High-temperature failure mechanism and defect sensitivity of TC17 titanium alloy in high cycle fatigue

doi:10.1016/j.jmst.2022.01.010

Abstract

Abstract:

Crack initiation is an essential stage of fatigue process due to its direct effect on fatigue failure. However, for titanium alloys in high-temperature high cycle fatigue (HCF), the crack initiation mechanisms remain unclear and the understanding for the defect sensitivity is also lacking. In this study, a series of fatigue tests and multi-scale microstructure characterizations were conducted to explore the high-temperature failure mechanism, and the coupled effect of temperature and defect on TC17 titanium alloy in HCF. It was found that an oxygen-rich layer (ORL) was produced at specimen surface at elevated temperatures, and brittle fracture of ORL at surface played a critical role for surface crack initiation in HCF. Besides, internal crack initiation with nanograins at high temperatures was a novel finding for the titanium alloy. Based on energy dispersive spectroscopy, electron backscatter diffraction and transmission electron microscope characterizations, the competition between surface and internal crack initiations at high temperatures was related to ORL at surface and dislocation resistance in inner microstructure. The fatigue strengths of smooth specimens decreased at elevated temperatures due to the lower dislocation resistance. While the fatigue strengths of the specimens with defect were not very sensitive to the temperatures. Finally, a fatigue strength model considering the coupled effect of temperature and defect was proposed for TC17 titanium alloy.

Key words: TC17 titanium alloy, High temperature, Defect, High cycle fatigue, Oxygen-rich layer, Rough area

Gen Li, Chengqi Sun. High-temperature failure mechanism and defect sensitivity of TC17 titanium alloy in high cycle fatigue[J]. J. Mater. Sci. Technol., 2022, 122: 128-140.

Figures/Tables 14

Fig. 1. EBSD microstructural characterization of TC17 titanium alloy. (a, b) Band contrast map and IPF. (c, d) Enlarged view of box 1 and 2 in (b), respectively, showing α cluster and anisotropic α grains. Inset in (c) the color legend for IPF. (e) Phase map (upper part) and IPF (lower part) near the defect. (f) Enlarged view of phase map (upper part) and IPF (lower part) of box 3 in (e).

Table 1. Mechanical properties of TC17 alloy.

Temperature (°C)	Elastic modulus (GPa)	Yield strength (MPa)	Tensile strength (MPa)	Elongation to failure (%)
25	118	1047	1141	11.9
200	112	920	1054	11.3
400	103	735	912	12.1

Fig. 2. Test results: (a) S-N data of smooth specimens tested at 25 °C, 200 °C and 400 °C. (b) Variation of fatigue strengths and tensile strengths of smooth specimens with temperatures. (c)-(e) S-N data of notched specimens at 25 °C, 200 °C and 400 °C, respectively. (f) Variation of fatigue strengths of smooth and notched specimens with temperatures.

Table 2. Tensile strength and fatigue strength of the TC17 alloys.

Tests	Temperature (°C)	σts (MPa)	σfs (MPa)	σfsσts
Current	25	1141	629	0.55
	200	1054	530	0.50
	400	912	416	0.46
Liu et al [4]	25*	1078	530	0.49
Liu et al [4]	400	853	450	0.53

Fig. 3 Fracture surfaces of smooth specimens at different temperatures. (a1) Surface crack initiation at 25 °C (σa=650 MPa, Nf=3.9 × 104 cycles). (a2) Surface crack initiation with facet. (a3) An enlarged view of the facet in (a2). (b1) Internal crack initiation at 25 °C (σa=620 MPa, Nf=6.0 × 106 cycles). (b2) Internal crack initiation with rough area. (b3) An enlarged view of the rough area in (b2). (c1) Multiple surface crack initiations by oxide intrusion at 200 °C (σa=650 MPa, Nf=2.7 × 104 cycles). (c2, c3) Close-ups of the upper and the right crack initiation regions in (c1), respectively. (d1) Surface crack initiation by oxide shedding at 400 °C (σa=520 MPa, Nf=7.6 × 105 cycles). (d2) An enlarged view of crack initiation region in (d1). (d3, d4) Close-ups of the oxide shedding regions in (d2). (e1) Internal crack initiation at 400 °C (σa=520 MPa, Nf=1.0 × 106 cycles). (e2) Internal crack initiation with rough area. (e3) An enlarged view of the rough area in (e2).

