Under specific conditions of specimen thickness and experimental settings, aberration-corrected high-resolution transmission electron microscopy images of a SiGe alloy grown on a silicon substrate display strong intensity variation from one atomic column to the other. Combining TEM image processing, semi-empirical atomic simulations of large three-dimensional structures including the SiGe/Si interface and TEM image simulations, it is demonstrated that the observed contrast is strongly correlated to the Ge content in the different atomic columns. From a theoretical point of view, this reveals new possibilities for Cs-corrected transmission electron microscopy to observe chemical contrast, and more generally opens new routes for chemical mapping in nanoalloys.