Investigation of Crack Initiation and Internal Stresses in Multi Metal Matrix Composites by Synchrotron Radiation

Kurzfassung

Titanium Matrix Composites (TMC) have been developed for demanding aerospace applications. The outstanding mechanical properties like high specific strength and stiffness up to high temperatures have been demonstrated in various studies. However, the high production costs of this material limit the potential applications to very few niche products. Furthermore, the geometries of TMC are limited due to the fact that shrinkage, distortion and even fibre breakage may occur during consolidation by pressure, e.g. hot isostatic pressing. Shrinkage can be avoided by using pressureless consolidation techniques. This can be realized by infiltrating the gaps between the matrix coated fibres with a lower melting filler material under high vacuum. These titanium-based metal matrix composites contain a primary matrix (PM) surrounding the SiC fibres and a secondary matrix (SM) which corresponds to the filler material. A complex transition zone (TZ) between PM and SM develops that mainly consist of intermetallic phases. The aim of the present study was to investigate the behavior of these intermetallic phases under stepwise increased tensile stresses. High energy X-ray radiation was used to detect cracks by computer tomography as well as X-ray diffraction (XRD) to measure internal stresses. By means of these methods and complementary microstructural and fractographic investigations it could be substantiated that transversal cracks originate in the intermetallic TZ far below the ultimate tensile strength of the composite. The evaluation of the XRD data demonstrated that, in contrast to the alpha titanium phase and the SiC fibre, there was no significant elastic deformation in the silver phase of the secondary matrix indicating that the process induced thermal residual stresses in the silver phase exceeded the yield strength. In that case tensile loading causes plastic deformation but no changes of the internal strains can be detected by XRD.