The effect of d 0 reference value on a neutron diffraction study of residual stress in a γ / γ ′ nickel-base superalloy

A nickel-base superalloy sub-scale disc has been studied using neutron diffraction to characterise the residual stress distribution following heat treatment. Strain has been calculated using measured neutron diffraction d-spacings in combination with d₀ reference values obtained using three distinct methods, namely, measurement of a stress-free cube, a sin²ψ characterisation (used independently and as a correction factor), and an axial stress balance. In addition, data obtained using the contour method have been included since it allows comparison of diffraction-based data to a technique that does not require a d₀ value. In terms of residual stress, agreement between all d₀ measurement techniques and the contour method was high, with the exception being those made using the sin²ψ technique. An offset of approximately 350 MPa was observed in these data, and a possible systematic error was ascribed to a unique diffraction characteristic of a γ/γ′ nickel-base superalloy. A global d₀ value obtained using a stress balance or stress-free cube was found to be reliable throughout the sample bulk in cases where thermal processing was carried out at a slow rate, such as ageing. However, d₀ differences equivalent to 400 µε were observed between central and surface regions when a global d₀ was applied to a water-quenched sample.