Rafting microstructure during creep of the MC2 nickel-based superalloy at very high temperature

Directional coarsening of the single-crystalline nickel-based superalloy MC2 has been investigated by means of tensile creep tests at 1100 °C. Two specific specimen geometries were designed in order to generate a variety of stress and strain states. Different coarsening microstructures are observed: N- and P-type classical rafting but also coarsening oriented 45° away from the load axis. The comparison of microstructure maps with the local mechanical state evaluated by finite element calculations shows that the 45° directional coarsening appears in case of very high cumulated strain values (above 10%), independent of the stress sign. Transmission electron microscopy investigations show that the dislocation microstructure is similar in both N-type and 45° coarsened areas.