Effect of stress ratio on very high cycle fatigue properties of Ti-10V-2Fe-3Al alloy with duplex microstructure

doi:10.1016/j.jmst.2017.11.036

Abstract

Abstract:

In fatigue critical applications, Ti-10V-2Fe-3Al alloy components are expected to endure cyclic loading with cycles above 10⁹. To assess their operating safety, S-N relations of Ti-10V-2Fe-3Al alloy in very high cycle fatigue (VHCF) regime are of concern and have been investigated in this work. Fatigue behavior including S-N curves and crack initiation mechanisms is reported. Two transitions of fatigue crack initiation mechanism, from internal crack initiation to surface crack initiation and from α_p cleavage to α_s/β decohesion, occur when the stress ratio (R) and stress level are reduced. Fatigue limits exist at N_f = 6 × 10⁷ cycles for all stress ratios except for 0.5. In the VHCF regime two kinds of internal crack initiation mechanisms exist, i.e., coalescence of cluster of α_p facets and α_s/β decohesion. Their mutual competition depends on the stress ratio and can be interpreted in terms of different stress character required for promotion on different internal crack initiation mechanism. Small crack propagation is discussed to be life controlling process under the stress ratio range from -0.5 to 0.1 during VHCF regime while under the stress ratio 0.5 VHCF, life almost refers to the life required for crack initiation.

Key words: Ti-10V-2Fe-3Al alloy, Very high cycle fatigue, S-N curve, Fatigue crack initiation, Life controlling process

Ying Wu, Jianrong Liu, Hao Wang, Shaoxuan Guan, Rui Yang, Hongfu Xiang. Effect of stress ratio on very high cycle fatigue properties of Ti-10V-2Fe-3Al alloy with duplex microstructure[J]. J. Mater. Sci. Technol., 2018, 34(7): 1189-1195.

Figures/Tables 11

Fig. 1. Microstructure of the bimodal Ti-1023 alloy: (a) SEM image showing αp particles and transformed β sub-grains (αs + β), (b) TEM bright-field image showing αs precipitates.

Table 1 Mechanical properties of the Ti-1023 alloy.

Alloy	UTS (MPa)	YS (MPa)	El (%)	AR (%)
Ti-1023	1270	1210	8.5	31

Fig. 2. Schematic of fatigue test specimens.

Fig. 3. S-N curves of Ti-1023 alloy vs σmax under stress ratios 0.5, 0.1, -0.3, -0.5, -0.7 and -1, with open and closed symbols corresponding to surface crack initiation and internal crack initiation respectively, and symbols with arrow denoting running out.

Fig. 4. Fractographs of the surface crack nucleation site at R = 0.1, σmax = 1100 MPa (The locations where the compositions were taken by EDS are marked and labeled): (a) one half fracture surface, (b) the other half fracture surface, (c) image at higher magnification of location framed in (b).

Table 2 EDS analysis of facet, gutter belts and ridge belts.

Location	Ti	Al	V	Fe
Facet A	91.4-91.5	5.7-5.9	2.6-2.8	no trace
Facet B	92.5-92.7	4.8-4.9	2.5-2.7	no trace
Gutter belt A1	85.6-86.6	0.3-0.7	11.5-12.0	1.6-1.8
Ridge belt B1	81.7-82.7	3.6-3.9	10.9-12.2	2.1-2.5
Gutter belt B2	83.9-86.5	0.5-1.1	11.6-12.0	1.7-2.4
Ridge belt A2	82.5-83.4	3.04-3.26	11.1-12.0	2.3-2.4
Transformed β matrix	82.1-84.7	2.25-2.85	10.9-11.9	1.9-3.43

Fig. 5. Fractographs of the internal crack nucleation sites at R = 0.5: (a) σmax = 1000 MPa, Nf = 2.4 × 107, (b) σmax = 950 MPa, Nf = 1.7 × 108.

Fig. 6. Fractographs of the internal crack nucleation sites at R = 0.1: (a) σmax = 950 MPa, Nf = 4.8 × 106, (b) σmax = 900 MPa, Nf = 2.7 × 107, (c) σmax = 900 MPa, Nf = 2.1 × 107, (d) σmax = 850 MPa, Nf = 4.8×107.

Fig. 7. Fractography of the crack nucleation sites: (a) R = -0.5, σmax = 750 MPa, Nf = 5.2 × 107, (b) R = -1, σmax = 725 MPa, Nf = 4.8 × 107.

Fig. 8. Distance of crack initiation sites from surface vs maximum stress.

Fig. 9. Schematic presentation of αp cleavage facilitated by slip localization in β sub-grain and a stage I crack propagation by αs/β decohesion.

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