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Acta Metall Sin  2017, Vol. 53 Issue (9): 1047-1054    DOI: 10.11900/0412.1961.2016.00561
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Very High Cycle Fatigue Failure Mechanism of TC17 Alloy
Hanqing LIU1, Chao HE2, Zhiyong HUANG1(), Qingyuan WANG1,2
1 School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065, China
2 School of Architecture and Civil Engineering, Chengdu University, Chengdu 610106, China
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Hanqing LIU, Chao HE, Zhiyong HUANG, Qingyuan WANG. Very High Cycle Fatigue Failure Mechanism of TC17 Alloy. Acta Metall Sin, 2017, 53(9): 1047-1054.

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Abstract  

Titanium alloys have been widely used in bearing force components in aeronautical structures, such as blades and beams to withstand the high frequency dynamic loads, which requires an outstanding fatigue resistance performance in very high cycle regime during their service life. In this work, very high cycle fatigue failure property of TC17 alloy used as aircraft engine blade material was studied by ultrasonic fatigue test and electromagnetic resonance fatigue test under 110 Hz and 20 kHz sinusoidal load, and crack initiation mechanism of different failure mode was analyzed. The results showed that, fatigue failure modes of TC17 alloy could be divided into surface induced failure and interior induced failure. Surface induced failure was caused by the machine defect and surface slide trace that triggered by the asymmetric loading. Interior induced failure was caused by slid fracture of primary α phase under asymmetric loading. Fatigue resistance of TC17 alloy was influenced by the fatigue crack initiation mechanism but concerned little about the loading frequency. The variation of the fatigue failure mechanism resulted in the S-N curves presenting bilinear. A fatigue strength predicted model is established based on the parameter of the weak crystal orientation area, which is in good agreement with the fatigue test result.
Key words:  TC17 alloy      very high cycle fatigue      fatigue failure mechanism      slip fracture     
Received:  15 December 2016     
ZTFLH:  O346.2  
  V252.2  
Fund: Supported by National Natural Science Foundation of China (No.11372201)
About author: 

1 The authors contributed equally to this work.
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Hanqing LIU
Chao HE
Zhiyong HUANG
Qingyuan WANG

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https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00561     OR     https://www.ams.org.cn/EN/Y2017/V53/I9/1047

Fig.1  Geometry parameters of fatigue specimens (unit: mm) (a) ultrasonic fatigue specimen (b) high frequency fatigue specimen
Fig.2  SEM image of microstructure of TC17 alloy after heat treatment
Fig.3  Microhardness of TC17 alloy fatigue specimen with 3 mm cross sectional alone radius from surface to center
Fig.4  Fatigue test results of TC17 alloy under 110 Hz and 20 kHz sinusoidal loading (σa—stress amplitude, Nf—number of fatigue loading cycles, int.—interior initiation fatigue crack, sur.—surface initiation fatigue crack)
Fig.5  SEM images of surface induced fracture of TC17 alloy axial loading at R=0.1 (R—stress ratio)
(a) σa=470 MPa, Nf=1.7×104 cyc, 110 Hz
(b) σa=400 MPa, Nf=8.8×105 cyc, 20 kHz
Fig.6  SEM images and local magnifications (insets)of interior induced fracture of TC17 alloy axial loading at R=0.1 (Black arrows show the facets) (a) σa=460 MPa, Nf=1.2×106 cyc, 110 Hz (b) σa=430 MPa, Nf=9.4×106 cyc, 20 kHz
Fig.7  Slip trace and vertical transversal micro crack of TC17 alloy axial loading at R=0.1, 20 kHz (White arrows show the slip traces, black arrows show the micro cracks) (a) σa=350 MPa, Nf=8.8× 107 cyc (b) σa=400 MPa, Nf=8.8×105 cyc
Fig.8  Schematic of interaction between slip trace and local stress[19] (σm—mean stress, L—length of slip trace, r—horizontal distance, θ—included angle between slip trace and cross section, σr—local stress )
Fig.9  Morphologies of surface initiation cracks of TC17 alloy axial loading at R=0.1 (Black arrows show the facets)
(a) σa=400 MPa, Nf=1.2×106 cyc, 110 Hz (b) σa=340 MPa, Nf=1.2×108 cyc, 20 kHz
Fig.10  Facet morphology of interior initiation fatigue crack of TC17 alloy axial loading at R=0.1, σa=400 MPa, Nf=1.31×107 cyc, 20 kHz
Fig.11  Side view of interior crack initiation area axial loading at R=0.1, σa=350 MPa, Nf=8.8×107 cyc, 20 kHz (Black arrows show the facets)
Fig.12  S-N curves of TC17 alloy sorted by morphology of crack initiation area (1—S-N curve contains unfaceted fatigue failure data, 2—S-N curve contains faceted fatigue failure data, black arrows show the surface initiation faceted cracks)
Fig.13  Fatigue crack initiation area vs initiation depth
Fig.14  Relative error between predicted values and experimental values
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