Visco-plasticity during in-situ cooling from solidification of a nickel-base single crystal superalloy using neutron diffraction

In-situ neutron diffractometry is performed to study the visco-plasticity in a nickel-base single crystal superalloy during in-situ cooling from high temperatures. It is found that visco-plastic deformation has two contributions from creep and stress relaxation, which are subject to the accumulation of dislocation activity and dislocation annihilation, respectively. Use has been made of the lattice strain evolution of the (200) γ+γ′ peak to confirm this effect. The decrease in lattice strain and macro-stress during in-situ cooling has been observed and confirms that there was softening taking place before thermal strain dominates at lower temperatures. Therefore, in-situ isothermal cyclic loading and relaxation tests under strain control, akin to thermal contraction during casting, have been carried out. A visco-plasticity law was then developed based on macro-strain development during creep and lattice strain evolution during stress relaxation within an appropriate timescale, where transient effects are captured. The constitutive law developed has been used to independently determine the evolution of stress and strain during in-situ cooling. The implementation of these findings into thermo-mechanical modelling during cooling from solidification is also discussed.