The effect of volume percent of fine γ’ on creep in Ds Mar-M200 + Hf

A correlation was found to exist between creep life at 1800∮F (1255 K) and the volume percent of uniformly dispersed fine (<0.5 μm) gamma prime (γ’) precipitate in direction-ally solidified (DS) Mar-M200 + Hf turbine blade castings. A three-fold increase in creep-rupture life was realized when the amount of fine γ’ increased from about the 30 pct level to approximately a 45 vol pct level. Heat treatment studies revealed that the amount of finey’ developed in this alloy is strongly dependent on solution temperature, especially in the range from 2160 to 2210∮F (1455 to 1483 K). Higher solution temperatures promote the development of greater amounts of fine y’ by more complete solutioning of the coarse γ’ and eutectic γ-γ’ constituents which subsequently reprecipitate as fine y’ upon cooling and aging.