In-situ observations of titanium metal-matrix composites under transverse tensile loading

The transverse stress-strain behavior of several titanium metal-matrix comp osites (TiMMCs) has been studied in-situ. Debonding of 1140+/Ti-6-4 composi tes occurs over a range of stresses. The sharpness of the first "knee" is a ffected by the fiber volume fraction and by the relative moduli of the matr ix regions and the reinforced composite. It has been observed that debondin g occurs mainly at the interface between two sublayers of carbon/carbon coa tings in 1140+/Ti-6-4 composites and mainly at the interface between the ca rbon/reaction zone in the as processed and peak-aged 35 pct SCS-6/Ti beta 2 1s composites. At surface positions, this process starts at very low stress es (greater than or equal to 50 MPa) from the positions with sharp changes of curvatures (or undulations), voids, or debris at the periphery of the in terface. Cracking of the outermost carbon sublayer and of the reaction zone in the 1140+/Ti-6-4 composites and the reaction zone in the SCS-6/Ti beta 21s composites occurs during elastic deformation of the matrix. This has be en directly observed in a field-emission gun (FEG)-scanning electron micros cope (SEM) under incremental loading. Although these cracks are arrested an t blunted by the matrix material, they cause local stresses and, thus, stim ulate local plastic deformation of the matrix and subsequent development of a second knee on the stress-strain curve. The in-situ observations are dis cussed in terms of the effects of fiber volume fraction and fiber type on t he loci and dynamic processes of interfacial debonding, cracking of carbon coatings and reaction zones, and plastic deformation of the matrix.