J. Mater. Sci. Technol. ›› 2020, Vol. 50: 204-214.DOI: 10.1016/j.jmst.2020.01.060

• Research Article • Previous Articles     Next Articles

Duality of the fatigue behavior and failure mechanism in notched specimens of Ti-7Mo-3Nb-3Cr-3Al alloy

Zhihong Wu, Hongchao Kou*(), Nana Chen, Mengqi Zhang, Ke Hua, Jiangkun Fan, Bin Tang, Jinshan Li   

  1. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China
  • Received:2019-12-11 Revised:2020-01-11 Accepted:2020-01-27 Published:2020-08-01 Online:2020-08-10
  • Contact: Hongchao Kou

Abstract:

An interesting phenomenon of dual S-N fatigue behavior is investigated in a metastable β titanium alloy, Ti-7Mo-3Nb-3Cr-3Al notched cylindrical specimens with an elastic stress concentration factor of Kt = 3. Fractographic studies revealed all specimens, and irrespective of lifetime, failed from the specimen surface because of stress concentration occurs at the notch root. Typically, the short-life-distribution is usually associated with surface-failure-without-facets and the long-life-distribution generally occurs due to surface-failure-with-facets. This competing failure leads to increasing the variability in fatigue lifetime and further facilitates the difficulty in prediction of fatigue lifetime. Crack-initiation area characterization was conducted by using mechanical grinding, focused ion beam milling and subsequent electron back-scattered diffraction (EBSD) analysis of the 2D section across faceted grains. Results show that the αp particles (especially the elongated αp particles) well-oriented for basal slip activation is a preferential fatigue-critical microstructural configuration. Additionally, the β+αs matrix has a higher KAM value than the αp particles in fatigued microstructures and significant dislocation activity in the form of dislocation tangles is observed in αp boundaries.

Key words: High cycle fatigue, Metastable β titanium alloys;, Notch effects, S-N curves, Surface crack initiation