Elastic microstrains during tension and creep of superalloys: Results from in situ neutron diffraction

Abstract

In situ neutron diffraction experiments were conducted to monitor elastic microstrains during tension and creep of a directionally solidified (DS) superalloy PWA1422 and a single crystal (SX) alloy PWA 1484. The rationale was to obtain insight on internal stress evolution, and thereby improved understanding of deformation mechanisms. The misfit between the gamma (γ) and gamma-prime (γ′ ) phases were determined as a function of temperature and deformation. The microstrains in the tension test were used to obtain the critical resolved shear stress (CRSS) of the phases. Results show a dramatic increase in the CRSS of the γ-phase compared to the bulk, and are consistent with dislocations bowing (Orowan type) between narrow γ-channels. A threshold stress is calculated for dislocation activity in the horizontal channel, and is suggested as a minimum stress for initiating any creep or rafting. Microstrains from the creep test at 900°C/425 MPa are consistent with the formation of dislocation networks, but the data do not suggest that the CRSS is exceeded for cutting of the γ′ . Rather, rafting is suggested for the onset of tertiary creep. At 1092°C/120 MPa, the far-field creep strain exhibits an unusual behavior in the pre-steady-state low strain (<0.5%) regime. This is also accompanied with creep acceleration and decrease of elastic strain in γ′ . The response is accompanied with the formation of a completely rafted microstructure. An explanation is provided in terms of deposition of actual misfit dislocations preferentially on the vertical interfaces.