Decreased Ti/Al ratio modifies the precipitate behavior of Cr23C6 and improves long-term stress-rupture life

Abstract

The long-term stress-rupture life of Nimonic 80A with varied Ti/Al ratios was investigated at 650 °C/300 MPa. The stress-rupture life increases from about 6479 to 7734 and 11713 h, and the duplex grain size distribution evolves from bimodal morphology to near-homogeneous morphology with the decreased Ti/Al ratio. Although the stress-rupture life increases significantly, the stable γ′ homogeneously distributes in γ matrix with a coherent orientation relationship with γ on {100}γ′/γ and {110}γ′/γ atomic planes. With the decreased Ti/Al ratio, more precipitation of TiC particles leads to the depletion of C element near grain boundary, which suppresses precipitation of Cr23C6. A large amount of plate-like Cr23C6 carbide which precipitates at grain boundaries can easily lead to the intergranular cracking, but the blocky Cr23C6 carbide which shows an orientation relationship on {111}γ atomic planes with the nearby matrix at the twin boundary does not cause grain boundary cracking. Lower Ti/Al ratio benefits longer stress-rupture life based on the near-homogeneous duplex grain size distribution and none precipitation of plate-like Cr23C6.