Crack Propagation Properties of Ni-Base Superalloys Under Creep-Fatigue Londing.

Abstract

This paper describes an experimental study on crack propagation properties of conventionally cast and directionally solidified Ni-base superalloys, Mar-M247CC and DS, under creep-fatigue loading. The following points were made clear: ( 1 )The crack propagation properties of Ni-base superalloys under creep-fatigue loading were divided into cyclic and time-dependent types as reported before. The result is due to a difference in fracture modes, transdendritic fracture in cyclic-dependent crack propagation and interdendritic fracture in time-dependent crack propagation. ( 2 )In time-dependent crack propagation, the resistance to crack propagation under CP loading was smaller than that under CC loading because of a difference in creep in the process of compression, similar to those of polycrystalline austenitic stainless steel and single-crystal Ni-base superalloy at elevated temperatures. ( 3 )In time-dependent crack propagation, nominal crack propagation rates of Mar-M247CC were lower than those of Mar-M247DS, because the crack traces a zigzag course along the dendrite boundaries in the case of Mar-M247CC crack propagation. ( 4 ) From conventional and present studies, it was clarified that the transitions of fracture modes from polycrystalline heat-resisting steels to a single crystal changed from transgranular through transdendritic to gamma prime transphasic ftactures in cyclic-dependent crack propagation, and from intergranular through interdendritic to gamma prime interphasic fractures in time-dependent crack propagation.