The paper presents the investigation of the expansibility of an Al-Si alloy in the form of a protective coating applied on a steel substrate and as a conventional cast, respectively. Examinations were carried out in the temperature range of troom÷220°C using the diffractometric technique based on reflections (311), (222), (400), and (331) from the Al solid solution. It was demonstrated that the coating exhibited better thermal expansibility compared to the cast, and its thermal expansion coefficient was close to the standard values for alloys of the same chemical composition. For the cast, the thermal expansion coefficient was lower by approx. 25% in relation to the standard value. It was also found that the soaking of alloys during X-ray examination reduced their texturing and caused changes in the microstructure of both the coating and the cast by thinning the silicon crystals and rounding their edges.