Creep tests at 1150 ℃

80 – 150 MPa were carried out in a 3rd generation single crystal superalloy DD33. The fracture morphology and damage characteristics in longitudinal section were studied by SEM

EBSD

EDS and TEM. The effect of stress on the fracture mechanisms was analyzed. The results show that the casting pores near the fracture are almost polyhedral

and cracks initiated at the corner of pores. Compared with the lower stress

more creep pores and cracks were induced at 150 MPa. Besides

at 80 MPa

the γ/γ′ matrix bears an extra 35 MPa effective stress due to the thick oxidation affected zone. The highly-localized topologically close-packed (TCP) phase can be found nearby the fracture

and some pores as well as cracks exist at TCP/γ′ interface. With the further increase of applied stress

more surface cracks penetrate into the matrix and more carbides are cracked. Thus

the final failure of the alloy not only directly relates to the internal pores and the pore-induced cracks

but also the effect of the oxidation affected zone

surface cracks penetrating into the matrix

cracks at TCP/γ′ interface and cracked carbides should be considered.