Influence of Multiaxial Stress and Crystal Control on Creep Deformation / Fracture of Ni-based Superalloy Rene80

Conventional Casting (CC) and Directional Solidified Casting (DS) Ni-based superalloy Rene80 are being widely used for high temperature components such as gas turbine blades due to their superior high temperature strength and corrosion resistance. Creep damage gradually proceeds especially at stress concentration portion under multiaxial stress with stress gradient in high temperature components. Therefore, it is important to clarify creep damage extension process and to develop a creep rupture time prediction method for maintaining reliable operation. In this study, smooth and two kinds of circular notch specimens were machined from both CC and DS Rene80, and creep tests using these specimens were performed at 900℃. Creep deformation until steady state creep regime of both CC and DS smooth specimens are similar, while transition to acceleration creep of the CC specimens is much shorter period than that of the DS specimens resulting in lower rupture ductility and shorter rupture time in the CC specimens. Creep rupture times of notch specimens of both CC and DS are longer than those of smooth specimens indicating notch strengthen effect. Although creep voids and grew on grain boundaries causing intergranular failure in CC specimens, no void was observed in DS specimens. From finite element creep analysis, multiaxial stress states occurred across notch root section in notch specimens causing lower axial strain rate than smooth specimens. It was demonstrated that creep rupture times of both CC and DS notch specimens were accurately predicted based on the area average creep damage concept.