The role of stress relaxation and creep during high temperature deformation in Ni-base single crystal superalloys - Implications to strain build-up during directional solidification

The study of high temperature deformation of Ni-base superalloys finds a number of applications. In this study we consider how stress/strain resulting from directional solidification can be alleviated through phenomena such as relaxation and how this can be measured. Based on a modelling study, a range of stresses between 550 and 650 MPa were chosen for tensile testing at 900°C in the Ni-base superalloy, CMSX4. In-situ neutron diffractometry together with ex-situ transmission electron microscopy have been used to study relaxation occurring during high-temperature deformation. The behaviour of (100) and (200) lattice strains with macroscopic stress has been quantitatively analysed for single crystals with axial orientations within 5°from [100]. The (200) γ+γ^/ fundamental peak has been used to relate the decay in applied macroscopic stress with lattice strain. At high stresses (above 600 MPa), relaxation was particularly pronounced with an immediate appreciable decay in lattice strain (and stress) within 20 min. At lower stresses, significant relaxation is only observed after prolonged hold. Relaxation occurs in both γ and γ^/, as confirmed by presence of dislocations within both phases and it was also with minimal lattice rotation (<3°). Also, the decrease in lattice strain from relaxation was at least two/threefold lower than the creep strain, obtained from sample elongation. It is shown that in modelling of strain during solidification, it is important to consider the relaxation of lattice strain, rather than just creep. Implications of this study to the critical plastic strain for re-crystallisation are addressed.