Influences of boron contents on microstructures and mechanical properties of as-casted near α titanium alloy

doi:10.1016/j.jmst.2020.11.010

Abstract

Abstract:

Ti-6.5Al-2.5Sn-9Zr-0.5Mo-0.25Si-1Nb-1W-0.1Er-xB (x = 0, 0.1, 0.2, 0.4, 0.6 and 0.8 wt%) with different boron contents are fabricated for investigation. Influences of boron element and its amounts on phase constitutions, microstructures, textures and compression mechanical properties are carefully studied. With the increasing boron additions, contents of TiB phase increase, and the maximum intensities of textures decrease. Microstructures are significantly and continuously refined after adding boron element and with the increasing boron additions, and turning point of refinement rates for different microstructural parameters is found at 0.2 wt% boron content. Strengths increase monotonously, while elongations increase firstly and decrease afterwards. The maximum value of elongation is acquired at boron content of 0.2 wt%. On the premise of ensuring plasticity, 0.2 wt% boron content is the most appropriate amount for microstructural refinement and mechanical properties enhancement for current near α titanium alloys. Meanwhile, aspect ratios of TiB whiskers increase with the increasing boron contents. Lots of near equiaxed α grains or α grains with irregular morphologies are discovered around TiB phase. Distorted and twisted α grain boundaries are also obviously detected in boron containing alloys. Moreover, premature fracture of alloys containing TiB whiskers is largely influenced by the fracture of these brittle reinforcements.

Key words: Boron element, Mechanical properties, Microstructures, Fracture behaviors, Near α titanium alloy, Textures

Xiaozhao Ma, Zhilei Xiang, Chao Tan, Zhitian Wang, Yingying Liu, Ziyong Chen, Qun Shu. Influences of boron contents on microstructures and mechanical properties of as-casted near α titanium alloy[J]. J. Mater. Sci. Technol., 2021, 77: 1-18.

Figures/Tables 23

Table 1. Nominal compositions of different alloys (wt%).

Alloys	Ti	Al	Sn	Zr	Mo	Si	Nb	W	Er	B
TA6.5-0B	Bal.	6.5	2.5	9.0	0.5	0.25	1.0	1.0	0.1	0
TA6.5-0.1B	Bal.	6.5	2.5	9.0	0.5	0.25	1.0	1.0	0.1	0.1
TA6.5-0.2B	Bal.	6.5	2.5	9.0	0.5	0.25	1.0	1.0	0.1	0.2
TA6.5-0.4B	Bal.	6.5	2.5	9.0	0.5	0.25	1.0	1.0	0.1	0.4
TA6.5-0.6B	Bal.	6.5	2.5	9.0	0.5	0.25	1.0	1.0	0.1	0.6
TA6.5-0.8B	Bal.	6.5	2.5	9.0	0.5	0.25	1.0	1.0	0.1	0.8

Fig. 1. (a) XRD patterns of as-casted alloys; (b) enlarged patterns of (a); (c) contents of β phase acquired by XRD analyses; (d) contents of TiB phase acquired by XRD analyses.

Fig. 2. {0001}α PF maps of alloys with different boron contents acquired by XRD analyses: (a) TA6.5-0B; (b) TA6.5-0.1B; (c) TA6.5-0.2B; (d) TA6.5-0.4B; (e) TA6.5-0.8B; (f) evolution law of texture intensities.

Fig. 3. As-casted OM microstructures of alloys at low magnification: (a) TA6.5-0B; (b) TA6.5-0.1B; (c) TA6.5-0.2B; (d) TA6.5-0.4B; (e) TA6.5-0.6B; (f) TA6.5-0.8B.

Fig. 4. Typical OM microstructures of as-casted alloys at high magnification: (a) TA6.5-0B; (b) TA6.5-0.1B; (c) TA6.5-0.2B; (d) TA6.5-0.4B; (e) TA6.5-0.6B; (f) TA6.5-0.8B.

Table 2. Corresponding microstructure parameters.

Alloys	P_β(μm)	W_l(μm)	L_l(μm)	AR-l	AR-TiB_w
TA6.5-0B [22]	503.3	1.25	107.5	86.0	0
TA6.5-0.1B	195.0	1.78	58.4	32.8	5.5
TA6.5-0.2B [22]	131.5	1.99	47.6	23.9	6.8
TA6.5-0.4B	118.3	2.10	38.1	18.1	8.4
TA6.5-0.6B	---	2.21	34.1	15.4	11.0
TA6.5-0.8B	---	2.31	32.0	13.8	14.9

Fig. 5. Evolution laws of microstructure parameters for as-casted alloys.

