High-temperature materials and creep
Why are directionally solidified columnar-grained or single-crystal nickel-based superalloys used for jet engine turbine blades?
- ABecause a single crystal contains no dislocations at all, so creep deformation cannot occur
- BBecause a single crystal allows more carbon and boron, which are used to strengthen grain boundaries, to be added, so the alloy can be strengthened by their carbides and borides
- CBecause removing grain boundaries perpendicular to the stress eliminates the sites where creep damage starts, and a single crystal can do without grain-boundary-strengthening elements, so it can be solution treated at high temperature to make γ' uniform and fine
Show answer and explanation
Answer: C. Because removing grain boundaries perpendicular to the stress eliminates the sites where creep damage starts, and a single crystal can do without grain-boundary-strengthening elements, so it can be solution treated at high temperature to make γ' uniform and fine
Turbine blades are pulled along their length by centrifugal force, and grain boundaries perpendicular to the stress become the starting points of creep voids and cracks. Directional solidification produces columnar grains aligned with the stress, removing the perpendicular boundaries. In single-crystal blades, grown so that only one crystal is selected, there are no grain boundaries at all. When C, B, Zr and other grain-boundary strengtheners are no longer needed, the temperature at which the alloy begins to melt rises, so solution treatment can be done at high temperature to dissolve γ' fully and then reprecipitate it uniformly and finely. The statement about adding more strengtheners reverses this relationship. Single crystals still contain dislocations, and creep proceeds by their motion within the crystal.
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