Thanks to their ability to switch reversibly between an amorphous and a crystalline state, Phase Change Materials (PCM; typically Ge-Sb-Te alloys) are attracting enormous attention for application in various types of memories (embedded, neuromorphic…). Many studies aimed at investigating in situ the phase transition are reported in the literature but almost none at time scales comparable to those used in real devices (ns). In a previous experiment we have been able to induce by laser irradiation different crystallized states with nanocrystalline clusters of Ge or GST embedded in an amorphous matrix. In the present experiment we plan to investigate the dynamics of these very early stages of crystallization at ultimate time scales using a laser pump – X-ray probe experimental scheme.