Toward an understanding of dwell fatigue damage mechanism of bimodal Ti-6Al-4V alloys

doi:10.1016/j.jmst.2021.08.041

Abstract

Abstract:

Dwell fatigue effect is a long-standing problem threatening the long-term service reliability for fan blades and fan disks of an aircraft engine. To understand the basic mechanism of dwell fatigue damage, pure fatigue and 60 s dwell fatigue properties of bimodal Ti-6Al-4V alloys with different volume fractions of the primary α (α_p) phase were examined comparatively. The results showed that both pure fatigue and dwell fatigue life decreased with increasing the volume fraction of the α_p phase and the dwell fatigue life was lower than the pure fatigue one. The quasi-in-situ test results and the quantitative characterization of damage behaviors of the local microstructure units defined by the α_p-secondary α (α_s) combination reveal that the α_s phase close to the α_p phase with extensively slip activities was gradually damaged under dwell fatigue loading, while that under pure fatigue loading was undamaged, demonstrating that the dwell loading induced the damage of the α_s phase, and further reduced the fatigue life. A stress relaxation-based model is proposed to describe the physical mechanism on dwell fatigue damage of the bimodal Ti-6Al-4V alloy, i.e. the elastic deformation of the α_s phase caused by the strain incompatibility would be gradually transformed into plastic deformation during the dwell stage, and thus promotes fatigue damage. The model provides new insights into the microscopic process of stress/strain transfer between the soft and hard microstructure units under dwell fatigue loading.

Key words: Dwell effect, Fatigue damage, Stress relaxation, Volume fraction of primary α phase, Quasi-in-situ testing

L.R. Zeng, L.M. Lei, X.M. Luo, G.P. Zhang. Toward an understanding of dwell fatigue damage mechanism of bimodal Ti-6Al-4V alloys[J]. J. Mater. Sci. Technol., 2022, 108: 244-255.

Figures/Tables 13

Fig. 1. Schematic illustrations of (a) fatigue specimens under loading conditions of (b) pure fatigue and (c) dwell fatigue.

Fig. 2. Inverse pole figure coded with respect to the loading direction of the microstructure of the Ti-6Al-4V alloys with different Vαp: (a) 36%, (b) 50%, (c) 60% and (d) 76%.

Table 1. Characteristic Vαp, yield strength, size of αp phase and the width of lamellar αs phase in the Ti-6Al-4V alloys subjected to different heat treatments.

Thermal treatment		V_αp (%)	Y.S. (MPa)	d_αp (μm)	w_αs (nm)
945 °C × 1 h	+ 700 °C × 2 h	36	963 ± 23	7.1 ± 2.5	368.7 ± 103.4
930 °C × 1 h		50	929 ± 19	9.5 ± 3.3	335.9 ± 91.73
915 °C × 1 h		60	901 ± 18	8.2 ± 2.4	335.3 ± 119.8
895 °C × 1 h		76	870 ± 20	7.7 ± 2.6	252.3 ± 74.2

Fig. 3. TEM micrographs of the αs phase in the alloy with (a) Vαp = 36% and (b) Vαp = 76%. (c) Statistical distribution of the width of αs phase in the Ti-6Al-4V alloys with different Vαp. (d) Electron diffraction pattern of αs phase with ultra-thin layer β phase, indicating the existence of the Burgers orientation relationship between αs and the β phases.

Fig. 4. Comparison of pure fatigue life and dwell fatigue life of Ti-6Al-4 V alloys with different Vαp at σmax = 820 MPa.

Fig. 5. Evolution of damage behavior of the Vαp = 36% specimens under (a-c) pure fatigue and (d-f) dwell fatigue; of the Vαp = 76% specimens under (g-i) pure fatigue and (j-l) dwell fatigue. Ori: the specimen before loading. Nf: the number of cycles to failure (total fatigue life), and xNf: a fraction of the total fatigue life (x = 0.47, 0.39, 0.42, and 0.6). Loading is along the horizontal direction.

Fig. 6. Damage behavior nearby the fracture under pure fatigue with (a) Vαp = 36% and (b) Vαp = 76%; under dwell fatigue with (c) Vαp = 36% and (d) Vαp = 76%, respectively. (e) Crack density (ρ) in the αs phase under pure fatigue and dwell fatigue loading as a function of Vαp.

Fig. 7. Fracture feature under pure fatigue loading of Vαp = 36% specimen at (a) crack initiation region, (b) crack growth region; Vαp = 76% specimen at (c) crack initiation region, (d) crack growth region. Fracture feature under dwell fatigue loading of Vαp = 36% specimen at (e) crack initiation region, (f) crack growth region; Vαp = 76% specimen at (g) crack initiation region, (h) crack growth region.

Fig. 8. (a) Surface height variation of Vαp = 36% specimen subjected to different dwell fatigue cycles. Line scanning is along the dash line in the SEM image, and every scanning is at the same location. (b) Schematic illustration of fractal dimension, and the curve is a close observation of the dash-line region in (a).

Fig. 9. Fractal dimension increment with fatigue cycles in specimens with Vαp = 36% under (a) pure fatigue, (b) dwell fatigue, and in specimens with Vαp = 76% under (c) pure fatigue, (d) dwell fatigue.

Fig. 10. Microstructure connectivity in the specimens with two types of combination of configurations of the αp and αs phases: (a) low and (b) high volume fraction of the αp phase under pure fatigue loading, (c) low and (d) high volume fraction of the αp phase under dwell fatigue loading.

Fig. 11. (a) Maxwell model, (b) diagrammatic sketch of plastic deformation of the αs phase, (c) stress variation with time during one dwell fatigue cycle.

Fig. 12. Comparison of measurement results and calculated results of strain of αs after dwell loading: (a) Vαp = 36%, (b) Vαp = 76%.

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