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

The transverse stress-strain behavior of several titanium metal-matrix composites (TiMMCs) has been studied in-situ. Debonding of 1140+/Ti-6-4 composites occurs over a range of stresses. The sharpness of the first "knee" is affected by the fiber volume fraction and by the relative moduli of the matrix regions and the reinforced composite. It has been observed that debonding occurs mainly at the interface between two sublayers of carbon/carbon coatings in 1140+/Ti-6-4 composites and mainly at the interface between the carbon/reaction zone in the as-processed and peak-aged 35 pct SCS-6/tiβ21s composites. At surface positions, this process starts at very low stresses (≥50 MPa) from the positions with sharp changes of curvatures (or undulations), voids, or debris at the periphery of the interface. 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/Tiβ21s composites occurs during elastic deformation of the matrix. This has been directly observed in a field-emission gun (FEG)-scanning electron microscope (SEM) under incremental loading. Although these cracks are arrested and blunted by the matrix material, they cause local stresses and, thus, stimulate local plastic deformation of the matrix and subsequent development of a second knee on the stress-strain curve. The in-situ observations are discussed in terms of the effects of fiber volume fraction and fiber type on the loci and dynamic processes of interfacial debonding, cracking of carbon coatings and reaction zones, and plastic deformation of the matrix.