The Role of Interfaces in Ni-Base Superalloys
A typical commercial Ni-base superalloy consists of a coherent precipitate of γ′ (basically Ni3(Al,Ti)) in a matrix of γ (Ni-solid solution), interspersed with minor carbides and eutectics. It contains, therefore, three types of interfaces; (1) grain boundaries, (2) coherent phase boundaries γ/γ′ and (3) incoherent phase boundaries γ + γ′Vcarbide. The structure is further complicated by dendritic segregation and a partitioning of solid solution elements between γ and γ′.
Grain boundaries in Ni-base superalloys are a source of weakness at high temperatures. Carbides can be crack-initiators in fatigue. Coherent γ/γ′ interfaces enhance the strength of the superalloy. Specifically, a force must be applied to constrict a superlattice dislocation pair at γ/γ′ interfaces in order to shear the γ′ particles. The magnitude of the force depends primarily on the antiphase boundary energy of γ′.
In uni-axially stressed components, the influence of weak transverse grain boundaries can be eliminated by making use of a columnar grain structure. Further improvement in properties can be realized by eliminating grain boundaries altogether to produce a single crystal. In this case, the properties of the material can be optimized by appropriate choice of orientation, taking advantage of the intrinsic anisotropy of a crystal. The elimination of carbides in alloys designed specifically for single crystal application should improve fatigue resistance. Further improvement in the strength of these alloys can be realized by alloying to enhance the antiphase boundary energy of γ′.
KeywordsCritical Resolve Shear Stress Intergranular Crack Single Crystal Alloy Advance Material Research Dendritic Segregation
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