Quantitative characterisation of γ′ precipitation and its role in governing robust properties in the Alloy 925 material system

This work quantitatively investigates the γ′ precipitation behaviour and mechanical properties of the Alloy 925 (UNS N09925) material system to elucidate its resilience to compositional variations and aging treatments. Using advanced characterisation techniques, it is shown that both composition and aging parameters have a distinct influence on the volume fraction and size of γ′ precipitates during production-relevant heat treatments, including one-step aging at 720–760°C and subsequent second aging steps at 621–635°C. Quantitative analysis of the γ′ volume fraction, implementing a novel analysis procedure for high-energy X-ray diffraction data proposed in this work, shows that minor changes to the Ti and Al contents result in changes of up to 2 vol.% in the γ′ phase during aging. In addition, the introduction of a second aging step at lower temperatures increases the γ′ fraction by approximately 2.5 vol.%. Small-angle X-ray scattering captures not only the progressive coarsening with increasing temperature and time during one- and two-step aging, resulting in precipitate diameters ranging from 4 to 22 nm, but also the compositional effect on precipitate size. The experimentally obtained quantitative data for the γ′ precipitates allow the prediction of the yield strength increase associated with precipitation strengthening as well as the validation of precipitation kinetics modelling. Comparison with the results of hardness measurements reveals a strong correlation between the γ′ precipitation behaviour and the mechanical properties due to order-type strengthening associated with the weakly coupled dislocations mechanism.