Do the fine grain structure provide better tensile strength properties but reduction in creep resistance, if yes do we need coarse grains for turbine disk application. If yes, what is the optimized size of grains to balance both properties and how one can achieve that.
Best is single crystal, to prevent Grain Boundary (GB) rotation/ sliding, but more importantly to prevent diffusion of point defects as well as segregation along GB (Compare Nabarro-Herring vs Coble Creep and their temperature dependence with rising temperature), both are detrimental for creep resistance. High-T superalloys are rarely made into fine-grained structure as-is for strength raising- and even if such is done, GB is made free of brittle precipitate and microalloying agents that retard grain growth and prevent bulk/GB diffusion by segregating "useful ones" at GB- for Ni-based superalloys, they are Mg,B,C,Zr.
You may use the Creep relations (like NH, Coble, Harper-Dorn; latter one is important only in case of wish for persistent work-hardening retainment) to estimate creef life from T and grain size, and good-old hall-petch and orowan relations (for precipitates and dislocations -both) to relate grain size with strength. Thus it reduces to balance between strength and time
ASTM6 is the optimal grain size for superalloys. This size is a very suitable size for the inconel 718, for example under service conditions.
Best
There is no simple answer, since you have different superalloys for different parts. Blades are rotating at high temperature, where creep is a primary issue. There you need large grain or directionally solidified or single crystal, if possible. Disk works at little bit lower temperature and other issues are important too - as high temperature fatigue and strength. Then smaller grain is appropriate. Disks are often hot forged, blades are cast, etc.
Someone will probably know more about creep in case of disks.
That really depends on the component design and material. As you mentioned, larger grain size is better for better creep, stress rupture performance. But, turbine disks also need a high static strength and fatigue resistance, which is promoted by small grain size. Thus, one needs to access design constraints to choose the best grain size.
For the disk application, as it doesn't undergo high pressure as turbine blades, it is not required to have a single crystal structure due to cost reasons. But it is always good to have coarse grain for high-temperature application due to creep resistance requirement. With increasing grain size, you can compromise on many other required mechanical properties such as fatigue, tensile strength etc. Therefore, you need to optimize it further based on the phases present in your alloy and its roles. Based on my literature survey so far, most polycrystalline superalloys for turbine disk and blade application have 40-50 micrometer grain size for optimized properties.
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