The effect of temperature on the Deformation and Fracture of SiC/Ti-24Al-11Nb

Abstract

The tensile stress-strain behavior and failure mechanisms of Ti-24Al-11Nb and a SiC/ Ti-24Al-11Nb composite with continuous SCS-6 fibers oriented parallel to the loading direction have been examined over a range of temperatures from 23 °C to 815°C in air. Failure in Ti- 24Al-11Nb occurred at strains of approximately 4 pct soon after crack initiation at low tem- peratures. Ductility increased with temperature up to a maximum of 20 pct elongation at 600 °C, as surface-initiated cracks did not propagate readily at intermediate temperatures. At higher temperatures, the onset of grain boundary and interfacial void nucleation limited ductility. Com- posite failure appeared to be controlled by fiber fracture at all temperatures; for practical en- gineering purposes, composite failure occurred at 0.8 pct strain at all temperatures. At temperatures of 425 °C and less, fiber fractures occurred at intervals along the lengths of the fibers and appeared to be cumulative, while at temperatures of 650 °C and greater, fiber fractures were only observed locally to the fracture surfaces. The decreased radial residual stresses, interfacial strengths, and matrix properties at 650 °C and 815 °C allowed the composite to unload at 0.8 pct strain, due to fiber fractures, followed by a reloading in which fibers pulled out and the matrix failed, resulting in composite failure. The decreasing residual stresses with increasing temper- ature determined from an elastic-plastic concentric cylinder model were shown to affect the stress-strain response of the composite and were consistent with the measured decreasing inter- facial shear stresses, the increased fiber pullout with temperature, and the circumferential de- bonding observed around the fibers at higher temperatures.