CRSS of γ/γ′ phases from in situ neutron diffraction of a directionally solidified superalloy tension tested at 900 °C

Neutron diffraction was used to monitor elastic strains during in situ tension testing of a directionally solidified (DS) superalloy at 900 °C. Changes in misfit and thermal expansion coefficients of individual phases were obtained. In the γ phase, it is demonstrated that elastic strains saturate at 350 MPa, which is well below the yield strength of the alloy. This is interpreted as the onset of dislocation glide through less stressed vertical channels. The critical resolved shear stress (CRSS) of γ is found to be 143 ± 11 MPa, in agreement with a calculated CRSS that is dominated by Orowan bowing of dislocations through nanoscale-wide γ channels. This provides confirmation of Orowan bowing in plasticity/creep of the γ phase. Implications of CRSS and misfit in a “threshold stress” for creep and rafting are discussed. The CRSS of γ′ is found to be consistent with pairwise penetration of dislocations into γ′.