Detecting the onset and progression of spinodal decomposition using transient grating spectroscopy

Spinodal decomposition can degrade corrosion resistance and embrittle materials. The ability to quickly, non-destructively detect spinodal decomposition via elastic property measurements before catastrophic materials degradation would represent a significant advance in materials testing. We demonstrate that spinodal decomposition can be detected in a binary Fe-Cr alloy via modulus stiffening using ex situ (and in situ) transient grating spectroscopy (TGS). The modulus stiffening is attributable to the convex dependence of the C C ′ elastic modulus on chromium content within certain compositional limits, which has been established by prior density functional theory studies. Over this range of initial chromium compositions, chromium partitioning due to spinodal decomposition raises the average C C ′ modulus such that the decomposed Fe-Cr alloy is stiffer than the initial solid solution. The presence and extent of spinodal decomposition in a 36 at. % chromium alloy are confirmed using atom probe tomography (APT), and are consistent with atomistic simulations that predict elastic modulus stiffening in the same alloy. The observed spinodal decomposition via APT is used to predict an expected modulus stiffening which, based on a simple model, agrees in sign and order of magnitude with the observed stiffening via TGS. The quantitative link between an observed composition change and a measured elastic response suggests TGS as a potential tool for non-destructive evaluation of spinodal decomposition in high-chromium ferritic phases in steels.