Abstract: An investigation has been made into strengthening mechanism in a single crystal nickel-base superalloy DD8 by transmission electron microscopy. The results show that the stress rupture strength of the alloy increases with decreasing misfit, and the antiphase boundaries (APBs) formed in the ordered g' phase, rather than the misfits, play a dominate role in strengthening of the single crystal Ni-base superalloy DD8. There are three kinds of mechanisms for forming the APBs which were observed in the present materials. One is mis-arrangement of the local ordered atoms in the g'precipitates due to the local strain; the second arises from the 1/2<110> dislocations cutting into the g', and the third is the formation of the APBs induced by the 1/2<110> matrix dislocation network. The contribution of the antiphase boundary energy to the strength of the alloy can be expressed by: where t is the resistance to deformation provided by the APB energy; S is the long-range order degree in g'; TC is the transition temperature from order to disorder; f is the volume fraction of g'; rs is the radius of g'; b is the vector; a is the lattice constant; G is the shear modulus, and k is the proportional constant.

Key words: Single crystal nickel-base superalloy, g' phase, Strengthening