5.2.2. Complex Types of Rafting

The types of raft structures formed are largely dependent on the loading condition (direction, tension/compression, etc.), such as the 45-degree rafting in <011> direction [37] (Figure 4). Kamaraj [40] reported the formation of raft structures with 45◦ angle to the shear stress in a double shear creep test. Such a type of raft structure was also observed experimentally during the creep of MC2 nickel-based superalloy at high temperature and attributed to the highly localized creep strain [90]. However, the angle between the direction of the raft structure and the mechanical loading varied from the region near the fracture with highly localized creep strain to the region far away from the fracture.

Gaubert et al. [91] analyzed the microstructure evolution of <011>-oriented Ni-based superalloys using a 3D mean-field visco-plasticity model. Their results demonstrated the directional coarsening of raft structures along <100> direction. A slight deviation between the loading direction and <011> direction drove initial cubic γ' precipitates to coarsen firstly along <100> direction and extended then along <001> or <010> direction. Yang et al. [92] performed a similar study on the microstructure evolution of nickel-based superalloys with the loading direction deviated from <001> orientation. They referred to this loading mode as monoclinic loading, which can be equivalent to a shear-loading mode and a tension-loading mode. They analyzed the microstructure evolution under monoclinic, shear, and tension loadings, respectively, and found nearly no rafting behavior under the shear loading and synchronous N-type rafting for the monoclinic and tension loading under small stress. Increasing applied stress led to the formation of 45-degree rafting in the overall region under the shear loading and in the partial region under the monoclinic loading. A N-type rafting was still found under the tensile loading, and its rafting rate was smaller than that under the monoclinic loading. Increasing applied stress increased the role of the shear stress component in the rafting process under the monoclinic loading.

Ali et al. [93] combined the phase-field method with physics-based crystal plasticity model to explain the formation of a 45-degree rafting in the region with local creep strain larger than 10% under a tensile loading in <001> direction. Their results showed that creep strain in some regions was significantly higher than average creep strain. A large amount of geometrically necessary dislocations was found in these regions, which caused the change of the original direction of the raft structures and led to the formation of the 45-degree rafting.
