Cracking mechanisms in thermally cycled Ti-6Al-4V reinforced with SiC fibers

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 1995-04, Vol.26 (4), p.883-895
• Main Authors: THOMIN, S. H, NOËL, P. A, DUNAND, D. C
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Fractures; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/BF02649085

A titanium alloy (Ti-6Al-4V) reinforced with continuous SiC fibers (SCS-6) was thermally cycled between 200 and 700 C in air and argon. The composite mechanical properties deteriorate with an increasing number of cycles in air because of matrix cracks emanating from the specimen surface. These cracks also give oxygen access to fibers, further resulting in fiber degradation. The following matrix cracking mechanisms are examined: (1) thermal fatigue by internal stresses resulting from the mismatch of thermal expansion between fibers and matrix, (2) matrix oxygen embrittlement, and (3) ratcheting from oxide accumulating within cracks. Matrix stresses are determined using an analytical model that considers stress relaxation by matrix creep and the temperature dependence of materials properties. Matrix fatigue from these cyclically varying stresses (mechanism (1)) cannot solely account for the observed crack depth; oxygen embrittlement of the crack tip (mechanism (2)) is concluded to be another necessary damage mechanism. An approximate solution for the stress intensity resulting from crack wedging by oxide formation (mechanism (3)) is given, which may be an operating mechanism as well for long cracks. (Author)