Table 3. Prior β grain sizes in other studies.

Alloys	Diameter (μm)
Ti 64 [13]	1700
Ti 64-0.02B [13]	∼1130
Ti 64-0.1B [13]	200
Ti 64-0.2B [13]	∼130
Ti-B20 [30]	1175 ± 179
Ti-B20-0.1B [30]	176 ± 27
Ti 1100 [31]	1446 ± 91.9
Ti 1100-0.1B [31]	335 ± 20.3
Ti 1100-0.2B [31]	190 ± 12.3

Fig. 6. Distributions of elements by EDS: (a) SEM image used for analyses; (b) results of line scanning.

Fig. 7. Elemental distributions at high magnification by EDS: (a) TEM image of α/β boundary; (b) corresponding electron image of (a) used for mapping scanning; (c) mapping results of elements.

Fig. 8. TEM analyses of TiB phase (TA6.5-0.4B): (a) morphology of TiB whisker; (b) corresponding SAED pattern; (c) HRTEM pattern of circled area in (a); (d) corresponding FFT pattern.

Fig. 9. IPF maps of as-casted alloys: (a) TA6.5-0B; (b) TA6.5-0.1B; (c) TA6.5-0.2B; (d) TA6.5-0.4B; (e) TA6.5-0.6B; (f) TA6.5-0.8B.

Fig. 10. Evolution laws of average sizes of effective grains and sizes of maximum grains for different as-casted boron containing alloys.

Fig. 11. PF maps of boron containing alloys: (a) TA6.5-0B; (b) TA6.5-0.1B; (c) TA6.5-0.2B; (d) TA6.5-0.4B; (e) TA6.5-0.6B; (f) TA6.5-0.8B.

Fig. 12. Evolution law of maximum texture intensities of alloys with different boron contents.

Fig. 13. Typical TEM microstructures of alloys: (a, b) TA6.5-0B; (c-f) TA6.5-0.4B; (g, h) TA6.5-0.6B.

Fig. 14. Compression mechanical properties of as-casted alloys with different boron contents: (a) engineering stress vs. strain curves; (b) evolution laws of strengths and elongations.

Table 4. Corresponding mechanical properties parameters of as-casted alloys with different boron contents.

Alloys	Number	σ_s(MPa)		σ_b(MPa)		δ(%)
Alloys	Number	Values	Average values	Values	Average values	Values	Average values
TA6.5-0B	1	1013.7	1007.7	1909.8	1878.9	23.8	23.5
	2	986.3		1831.7		22.8
	3	1023.2		1895.1		23.9
TA6.5-0.2B	1	1036.9	1041.9	1990.2	2021.9	26.1	25.9
	2	1064.2		2065.6		25.9
	3	1024.6		2009.8		25.8
TA6.5-0.4B	1	1148.9	1160.1	2086.9	2090.7	25.5	25.5
	2	1188.0		2108.2		25.8
	3	1143.4		2077.1		25.2
TA6.5-0.8B	1	1196.7	1197.6	2050.0	2050.4	22.2	22.8
	2	1185.8		2052.0		23.0
	3	1210.4		2049.2		23.1

Fig. 15. Fractographs of as-casted boron containing alloy: (a, b) fracture characteristics; (c) crack on fracture surface; (d) magnified image of rectangle area in (c).

Fig. 16. Fractured lateral surfaces of alloys: (a-c) TA6.5-0B; (d) TA6.5-0.4B; (e-g) enlarged images of areas A, B and C in (e).

Fig. 17. EBSD analyses of lateral surfaces closing fracture surface after compression processes: (a) IPF map of TA6.5-0B; (b) corresponding grain boundaries map of (a); (c) IPF map of TA6.5-0.8B; (d) corresponding grain boundaries map of (c); (e) distribution frequency map of misorientation angles for TA6.5-0B; (f) distribution frequency map of misorientation angles for TA6.5-0.8B.

Fig. 18. EBSD analyses of lateral surface closing to fracture surface at high magnification: (a) IPF map of TA6.5-0.8B; (b) corresponding band contrast map of α phase (TiB phase is colored by red).

Fig. 19. (a, c, e) Deformation stages of boron free alloy; (b, d, f) deformation stages of boron containing alloy.

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