Decrease in grain size by heat treatment alone is possible when the material undergoes phase transformation, such as austenite to ferrite+ pearlite as in carbon steels. Nickel base and cobalt base alloys do not undergo phase transformations (phase stability is what makes the alloys suitable for high temperature applications). Therefore, reduction in grain size is not easy by heat treatment alone unless mechanical working (such as forging, rolling etc) precedes. However, grain growth can occur during solution treatment.

Decrease in grain size by heat treatment alone is possible when the material undergoes phase transformation, such as austenite to ferrite+ pearlite as in carbon steels. Nickel base and cobalt base alloys do not undergo phase transformations (phase stability is what makes the alloys suitable for high temperature applications). Therefore, reduction in grain size is not easy by heat treatment alone unless mechanical working (such as forging, rolling etc) precedes. However, grain growth can occur during solution treatment.

Krishnan Raja Thank you for your attention and answer.

Grain size reduction will have to be preceded by prior mechanical working below the recrystallization temperature

Lawrence O. Osoba , Thanks for pointing out the temperature of mechanical working for the grain size reduction.

Lawrence O. Osoba , Thanks for pointing out the temperature of mechanical working for the grain size reduction.

Lawrence O. Osoba Thank you for your guidance.

After reading other answers, it might be interesting to you to study the effects of static and dynamic recrystallization (alternatively, metadynamic one) on the grain size evolution and strength changes. Try to do it in general and later find studies specific to our type of alloy.

Additionally, it might be interesting to study the results of computational software that is simulating those effects. It was exactly the theme of my PhD that led to the development of the cellular automaton software simulating DRX, which is open-source.