Damage evolution and acoustic emission mechanisms in α2+β/SCS-6 titanium matrix composites

Damage evolution and acoustic emission mechanisms have been investigated during the tensile deformation of two alpha(2) + beta titanium aluminide matrix composites reinforced with SCS-6 silicon carbide fiber. The alloys had distinctly different beta phase morphologies and resulting ductilities. A Ti-14 Al-21 Nb matrix composite with a matrix failure strain significantly greater than the fiber exhibited annular microcracking of a brittle B-depleted matrix zone surrounding the fibers. Acoustic emission measurements indicated that this damage process increased rapidly near the composite yield point and continued at a constant rate thereafter. Acoustic emission detection of fiber fracture indicated that failure occurred after about four fiber fractures at a significantly lower stress than predicted by a global load sharing model. A Ti-13 Al-15 Nb-4 Mo-2 V-7 Ta matrix composite with a matrix failure strain less than the fiber exhibited multiple matrix cracking. Acoustic emission measurements indicated that matrix cracking initiated well below the stress where primary matrix cracks were first visually observed. Failure occurred after numerous fiber fractures at a significantly lower fiber stress than predicted by a fiber bundle model. Damage evolution data obtained from the calibrated acoustic emission measurements were combined with a simple micro-mechanical model to predict the inelastic contribution of matrix cracking to the overall deformation behavior. (C) 1997 Acta Metallurgica Inc.