Metal matrix composites are developed as heat sink materials with superior thermal properties. Future applications like power electronics or fusion reactors require new solutions on thermal management. In this work several composite types with fiber and particle reinforcements were investigated under simulated operation conditions, concerning their internal stresses and thermal fatigue damage. New non-destructive methods such as neutron/synchrotron diffraction and synchrotron tomography were combined for in situ experiments. The phase sensitive study of elastic as well as plastic deformation processes on the microscopic scale allowed conclusions on reinforcement architectures, their types and effects on thermal fatigue damage in composites. A comparison of neutron with synchrotron radiation was given by the example of diffraction and its experimental capabilities for heterogeneous materials.