Contribution of ultrasonic surface rolling process to the fatigue properties of TB8 alloy with body-centered cubic structure

doi:10.1016/j.jmst.2020.05.047

Abstract

Abstract:

The effect of ultrasonic surface rolling process (USRP) as a severe plastic deformation technology was investigated on the evolution of microstructure, residual stress and surface morphology of TB8 alloys with body-centered cubic structure. Stress-controlled rotating-bending fatigue tests indicated increased fatigue strength in USRP samples prepared using different number of passes compared to the base material, which was attributed to the presence of gradient structure surface layers. Five subsequent USRP passes resulted in the highest fatigue strength, due to the optimal surface properties including higher extent of grain refinement, larger compressive residual stresses, “smoother” surface morphology and increased micro-hardness. However, the effect of USRP technology on improving fatigue strength of TB8 alloy was not significant in comparison with that of other titanium alloys (for example, Ti6Al4V), which was attributed to the notable surface residual stresses relaxation revealed from measurements on post-fatigued USRP samples. Electron backscatter diffraction analysis confirmed that fatigue crack initiation occurred in the larger grains on the surface with high Schmid factor. Small cracks were found to propagate into the core material in a mixed transgranular and intergranular mode. Further analysis indicated that grain growth existed in post-fatigued USRP-treated TB8 samples and that the average geometrically necessary dislocations value reduced after fatigue loading.

Key words: Ultrasonic surface rolling process, Gradient structure layers, Fatigue crack, Rotating-bending fatigue, TB8 alloys

Dan Liu, Daoxin Liu, Mario Guagliano, Xingchen Xu, Kaifa Fan, Sara Bagherifard. Contribution of ultrasonic surface rolling process to the fatigue properties of TB8 alloy with body-centered cubic structure[J]. J. Mater. Sci. Technol., 2021, 61: 63-74.

Figures/Tables 19

Fig. 1. (a) Microstructure of the base TB8 alloy, (b) EBSD image with IPF color coding, (c) Stress-strain curve obtained from the base material, (d) (001), (110), and (111) pole figures obtained from examining the axial-sectional plane, (e) Inverse pole figure along the TD1 direction.

Table 1 Chemical composition (wt%) of TB8 alloy.

Position	Al	Si	Nb	Mo	Ti
a	3.29	0.21	2.88	15.10	balance
b	3.22	0.19	2.82	14.95	balance
c	3.27	0.19	2.90	15.22	balance

Table 2 Mechanical properties of the base TB8 alloy.

Sample	UTS (MPa)	σ_0.2 (MPa)	ε_f (%)	U_T (J/cm³)	n
TB8	891.3	855.87	26.0	218.4	0.40

Fig. 2. Schematic diagrams of (a) USRP set-up, (b) fatigue sample.

Table 3 USRP parameters and the corresponding fatigue lives in the initial tests performed (at a maximum stress amplitude of 450 MPa) to identify the optimized parameters.

Sample number	Lathe rotational speed (r/min)	Feed rate (mm/rev)	Static stress (MPa)	Fatigue life (cycle)
1	55	0.08	480	85853
2	55	0.10	630	99228
3	55	0.12	780	66395
4	75	0.08	630	91504
5	75	0.10	780	68176
6	75	0.12	480	176080
7	95	0.08	780	57918
8	95	0.10	480	110210
9	95	0.12	630	75084

Fig. 3. (a) EBSD image with BD map of USRP-1sample, (b) IPF of USRP-5sample, (c) BD map of USRP-5 sample, (d) IPF of USRP-15sample, (e) BD map of USRP-15 sample, (f) BD map of the base material, (g) (001), (110), and (111) pole figures obtained from examining the axial-section plane of USRP-15 sample, (h) Inverse pole figure along the TD1 direction of USRP-15 sample.

Fig. 4. (a) Surface phase compositions (b) Variations of Vickers microhardness along the depth from the USRP processed surface.

Fig. 5. Surface morphologies of (a) BM, (b) USRP-1, (c) USRP-5, and (d) USRP-15 samples, (e) The corresponding surface roughness of TB8 alloy and USRP samples.

