Creep anisotropy in nickel base γ, γ′ and γ/γ′ superalloy single crystals

A comprehensive study of creep deformation as a function of orientation for a broad range of single crystal nickel base alloys, is presented. The alloys include a wide range of precipitation hardenable gamma/gamma' superalloys, as well as single phase gamma and gamma'. To provide a complete overview, results of other critical studies are also cited. In general it is observed that superalloys, predominantly strengthened with elements such as Mo, partitioning strongly to the gamma matrix. behave very anisotropically, as do single phase gamma alloy. In contrast most superalloys strengthened with refractory elements such as Ta, which partition to the gamma' phase, tend to display isotropic creep behavior, as do most single phase gamma' alloys. The seemingly wide variation in creep anisotropy with composition, as well as with temperature, and microstructure, is in most cases quantitatively rationalized based on a model requiring only a single parameter. The parameter is defined as the ratio of creep resistance of cube slip to octahedral slip for a given alloy, at the temperature and microstructure of interest. The model shows that increased participation of cube slip is responsible for the isotropic behavior of single phase gamma' and several superalloys. An eight-fold enhancement in creep resistance for [111] orientation over [100] orientation is predicted and observed, for a gamma alloy, which predominantly deforms by octahedral slip. A concept of threshold shear stress is proposed, to account for occasional observation of unusually high creep anisotropy at low stresses.