The Origin of Crack Initiation and Internal Stresses in SiC Fibre reinforced Multi Metal Matrix Composites investigated by Synchrotron Radiation

The use of synchrotron radiation has become an essential tool to investigate the properties of a wide range of materials. The highly energetic X-ray beam delivers high flux, short acquisition times and high penetration into the material. The present work focuses on using synchrotron tomography and X-ray diffraction to study multi-metal matrix (3MC) composites during in-situ loading. 3MCs were produced via a process in which SiC fibres are coated with a primary matrix of Ti and then infiltrated with a low melting point filler material as secondary matrix. Two different filler materials Ag28Cu and Zr26Ti25Ni were used as secondary matrix. During the consolidation process, intermetallic phases form in the matrix. These intermetallic phases and their stress state are expected to critically influence the mechanical properties of 3MCs. Beamline I12 at the Diamond Light Source (UK) provides the capability to conduct high resolution tomography (CT) and X-ray diffraction during in-situ loading. CT was used to investigate the microstructure, to locate the origin of appearing cracks and to follow their dissemination during loading. The CT measurements reveal that cracks occur in the brittle zones of the matrix far below the ultimate tensile strength of the composite material. In the case of Ag28Cu, cracks appear in the brittle intermetallic transition zone between primary matrix and AgCu secondary matrix. In the case of Zr26Ti25Ni, cracks are located mainly in the secondary matrix. The aim of the XRD measurements was to determine the internal stresses in the different constituents of the 3MC. Strains in the α-Ti phase follow the expected linear dependency when applying a tensile load. In contrast, the strain in other phases remains unchanged indicating that no load is transferred to these phases.