Directional solidification and single crystal technologies are well developed and impart better properties than the polycrystalline superalloys. then why is the polycrystalline technology is still relevant? Why are people still doing research on polycrystalline superalloys?
Hi
It has become more common to use single-crystal rather than poly-crystalturbine blades. The reason for this can be attributed to two things: enhancedcreep and fatigue properties.The casting of blades in single-crystal form is a VERY COMPLICATED PROCESS and is done by investment casting with directional solidification.
In Figure a simple drawing shows how investment castingleads to a single-crystal microstructure. In the process, the superalloy material is melted in a vacuum furnace before being retracted from the furnace in a controlled direction. The front edge of the cast is cooled during the retraction. During cooling, columnar grains start to grow parallel to the directionof the retraction. By use of a grain selector, only one grain is permitted to grow any further within the component. After the grain selector, the single grain continues to grow through a pig tail shaped spiral. The spiral is fol-lowed by the actual blade form where the melt continues to solidify into one grain. After casting the bottom part, the part with columnar grains and the pig tail shaped part, is removed by machining.
By Virtue of Highly Complexity and Sensitivity involved in Single Crystal Technologies,Poly Crystalline Technology is still Prevalent.
thanks & regards,
g.sudhakar
phd(materials engineering)
H.C.U
Hi
Suggested Reading :Nickel-Based Single-Crystal Superalloys -the crystal orientation influence on high temperature properties
thanks & regards,
g.sudhakar
phd(materials engineering)
H.C.U
These superalloys in polycrystalline condition are still superior than many steels / alloys in many of the elevated temperature properties. These alloys need not be used for Turbine blade applications. There is a small motor called Jet Fuel Starter / AFP which actually starts the main aero-engine of an aircraft. These alloys in polycrystalline condition are used for such applications. Further the manufacturing of single crystal superalloys are very costly too. In our country we hardly had 3 to 4 single crystal furnaces manufacturing single crystal components.
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