Micro-mechanism during long-term creep of a precipitation-strengthened Ni-based superalloy

Abstract

The long-term creep behavior of a Ni-based superalloy Haynes 282 at 700 and 750 °C was investigated. The creep curves exhibit the traditional shape with three creep stages. The coarsening of the γ′ phase during creep at 700 and 750 °C can be detected. The applied stress plays an important role in the coarsening of γ′ particles because of the lattice misfit and the difference of elastic modulus between the matrix and γ′ phase. Dislocation shearing into the γ′ phase and the Orowan process are the dominant creep deformation mechanisms at 700 °C/322 MPa. Dislocations tend to shearing into γ′ phase at first; nevertheless, the Orowan bowing mechanism replaces the process of shear as the coarsening of γ′ phase. The dominant deformation mechanism at 750 °C/187 MPa and 750 °C/215 MPa is dislocation gliding combined with dislocation climbing. Dislocation networks distributed in the interface of γ/γ′ phase may change the direction of dislocations and promote them to climb over the γ′ phase. The fracture surfaces were observed by scanning electron microscopy. Intergranular fracture is the dominant failure mode of the three samples because of the softening of grain boundary and stress concentration. Quasi-cleavage fracture, which are attributed to the stress concentration at the carbides/matrix interface, can be observed on the fracture surface of the specimen crept at 700 °C/322 MPa, whereas dimples with small precipitates inside can be detected on the fracture surface of the samples crept at 750 °C/187 MPa and 750 °C/215 MPa.