Measurement of temperature dependence of gamma/gamma’-lattice misfit in single-crystal Ni-base alloys with different misfit signs

The gamma/gamma’-lattice misfit delta is an important structural parameter of Ni-base superalloys determining the efficiency of precipitate strengthening and the thermal stability of the g/g’-microstructure. A fundamental question is: which sign of delta is beneficial for creep strength of superalloys? The sign of delta determines the orientation of g’-rafts forming under creep conditions. At high temperature under tensile load and delta<0 the g’-phase forms rafts Normal to axis (N-rafts), but if delta>0 the forming rafts are Parallel (P-rafts). Under compressive load it is vice versa: P-rafts at delta<0 and N-rafts at delta>0. From different arguments it was concluded that delta>0 should be beneficial for the creep strength. But until now no direct experimental proof of these theoretical predictions exists. Two equivalent Ni-base alloys with the same absolute values of misfit but opposite misfit signs are designed, cast as single-crystal bars and tested under creep conditions.