Table 4 The stress levels and corresponding fatigue lives (3 × 106 run-out).

BM		USRP-1		USRP-5		USRP-15
Stress (MPa)	Cycle	Stress (MPa)	Cycle	Stress (MPa)	Cycle	Stress (MPa)	Cycle
280	3000000	320	3000000	320	3000000	320	3000000
300	2697789	340	1141318	340	3000000	340	1149526
280	3000000	320	3000000	360	1039376	320	3000000
300	3000000	340	3000000	340	3000000	340	1369957
320	189967	360	621385	360	1267542	320	3000000
300	386545	340	829649	340	2082442	340	3000000
280	3000000	320	3000000	320	3000000	360	783116
300	429000	340	931604	340	1723102	340	1149526
280	3000000	320	3000000	320	3000000	320	3000000

Table 5 Fatigue strength calculated by ISO12017 method.

Sample	Fatigue strength (MPa) (ISO12017)
BM	295 ± 7
USRP-1	335 ± 7
USRP-5	338 ± 8
USRP-15	335 ± 7

Fig. 6. Comparing the fatigue lives of BM and USRP-treated samples under maximum stress levels of 340 MPa and 360 MPa.

Fig. 7. Fracture surface morphologies under a maximum stress level of 340 MPa of (a, a1) BM, (b, b1) USRP-1, (c, c1) USRP-5, (d, d1) USRP-15, (e) The schematic figure for calculating the actual stress at the crack initiation point, (f) Graph representing the depth of crack initiation and the corresponding calculated actual stress at the initiation point.

Table 6 The effect of USRP on fatigue life of different titanium alloys.

Materials	Thickness (μm) (deformation layer)	The related residual stress		Fatigue test conditions	Fatigue results (improvement degree compared with BM)		Reference
Materials	Thickness (μm) (deformation layer)	Maximum (MPa)	Depth (μm)	Fatigue test conditions	Fatigue strength	Fatigue life	Reference
Ti6Al4V	__	-963	550	Rotating-bending fatigue R=-1	39% (1 × 10⁷)	295-fold	[28]
Ti6Al4V	335	-1155	650	Rotating-bendi ng fatigue R=-1	22% (1 × 10⁷)	24.5-fold	[26]
HIP-Ti6Al4V	20	-1173	—	Rotating-bending fatigue R=-1	25% (1 × 10⁷)	—	[64,65]
TC11	70	-898	200	Tension-tension fatigue R=0.1	19.3% (5 × 10⁶)	—	[66]
TB8	—	-1200	550	Rotating-bending fatigue R=-1	14.6% (3 × 10⁶)	4.95-fold	Present work

Fig. 8. (a) EBSD image with IPF, (b) Schmid factor map, (c) Phase distribution of USRP-5sample after fatigue test under a maximum stress level of 340 MPa.

Fig. 9. (a) EBSD image with BD map of pre-fatigue USRP-15 sample, (b) BD map of post-fatigued USRP-15 sample under a maximum stress level of 340 MPa, (c) and (d) The corresponding grain size distribution figures of pre-fatigue USRP-15 and post-fatigued USRP-15 samples, respectively.

Fig. 10. EBSD image with KAM maps in (a) pre-fatigue USRP-15 sample, (b) post-fatigue USRP-15 sample under a maximum stress level of 340 MPa; KAM distributions in (c) pre-fatigue USRP-15 samples, (d) post-fatigue USRP-15 sample under a maximum stress level of 340 MPa.

Fig. 11. (a) and (b) EBSD images with KAM maps and the corresponding derived fractions for BM and USRP samples, (c) In-depth residual stress distributions in BM and USRP samples, and (d) The surface residual stress of pre- and post- fatigued USRP-5 sample.

Fig. 12. A representative surface defects of USRP-15 sample.

Fig. 13. Surface morphologies of BM and USRP samples after fatigue tests under a maximum stress level of 340 MPa of (a, b) BM, (c, d) USRP-1, (e, f) USRP-5 and (g, h) USRP-15 sample.

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