were observed in a gamma matrix. Coarsening of the gamma " phase seems to obey the Ostwald ripening mechanism. The act ivation enthalpy for the coarsening was obtained to be 418 kJ/mol. For the specimens with cold rolling, over-aging is rather restrained due to fine ga mma " particles precipitated along dislocations.