= Solution
At turbine operating <temperatures>, <grain boundaries> provide paths for rapid <diffusion>, <grain-boundary sliding> and cavity formation, encouraging <creep (deformation)> rupture. A <single-crystal turbine blade> removes these boundaries, especially boundaries unfavourably oriented across the tensile load. It retains <dislocations> and can still <creep (deformation)>; eliminating boundaries is useful in this high-temperature regime even though fine <crystal grains> can strengthen other materials at lower <temperatures>.
Use <directional solidification> of the cast alloy through a controlled thermal gradient. A suitably oriented seed or a narrow <grain selector> lets one crystal survive and grow into the blade mould while suppressing competing <nucleation>. A spiral selector is a common way to exclude competing <crystal grains>. The blade is generally oriented near a $\langle001\rangle$ axis for the intended <anisotropic> properties. <crystal grain> selection is experimentally verified by orientation mapping, rather than assuming a directionally elongated casting is automatically a <single crystal>.<Grain Selection in a High-Efficiency 2D Grain Selector During Casting of Single-Crystal Superalloys by Xintao Zhu and others>[Sections 1–3, directional casting, selector geometry and electron-backscatter diffraction of surviving grains]{ref}
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