Without going into too much detail. Metals have a type of crystal structure (definition of a metal) usually many crystals in a piece and this can be clearly seen in polished and etched specimens. A common example is old brass doorknobs polished by palms and etched by sweat. The reason etching shows the grains is that the grain boundaries often have residual stresses and impurities tend to congregate there so etching will dissolve the metal at a different rate at the boundary compared the the middle of a grain. One of the methods of getting the desired properties is fiddling with the grain size.
In the case of materials for high temperature the impurities at the grain boundaries can weaken the structure by in the worst case melting or by less dramatic means. For turbine blades the first step (30 yrs ago) was carefully controlling the composition,casting and cooling process in a vacuum to give a few long crystals the full length of the blade to avoid crystal boundaries across the direction of the main stress - Directionally Solidified Blades. Further development enabled the turbine blades to be cast as a single crystal.
The exact compostions tend to be secret but nickel/cobalt is often used as a base with all sorts of exotic additions eg hafnium, ytterium and rhenium