Fatigue crack tip plastic zone of α + β titanium alloy with Widmanstatten microstructure

doi:10.1016/j.jmst.2018.03.012

Abstract

Abstract:

The recent studies had focused on the fatigue crack propagation behaviors of α + β titanium alloys with Widmanstatten microstructure. The fascinated interest of this type of microstructure is due to the superior fatigue crack propagation resistance and fracture toughness as compared to other microstructures, which was believed to be related to the fatigue crack tip plastic zone (CTPZ). In this study, the plastic deformation in fatigue CTPZ of Ti-6Al-4V titanium alloy with Widmanstatten microstructure was characterized by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The results showed that large-scale slipping and deformation twinning were generated in fatigue CTPZ due to the crystallographic feature of the Widmanstatten microstructure. The activation of twinning was related to the rank of Schmid factor (SF) and the diversity of twin variants developing behaviors reflected the influence of SF rank. The sizes of CTPZ under different stress intensity factors (K) were examined by the white-light coherence method, and the results revealed that the range of the plastic zone is enlarged with the increasing K (or crack length), while the plastic strain decreased rapidly with the increasing distance from the crack surface. The large-scale slipping and deformation twinning in Widmannstatten microstructure remarkably expanded the range of fatigue CTPZ, which would lead to the obvious larger size of the observed CTPZ than that of the theoretically calculated size.

Key words: Titanium alloy, Widmanstatten microstructure, CTPZ, Slip, Deformation twinning

Yingjie Ma, Sabry S. Youssef, Xin Feng, Hao Wang, Sensen Huang, Jianke Qiu, Jiafeng Lei, Rui Yang. Fatigue crack tip plastic zone of α + β titanium alloy with Widmanstatten microstructure[J]. J. Mater. Sci. Technol., 2018, 34(11): 2107-2115.

Figures/Tables 16

Table 1 Chemical composition of Ti-6Al-4V alloy employed in this study.

Materials	Chemical composition (wt.%)
Materials	Al	V	O	Fe	Ti
Ti-6Al-4V	6.05	4.10	0.06	0.05	Bal.

Fig. 1. Optical (a) and SEM (b) morphology of Ti-6Al-4V alloy with Widmanstatten microstructure obtained by furnace cooling from β phase region.

Fig. 2. The EBSD morphology of Ti-6Al-4V alloy with Widmanstatten microstructure (a), and (b) the small misorientation between neighboring α lamellas in a single α colony.

Fig. 3. Single-edge specimen with prepared initial fatigue crack, dimensions in mm.

Fig. 4. SEM morphology of slipping in fatigue CTPZ including river-shape slipping and straight slipping (a), the river-shape slipping exhibits tortuous path (b).

Fig. 5. Large-scale deformation twinning (pointed by the white arrows) respectively activated in two regions which are relatively long distance from crack surface in CTPZ.

Fig. 6. Deformation twinning in Region A of Fig. 5, (a) EBSD image showing T1 and T2 twinning variants in P1 and P2 parent colony respectively, (b) the {10-12} twining plane indicated by the red line, (c) 3D crystal viewer of the corresponding parent and deformation twinning.

Fig. 7. The misorientation between α lattices of parent P1 and deformation twinning T1 in Fig. 6.

Fig. 8. SF distribution maps of Region A in Fig. 5 respectively based on (a) basal slip {0002} <11-20>, (b) prismatic slip {1-100} <11-20> and (c) pyramidal slip {1-101} <11-20>.

Fig. 9. EBSD characterization of deformation twinning variants in Region B of Fig. 5, (a) six types of twinning variants in two parent α colonies, (b) pole figures of the six {10-12} twinning plane, (c) 3D crystal viewer of the corresponding parents and deformation twins.

Table 2 The six types of twining variants in Fig. 9 and the corresponding SF based on {10-12} twinning.

Twin variant		SF
T1	(0-112) [01-11]	0.497
T2	(01-12) [0-111]	0.498
T3	(10-12) [-1011]	0.492
T4	(-1012) [10-11]	0.479
T5	(0-112) [01-11]	0.485
T6	(01-12) [0-111]	0.484

Fig. 10. Two types of deformation twinning in fatigue CTPZ, (a) SEM image, (b) EBSD image showing T1 and T2 twinning variants in P1 and P2 parent colony respectively, and (c) 3D crystal viewer of the corresponding parent and deformation twinning.

Fig. 11. Scanning white-light interferometry picture showing (a) two-dimensional and (b) three dimensional morphology of the plastic zone, with Kmax = 33.3 MPa m1/2, while the relatively dark color represents the high plastic deformation.

Fig. 12. Scanning white-light interferometry picture showing the range of fatigue CTPZ with three Kmax level: (a) 33.3 MPa m1/2, (b) 50 MPa m1/2, (c) 66.7 MPa m1/2.

Table 3 The measured and calculated PZS under three levels of stress intensity factor.

K_max (MPa m^1/2)	Measured Monotonic PZS (mm)	Calculated Monotonic PZS (mm)
33.3	～1	0.21
50.0	～2	0.47
66.7	～2.5	0.84

Fig. 13. Schematic representation of CTPZ in Widmannstatten microstructure, showing large-scale slipping and deformation twinning expand CTPZ range, then generate obviously larger size of CTPZ compared to the size of LEFM.

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