Fig. 4. SEM observation of fracture surface of notched specimens at different temperatures. (a1) The whole fracture surface at 25 °C (σa=105 MPa, Nf=2.6 × 105 cycles). (a2) A close-up of the coarse surface in black box in (a1). (a3) A close-up of crack initiation zone at notch root in (a1). (b1) The whole fracture surface at 200 °C (σa=95 MPa, Nf=4.5 × 105 cycles). (b2) and (b3) Close-ups of crack initiation zone at notch root in (b1). (c1) The whole fracture surface at 400 °C (σa=107 MPa, N=8.7 × 104 cycles). (c2, c3) Close-ups of crack initiation zone at notch root in (c1).

Fig. 5. Variation of hardness of the specimens at different temperatures. (a) Hardness values of specimen surface versus holding time. (b) Hardness versus depth from surface on the cross section of run-out specimens at different temperatures. Inset in (a) and (b) the sketches for the hardness tests.

Fig. 6. Analysis of ORL by EDS. (a1) SEM image of a section on the specimen tested at 200 °C (σa=540 MPa, Nf=8.4 × 105 cycles). (b1) SEM image of a section on the specimen tested at 400 °C (σa=460 MPa, Nf=6.2 × 105 cycles). (a2) and (b2) EDS maps of the selected zones in (a1) and (b1), respectively.

Fig. 7. Observations for the rough area of the sample tested at 25 °C (σa=620 MPa, Nf=6.0 × 106 cycles). (a) SEM image of the rough area. (b) SEM image of the sectional plane at position b in (a). (c-e) IPF, phase map and TEM image of the sectional plane at position c in (a). (f) Dark field image and SAD of box 1 in (e); (g) An enlarged image of box 2 in (e).

Fig. 8. Observations for the rough area of the sample tested at 400 °C (σa=520 MPa, Nf=1.0 × 106 cycles). (a) SEM image of the rough area. (b) SEM image of the sectional plane at position b in (a). (c-e) IPF, phase map and TEM image of the sectional sheet at position c in (a). (f) Dark field image and SAD of box 1 in (e). (g) An enlarged image of box 2 in (e).

Fig. 9. Competition mechanism between surface and internal initiations at 400 °C. (a1) and (a2) Section view and side view of surface crack initiation by brittle fracture of oxygen-rich site, respectively. (b1) and (b2) Section view and side view of surface crack initiation by oxide shedding, respectively. (c1) and (c2) Section view and side view of internal crack initiation with rough area, respectively.

Fig. 10. Cracks below the notch defect. (a) and (b) Band contrast map and IPF of a specimen with the notch depth of 0.5 mm tested at 25 °C. Inset in (b) the color legend of IPF. (c) and (d) Band contrast map and IPF of a specimen with the notch depth of 0.2 mm tested at 200 °C. Inset in (a) and (c) SEM images of cracks in (a) and (c), respectively. (e) SEM image of a specimen with the notch depth of 0.5 mm tested at 400 °C. (f) An enlarged view of yellow box in (e).

Fig. 11. Variation of fatigue strength with defect size. Hollow symbols and solid symbols in the figure represent experimental fatigue strengths of smooth specimens and notched specimens, respectively.

Table 3. The notch depth and defect size $\sqrt{area}$ of notched specimens.

Depth (mm)	$\sqrt{area}$ (μm)
0.05	182
0.2	513
0.5	1011

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