Fatigue crack growth behavior of beta-annealed Ti–6Al–2Sn–4Zr–xMo (x = 2, 4 and 6) alloys: Influence of microstructure and stress ratio

Significance Statement

A good understanding of the fatigue crack growth (FCG) behavior is an important precondition for the ‘run-on’ fatigue life assessment of service components and the formulation of the inspection and repair schemes. In this investigation, a comparative study was undertaken to characterize the long crack fatigue crack growth behavior in relation to the alloy types, different lamellar microstructures and stress ratios in the case of three α + β titanium alloys with nominal compositions of Ti–6Al–2Sn–4Zr–xMo (Ti624x, x = 2, 4 and 6), an important alloy system for aero engine application.

The effects of microstructure and stress ratio on the fatigue crack growth behavior in the three beta-annealed Ti alloys were separated through carefully-designed experiment. The changes in fatigue crack growth rate caused by increasing stress ratio were much larger compared to those due to microstructure variation. The most important microstructural parameter was identified as the α/β colony size which in turn depends strongly on the Mo-content. The results showed that the damage tolerance properties gradually decreased from Ti6242 to Ti6246. An increased stress ratio was found to reduce the roughness-induced crack closure (RICC) both through larger crack opening levels and through heavier crack bifurcation reducing crack deflection. A prominent transition on the da/dN vs. ΔK plots was observed in the extra-coarse microstructures of Ti6242 and Ti6244 alloys tested under R = 0.1, due to, in the early stage of Paris-region, long and straight cleavage propagation under low stress ratio causing strong RICC effect and obviously reducing the fatigue crack growth rates. 

Fatigue crack growth behavior of beta-annealed Ti–6Al–2Sn–4Zr–xMo (x = 2, 4 and 6) alloys: Influence of microstructure and stress ratio. Advances in Engineering

About the author

Jianke Qiu (BSc, Central South University; MSc, IMR-CAS; PhD, IMR-CAS) is a research associate of Titanium Alloys Division of IMR-CAS. His PhD thesis is on the cold dwell fatigue of titanium alloys.

About the author

Yingjie Ma (BSc, Northeastern University, Shenyang; MSc, IMR-CAS; PhD, IMR-CAS) is an Associate Professor at Titanium Alloys Division of IMR-CAS. He has worked on fatigue of titanium alloys for aircraft and marine applications.

About the author

Jiafeng Lei (BSc, Northeastern University, Shenyang; MSc, IMR-CAS; PhD, IMR-CAS) is the Head of Structural Titanium Alloys Group and a Deputy Head of Titanium Alloys Division at IMR-CAS.

About the author

Aijun Huang (BSc, Northeastern University, Shenyang; MSc, IMR-CAS; PhD, Birmingham University) is an R&D Manager at Skyrizon. He has previously worked as a Specialist on titanium alloys at Rolls-Royce and Central Research Institute of Baosteel.

About the author

David Rugg (BSc, Aston University; PhD, Sheffield University) is the Engineering Fellow in Materials Processing and a Titanium Specialist at Rolls-Royce. He is a Co-Chair of the Material Definition Panel responsible for introducing all new materials to Aerospace, Nuclear, Marine and Power Generation sectors within Rolls-Royce.

About the author

Rui Yang (BSc, Wuhan Institute of Hydraulic and Electric Engineering; MSc, IMR-CAS, PhD, Cambridge University) has been the Head of Titanium Alloys Division, IMR-CAS, since 1997. He was appointed the Director of IMR-CAS in 2012.

Journal Reference

International Journal of Fatigue, Volume 83, Part 2, 2016, Pages 150–160.

Jianke Qiu1, Xin Feng1, Yingjie Ma1, Jiafeng Lei1, Yuyin Liu1, Aijun Huang2, David Rugg3, Rui Yang1

Show Affiliations
  1. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
  2. Baosteel Co. Ltd., Shanghai 200940, China
  3. Rolls-Royce plc, Derby DE24 8BJ, UK.  

Abstract

The influence of microstructures (β grain size, α/β colony size and width of α lath) and stress ratios (R = 0.1, 0.7) on fatigue crack growth (FCG) behavior of beta-annealed Ti624x (x = 2, 4 and 6) alloys was investigated. The fatigue crack growth rate curves, crack paths, and the plastic zone ahead of the crack tip were compared. The reducing fatigue crack growth resistance from Ti6242 to Ti6246 was found to stem from decreasing α/β colony size. Increasing stress ratio also reduced crack deflection and crack closure, and thus decreased the fatigue crack growth resistance. The relevant mechanisms were discussed in terms of Kmax and crack closure effect.

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