Multiple α sub-variants and anisotropic mechanical properties of an additively-manufactured Ti-6Al-4V alloy

doi:10.1016/j.jmst.2020.06.039

Abstract

Abstract:

Additive manufacturing (AM) is a promising material processing method which gains significant momentum in the aerospace and biomedical industries. However, the anisotropy in the mechanical properties of additively-manufactured materials is still poorly understood. This study was aimed at elucidating crystallographic feature-anisotropy-mechanical property relationship for a Ti-6Al-4V alloy manufactured via selective electron beam melting (SEBM). Abundant α lamellae with six variants were present inside the columnar prior-β grains with a <100> fiber texture during β→α phase transformation. The six α variants followed the Burgers orientation relationship of {110}_β//{0001}_α and <1-11>_β//<11-20>_α. Multiple sub-variants in each α variant were observed for the first time. The anisotropy in the mechanical properties was mainly related to the relative amount of six α variants. While the horizontally-oriented samples had a lower yield strength, they exhibited a higher ductility and longer fatigue life than the vertically-oriented samples. Cyclic softening occurred at higher strain amplitudes, and cyclic stabilization sustained at lower strain amplitudes. Fatigue crack mainly initiated from the specimen surface at lower strain amplitudes, while multiple crack initiation tended to occur at higher strain amplitudes. Crack propagation was characterized by fatigue striations along with some secondary cracks.

Key words: Titanium alloy, 3D printing, Deformation behavior, Anisotropy, Burgers orientation relationship

Yinling Zhang, Zhuo Chen, Shoujiang Qu, Aihan Feng, Guangbao Mi, Jun Shen, Xu Huang, Daolun Chen. Multiple α sub-variants and anisotropic mechanical properties of an additively-manufactured Ti-6Al-4V alloy[J]. J. Mater. Sci. Technol., 2021, 70: 113-124.

Figures/Tables 17

Table 1 Chemical composition of Ti-6Al-4V powders (wt.%).

Ti	Al	V	Fe	O	N	C	H
Bal.	6.28	3.99	0.22	0.076	0.025	0.013	<0.0010

Fig. 1. (a) Schematic illustration of powder deposition geometry in the 3D-printing process via selective electron beam melting along with the extracted directions of the cylinders. (b) Test sample geometry and dimensions (in mm) used in the present tensile and fatigue experiments.

Fig. 2. (a) Orientation map of α (hcp) and β (bcc) phases with IPF-Y coloring for the 3D-printed Ti-6Al-4V alloy; (b) schematic of Burgers orientation relationship (BOR) between α (hcp) and β (bcc); (c) discrete orientation maps of six α variants extracted from (a); (d) {0001} pole figure of α; (e) {110} pole figure of β; (f) the corresponding pole figure about six α variants following the BOR.

Table 2 Six α variants (α1-α6) and the relevant sub-variants between β(bcc) matrix and α(hcp) phase based on the Burgers orientation relationship (BOR).

Variant	Plane parallel	Sub-variants defined by the direction parallel
α₁	(110)_β // (0001)_α	[1-11]_β // [2-1-10]_α	[1-11]_β // [11-20]_α	[1-11]_β // [-1210]_α
α₂	(1-10)_β // (0001)_α	[111]_β // [2-1-10]_α	[111]_β // [11-20]_α	[111]_β // [-1210]_α
α₃	(101)_β // (0001)_α	[11-1]_β // [2-1-10]_α	[11-1]_β // [11-20]_α	[11-1]_β // [-1210]_α
α₄	(10-1)_β // (0001)_α	[111]_β // [2-1-10]_α	[111]_β // [11-20]_α	[111]_β // [-1210]_α
α₅	(01-1)_β // (0001)_α	[111]_β // [2-1-10]_α	[111]_β // [11-20]_α	[111]_β // [-1210]_α
α₆	(011)_β // (0001)_α	[11-1]_β // [2-1-10]_α	[11-1]_β // [11-20]_α	[11-1]_β // [-1210]_α

Fig. 3. (a) Typical engineering stress-strain curves and (b) strain-rate sensitivity as a function of true strain of the present SEBM-manufactured horizontally and vertically-orientated Ti-6Al-4V alloy.

