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Acta Metall Sin  2016, Vol. 52 Issue (8): 923-930    DOI: 10.11900/0412.1961.2015.00581
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Xiaolong LIU1,Chengqi SUN1,Yantian ZHOU2,Youshi HONG1()
1 State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China 。
2 Heat Treatment Plant, Wafangdian Bearing Co. Ltd., Dalian 116300, China
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Titanium alloys have been widely used as superior engineering materials because of their high specific strength, high temperature resistance and high corrosion resistance. In their engineering applications such as used in aircraft engines, titanium alloys may experience even 1010 fatigue cycles. Recently, faceted crack initiation was observed in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes of titanium alloys, which resulted in a sharp decrease in fatigue strength. Therefore, the HCF and VHCF of titanium alloys have both scientific significance and engineering requirement. In this work, the effects of microstructure and stress ratio (R) on HCF and VHCF of a Ti-6Al-4V alloy have been investigated. Fatigue tests were conducted on a rotating-bending fatigue machine and an ultrasonic fatigue machine. All the fatigue fracture surfaces were observed by SEM. The results show that the HCF and VHCF behaviors of the fully-equiaxed and the bimodal Ti-6Al-4V alloy are similar. The observations of fracture surface indicate that two crack initiation mechanisms prevail, i.e. slip mechanism and cleavage mechanism. With the increase of stress ratio, the crack initiation mechanism switches from slip to cleavage. The S-N curves present the single-line type or the bilinear type. For the cases of rotating-bending and ultrasonic axial cycling with R= -1.0, -0.5 and 0.5, the S-N curves are single-line type corresponding to the slip mechanism or cleavage mechanism. For the cases of R= -0.1 and 0.1, the S-N curves are bilinear type corresponding to both slip and cleavage mechanisms. A model based on fatigue life and fatigue limit is proposed to describe the competition between the two mechanisms, which is in agreement with the experimental results.

Key words:  Ti-6Al-4V alloy      very-high-cycle fatigue      microstructure      stress ratio      slip mechanism      cleavage mechanism     
Received:  12 November 2015     
Fund: Supported by National Natural Science Foundation of China (Nos.11202210 and 11572325)

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Fig.1  Schematics of shape and dimensions of specimens (unit: mm, R—stress ratio)

(a) rotating bending (b) ultrasonic, R= -1.0 (c) ultrasonic, R>-1.0=2.26×107 cyc)

Fig.2  SEM images of Ti-6Al-4V alloy after annealing (700 ℃, 2 h, A.C.) (a) and solution-ageing (920 ℃, 1 h, A.C. + 550 ℃, 4 h, A.C.) (b) heat-treatments (αp—primary α grain, ββ grain, αs—secondary α grain)
Fig.3  EBSD results of Ti-6Al-4V alloy after annealing (a) and solution-ageing (b) heat-treatments
Fig.4  Fatigue crack initiation by slip mechanism (σa—fatigue strength, Nf—fatigue life)

(a) fully-equiaxed microstructure (rotating bending, σa=620 MPa, Nf=4.38×107 cyc)(b) bimodal microstructure (rotating bending, σa=690 MPa, Nf=2.47×105 cyc)

Fig.5  Fatigue crack initiation by cleavage mechanism (arrows show the facets)

(a) fully-equiaxed microstructure (axial loading at R=0.1, σa=339 MPa, Nf=3.09×107 cyc) (b) bimodal microstructure (axial loading at R=0.1, σa=339 MPa, Nf=2.26×107 cyc)

Fig.6  Schematic of interior crack initiation (a) and morphologies of facet (b), rough area (c) and fish-eye (d) by AFM (bimodal microstructure, axial loading at R=0.1, σa=250 MPa, Nf=2.02×107 cyc)
Fig.7  Morphology of cleavage facet (a) and EDS result of facet (b) (bimodal microstructure, axial loading at R=0.1, σa=339 MPa, Nf
Fig.8  Distributions of facet size and primary α grain size
Fig.9  S-N curves of Ti-6Al-4V alloy under rotating bending (R.B.—rotating bending, Sli—fatigue induced by slip mechanism, symbol with arrow—unbroken) (a) fully-equiaxed microstructure (b) bimodal microstructure
Fig.10  S-N curves of Ti-6Al-4V alloy under ultrasonic axial loading (Cle—fatigue induced by cleavage mechanism, symbol with arrow—unbroken) (a) fully-equiaxed microstructure (b) bimodal microstructure
Fig.11  Relation between lgD* and stress amplitude at different stress ratios
Fig.12  Schematic of competition between slip mechanism and cleavage mechanism
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