1999 Volume 85 Issue 3 Pages 241-248
Hardening behavior due to precipitation of a Ni3Nb(γ″) phase during aging was experimentally studied for a Ni-22Cr-9Mo-5Fe-4Nb alloy. The alloy was homogenized at 1373K for 3.6ks and then quenched in water. The homogenized alloy was aged at temperatures between 894 and 1073K for up to 720ks. The aging was also carried out at 893 to 1033K for up to 720ks for the alloys cold rolled with reductions of 4, 10, 30 and 50% after the homogenization. The micro Vickers (MV) hardness test was made for the aged specimens. The microstructures of the specimens were observed by transmission electron microscopy (TEM).
For the specimens without cold rolling, the MV hardness monotonically increases with increasing aging time at 893, 943 and 1073K. On the other hand, the MV hardness increases with increasing aging time and then reaches the maximum values at 36 and 72ks at 993 and 1033K, respectively. In the specimens with the maximum values of the MV hardness, disk-like γ″ precipitates with diameters of 30 to 35nm were observed in a γ matrix. Coarsening of the γ″ phase seems to obey the Ostwald ripening mechanism. The activation enthalpy for the coarsening was obtained to be 418kJ/mol. For the specimens with cold rolling, over-aging is rather restrained due to fine γ″ particles precipitated along dislocations.