To withstand extreme conditions of high temperature and stress, modern aircraft turbine engines require the use of single crystal (SC) nickel-base superalloy turbine blades. A microstructure of SC superalloys consists of a cuboidal coherent precipitates of g’ phase surrounded by g phase channels. The high temperature creep strength of SC alloys results both from the coherency and the nanoscale dimensions of the g phase. The aim of the present work was to investigate the microstructural changes during creep of second generation SC CMSX‑4 superalloy. Chemical composition of the alloy investigated is as follows: Ni ‑ 9.5Co - 6.4Cr - 6.4Ta - 6.4W - 5.6Al - 2.9Re - 1Ti - 0.6 Mo - 0.1Hf (wt %). The creep tests were carried out at 750ºC and 675 MPa as well as 950°C and 185 MPa. The temperature and stress values were selected to cover the range typical for operating conditions of turbine blades. The tests were terminated at the beginning of the secondary creep stage. TEM microstructural analysis were performed on thin foils prepared from {001} longitudinal sections of creep deformed specimens.

In the CMSX-4 deformed at 750°C and 675 MPa, stacking faults incorporated within extended dislocation ribbons passing through g matrix and g' particles were observed. Both extrinsic and intrinsic character stacking faults were identified. Inside the g channels the formation of dislocation networks was detected.

Microstructure of the specimen creep tested at 950°C and 185 MPa exhibited development of directional coarsening of γ’ particles from a cuboidal to a plate-like morphology, so-called rafting. TEM analysis showed the dislocation networks on g-g’ interfaces and rarely cutting of rafted g’ particles by single dislocations.

The results of TEM investigations indicated the two different creep mechanisms of CMSX-4 superalloy. It is of importance in interpreting the creep behavior of the single crystal aircraft turbine blades, and thus determining their life-time.

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