Relating micro-segregation to site specific high temperature deformation in single crystal nickel-base superalloy castings

Thermo-mechanical deformation of the solid on cooling following solidification has been studied quantitatively in a Ni-base single crystal superalloy, CMSX-4 used in turbine blade applications. In the as-cast state, the alloy has location specific properties due to micro-segregation of alloying elements during solidification; this effect become increasingly important with smaller specimen cross-section in thermo-mechanical tests. Accordingly, normalised resistance/resistivity tests have been used to classify samples with varying micro-segregation, given the different γ and γ′ phase resistivities. Increased normalised resistance corresponds to increased local solvus temperature, which determines the plastic strain and stress evolution during cooling. Upon cooling from above the γ′ solvus temperature, dislocation creep occurs within the γ phase accompanied by a small increase in stress. A critical precipitation γ′ volume fraction is reached as the material cools, leading to precipitation hardening as measured by a dramatic resistance change and thereby stress increase at lower temperatures. Short-term creep tests capturing the history-dependent deformation, as demonstrated by controlled cooling experiments, gives steady-state creep, enabling parameter measurement for a Norton-type constitutive equation in a given temperature range. Implications of these results to modelling of plastic strain and stress during cooling from close to solvus temperature during casting has been discussed.