Failure Mechanisms During Isothermal Fatigue of SiC/Ti-24Al-11Nb Composites

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 200; no. 2-Jan; pp. 55 - 67
• Main Authors: Brindley, P. K., Bartolotta, P. A.
• Format: Journal Article
• Language: English
• Published: Legacy CDMS Elsevier Science 01.01.1995
• Subjects: Crack initiation; Cracks; Failure (mechanical); Fatigue of materials; Materials testing; Metallic Materials; Microstructure; Strain; Surfaces; Thermal effects; Titanium alloys
• ISSN: 0921-5093
• DOI: 10.1016/0921-5093(95)07013-3

Failure mechanisms during isothermal fatigue of unidirectional SiC/Ti-24Al-11Nb (at.%) composites have been determined by microstructural analysis of samples from tests interrupted prior to the end of life and from tests conducted to failure. Specimens from three regions of life were examined based on the maximum strain from a fatigue life diagram: Region 1 (high strain), Region 2 (mid-strain) and Region 3 (low strain). Crack lengths were also measured from interrupted samples and compared based on temperature (23-815 C), region of life and numbers of cycles. Region 1 was controlled by fiber-dominated failure. A transition zone was observed between Regions 1 and 2 due to competition between failure mechanisms. Failure in Region 2 was generally described as surface-initiated cracking with varying amounts of fiber bridging. However, the specific descriptions of crack propagation through the fibers and matrix varied with strain and temperature over this broad region. Region 3 exhibited endurance behaviour at 23 C with no cracking after lO(exp 6) cycles. However at 425 C, surface-initiated cracking was observed after 10(exp 6) cycles with fractured fibers in the crack wake. If endurance behaviour exists for conditions of isothermal fatigue in air at temperatures of greater than or equal to 425 C, it may only be found at very low strains and at greater than 10(exp 6) cycles.