Table 3 Tensile properties of SEBM-manufactured Ti-6Al-4V alloy in the horizontal and vertical directions, which were obtained at different strain rates at room temperature.

Strain rate, s^-1	Orientation	σ_y, MPa	σ_UTS, MPa	Elongation, %
1 × 10^-4	Horizontal	893 ± 2.5	977 ± 1.5	11.7 ± 0.3
1 × 10^-4	Vertical	960 ± 3	1034 ± 3	10.0 ± 4
1 × 10^-3	Horizontal	953 ± 3	994 ± 3.5	13.4 ± 1.6
1 × 10^-3	Vertical	1020 ± 7	1040 ± 9	7.0 ± 3.5
1 × 10^-2	Horizontal	973 ± 7	1007 ± 8	13.6 ± 2.4
1 × 10^-2	Vertical	1022	1059	9.6

Fig. 4. Schematic illustration of slip systems in an hcp crystal structure.

Fig. 5. (a) Schmid factor (SF) map of α phase along the <11-20> directions on the (0001) plane with a stress applied along the horizontal direction and (b) the corresponding SF maps of individual α variants; (c) SF map of α phase along the <11-20> direction on the (0001) plane with a stress applied along the vertical direction and (d) the corresponding SF maps of individual α variants.

Fig. 6. Optical micrographs of the present SEBM-manufactured Ti-6Al-4V alloy in (a) XOZ plane and (b) XOY plane; schematic illustration showing the orientation of a lack-of-fusion defect in (c) horizontal and (d) vertical samples, respectively.

Fig. 7. (a) Stress amplitude and (b) plastic strain amplitude versus the number of cycles of SEBM-manufactured Ti-6Al-4V alloy in the horizontal direction tested at different total strain amplitudes at a strain ratio of Rε = -1.

Fig. 8. Total strain amplitude versus the number of cycles to failure for the SEBM-manufactured Ti-6Al-4V alloy in the horizontal direction, in comparison with the results in the vertical direction [48] and other additively manufactured Ti-6Al-4V alloys reported in the literature [73,74].

Fig. 9. Total, plastic, and elastic strain amplitude-fatigue life response of the SEBM-manufactured Ti-6Al-4V alloy along the horizontal direction.

Table 4 Low cycle fatigue parameters of the SEBM-manufactured Ti-6Al-4V alloy in the horizontal and vertical directions.

Low cycle fatigue parameters	Symbol	Horizontal direction	Vertical direction
Cycle strain hardening exponent	n’	0.088	0.063
Cyclic strength coefficient, MPa	K’	1460	1337
Fatigue strength coefficient, MPa	σ'_f	2327	3600
Fatigue strength exponent	b	-0.14	-0.19
Fatigue ductility coefficient	ε'_f	89.4	21.2
Fatigue ductility exponent	c	-1.49	-1.48

Fig. 10. Cyclic stress-strain curve (CSSC) for the SEBM-manufactured Ti-6Al-4V alloy along both horizontal and vertical directions, where the two corresponding monotonic stress-strain curves obtained at a strain rate of 1 × 10-2 s-1 are also potted for comparison.

Fig. 11. Fatigue life of the SEBM-manufactured Ti-6Al-4V alloy along both horizontal and vertical directions based on Coffin-Mason-Basquin equation.

Fig. 12. SEM micrographs of the SEBM-manufactured Ti-6Al-4V alloy in the vertical direction tested at a strain amplitude of 0.4 %: (a) overall fracture surface; (b) Place A in (a); (c) Place B in (a); (d) overall fracture surface of the SEBM-manufactured Ti-6Al-4V alloy in the horizontal direction also tested at a strain amplitude of 0.4 %.

Fig. 13. SEM micrographs of the SEBM-manufactured Ti-6Al-4V alloy in the horizontal direction tested at a strain amplitude of (a, b) 0.4 % and (c, d, e, f) 1.0 %, respectively, where (a) and (c) show an overall view containing the crack initiation site, while (b) and (d) show the typical fatigue crack propagation characteristics; (e) unmelted zones and (f) dimple feature in the fast propagation zone.

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