Abstract: The thermal fatigue behaviour and microstructure in a directionally solidified DZ38G alloy has been investigated.It was found that DZ28G alloy has larger thermal fatigue resistance than that of original M38G alloy because of the elimination of the transverse grain boundaries and the small elastic modulus.Detailed observation shows that microcrack is initiated at the grain boundaries or the phase interfaces and propagates along interdendrites,grain boundaries and,sometimes,along the defined crystallographic direction and that the amount of carbides at grain boundaries increases and the morphology of phase becomes irregular after thermal fa- tigue test. Both γ' denudation and recrystallization have been observed on both sides of the crack.

Key words: DZ38G superalloy, thermal fatigue, directional solidification, microstructure