Microstructure evolution and strengthening mechanism of wrought Ni-41Cr-7Al alloy

The microstructure/mechanical property relationships of Ni-41Cr-7Al (at%) have been examined. The alloy homogenized above 1473 K is very plastic. However, it exhibits an extremely high age-hardening at 973 K and the room-temperature tensile stress reaches more than 2.3 GPa with 2% elongation. The superior strength of the alloy is attributed to fine cellular microstructure consisting of gamma/gamma' lamellae. The formation of the cellular microstructure is caused by a discontinuous precipitation reaction (gamma-->gamma+gamma') from grain boundary toward the interior. The TTT diagram reveals that the reaction occurs from 773 K up to about 1200 K, and its start and finish temperatures show typical C curves with a nose at around 1000 K. Above 1200 K only alpha-Cr phase precipitates within the gamma grains. The gamma' phase in lamellar form is not an equilibrium phase containing Cr more than 40 at%, and it decomposes to gamma + gamma' + alpha-Cr phases after prolonged aging. The formation of the metastable gamma' phase is attributed to an existence of metastable gamma+gamma' two-phase region.