Acoustic emission and fiber damage in creep of a unidirectional [formula omitted] metal matrix composite at 450°C

Abstract

Metal matrix composites (i.e., MMCs) have potentially for high temperature applications because of light weight, high stiffness and good heat resistance. Especially titanium alloy matrix composites reinforced with continuous SiC fibers are expected to have excellent applications at high temperature, and such MMCs have been tested under tensile, compressive and cyclic loadings to evaluate high temperature strength. The present authors performed creep tests of a unidirectional SCS-6/Ti-15-3 MMC at 450 C. They found that it exhibits creep deformation and rupture in longitudinal creep at stress levels much lower than the tensile strength though the reinforcing phase, SCS-6 by Textron, does not creep at the test temperature. This experimental result can be explained as follows: At high temperature, stress in the matrix is relaxed, and accordingly stress in the fibers is increased, as was analyzed by Goto and McLean. Then, fiber breakage may occur progressively, starting from the weakest fiber, and if a certain number of fibers are thus broken, overall rupture can be induced. It is obvious that this creep rupture takes place more easily under higher applied stress, and that it is promoted if the fibers suffer from damage such as cracking in the reaction zone, degradation of themore » coating layer, and so on. Monitoring of AE (i.e., acoustic emission) can be effective in verifying the creep rupture process mentioned above. Neu and Roman monitored AE during thermomechanical fatigue tests of a unidirectional SCS-6/TIMETAL[reg sign]21S MMC, and they showed how effective this method is. In the present work, first the authors monitored AE in creep tests of a 6-ply unidirectional SCS-6/Ti-15-3 MMC at 450 C. Then, to examine the damage evaluation based on AE monitoring, the authors observed fibers exposed by etching the outer matrix layers in the crept specimens.« less