Variant selection in additively manufactured alpha-beta titanium alloys

doi:10.1016/j.jmst.2021.10.021

Abstract

Abstract:

The crystallographic arrangements of the α-phase variants in α-β titanium alloys remains less identified due to the crystallographic complexity involved while being essential to understand the α-β microstructural intricacy. To improve the current understanding, specimens of two columnar-grained α-β Ti alloys (Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo) and two equiaxed-grained α-β Ti alloys (Ti-6Al-4V and Ti-4Al-2V) were fabricated by laser metal powder deposition (LMD). Electron backscatter diffraction (EBSD) analyses were applied to more than 10⁵ α-phase variants in each alloy. The results revealed that the Type 4 α/α variant boundary ($\left[ \overline{10}55\bar{3} \right]/{{63.26}^{\circ }}$) is prevalent in the two columnar-grained α-β alloys while the Type 2 α/α variant boundary ($\left[ 11\bar{2}0 \right]/{{60}^{\circ }}$) is common in the two equiaxed-grained α-β alloys. Further EBSD characterisation indicates that α-variant selection tends to be more prevalent in equiaxed prior-β grains, featured by the Category I triple-α-variant clusters, which mostly terminate on dense {$10\bar{1}1$} planes with lower boundary energy. Conversely, columnar prior-β grains show significant Category II triple-α-variant clusters, which mostly terminate on less dense $\left\{ 4\bar{1}\bar{3}0 \right\}$ planes with higher boundary energy. Self-accommodation to compensate for the β→α transformation strain is assumed to be the major underlying mechanism. The implications of these findings for understanding the tensile strengths are discussed in conjunction with the Schmid factor of α-variants calculated in columnar- and equiaxed-grained Ti-6Al-4V.

Key words: Variant selection, Ti-6Al-4V, Additive manufacturing, Columnar, Equiaxed

S.L. Lu, C.J. Todaro, Y.Y. Sun, T. Sun, T. Song, M. Brandt, M. Qian. Variant selection in additively manufactured alpha-beta titanium alloys[J]. J. Mater. Sci. Technol., 2022, 113: 14-21.

Figures/Tables 10

Table 1. LMD conditions used to fabricate samples of each alloy.

Samples	Powder size (µm)	Laser power (W)	Laser spot size (mm)	Scan speed (mm/min)	Overlap ratio (%)	Powder supplier
Columnar Ti-6Al-4V	45-90	250	0.61	600	50%	TLS Technik GmbH, Germany
Equiaxed Ti-6Al-4V*	45-90	150	0.61	600	50%	TLS Technik GmbH, Germany
Columnar Ti-6Al-2Sn-4Zr-2Mo	45-106	1000	2.5	1000	50%	AP&C, Canada
Equiaxed Ti-4Al-2V	45-106	500	1.5	800	50%	RHENIUMET, China.

Table 2. The twelve α-variants in titanium defined by the Burgers orientation relationship [7].

Variants	Orientation relationship
A	${{\left( 1\bar{1}0 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 111 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
B	${{\left( 10\bar{1} \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 111 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
C	${{\left( 01\bar{1} \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 111 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
D	${{\left( 110 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ \bar{1}11 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
E	${{\left( 101 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ \bar{1}11 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
F	${{\left( 01\bar{1} \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ \bar{1}11 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
G	${{\left( 110 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 1\bar{1}1 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
H	${{\left( 10\bar{1} \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 1\bar{1}1 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
I	${{\left( 011 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 1\bar{1}1 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
J	${{\left( 1\bar{1}0 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 11\bar{1} \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
K	${{\left( 101 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 11\bar{1} \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
L	${{\left( 011 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 11\bar{1} \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$

Table 2. The twelve α-variants in titanium defined by the Burgers orientation relationship [7].

Variants	Orientation relationship
A	${{\left( 1\bar{1}0 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 111 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
B	${{\left( 10\bar{1} \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 111 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
C	${{\left( 01\bar{1} \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 111 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
D	${{\left( 110 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ \bar{1}11 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
E	${{\left( 101 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ \bar{1}11 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
F	${{\left( 01\bar{1} \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ \bar{1}11 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
G	${{\left( 110 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 1\bar{1}1 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
H	${{\left( 10\bar{1} \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 1\bar{1}1 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
I	${{\left( 011 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 1\bar{1}1 \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
J	${{\left( 1\bar{1}0 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 11\bar{1} \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
K	${{\left( 101 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 11\bar{1} \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$
L	${{\left( 011 \right)}_{\beta }}//{{\left( 0001 \right)}_{\alpha }};{{\left[ 11\bar{1} \right]}_{\beta }}//{{\left[ 11\bar{2}0 \right]}_{\alpha }}$

Fig. 1. Representative inverse pole figure maps of the α-phase variants along the build direction by LMD: (a) columnar Ti-6Al-4V [4]; (b) equiaxed Ti-6Al-4V [4] (under the terms of the Creative Commons CC BY license); (c) columnar Ti-6Al-2Sn-4Zr-2Mo, where the broken profiles indicate the prior-β grain boundaries; and (d) equiaxed Ti-4Al-2V.

Fig. 2. Statistical distribution of the neighbouring α-variant misorientation angle and misorientation axes in: (a) columnar Ti-6Al-4V; (b) equiaxed Ti-6Al-4V; (c) columnar Ti-6Al-2Sn-4Zr-2Mo; and (d) equiaxed Ti-4Al-2V. More than 105 α-variants or α-laths were analysed in each grain structure. The three inset inverse pole figures in each plot display the misorientation axes corresponding to misorientation angle ranges of 5-10°, 55-70° and 85-95°.

Fig. 3. (a) Comparison of the statistical distribution of the five types of α/α boundaries in columnar and equiaxed Ti-6Al-4V. Distribution maps of the five types of α/α boundaries in columnar prior-β grains (b) and equiaxed prior-β grains (c).

Fig. 4. Inverse pole figure (IPF) maps relative to the build direction of the reconstructed prior-β columnar grains in Ti-6Al-4V (a) and equiaxed grains in Ti-6Al-4V (b), adapted from Ref. [4] under the terms of the Creative Commons CC BY license. Contour {0001} pole figures of the α-phase in the columnar grain ① from (a) and in the equiaxed grain ④ from (b) are shown at the right sides of respective IPF maps. White circles in the pole figures indicate the (0001) pole of minimum intensity, while red circles for the (0001) pole of maximum intensity.

Fig. 5. (a) Inverse pole figure maps relative to the build direction of the reconstructed prior-β columnar grains in Ti-6Al-2Sn-4Zr-2Mo and contour {0001} pole figures of the α-phase in the columnar grain ⑦. (b) Inverse pole figure maps relative to the build direction of the reconstructed prior-β columnar grains in Ti-4Al-2V and contour {0001} pole figures of the α-phase in the equiaxed grain ⑧. White circles in the pole figures indicate the (0001) pole of minimum intensity, while red circles for the (0001) pole of maximum intensity.

Fig. 6. (a) Distribution of variant C in columnar prior-β grain ①. (b) Distribution of variant B in equiaxed prior-β grain ④. (c) Area fractions of the 12 α-variants in columnar prior-β grains ①, ② and ③. (d) Area fractions of the 12 α-variants in equiaxed prior-β grains ④, ⑤ and ⑥. The dashed lines in (c) and (d) refer to the random average distribution of 8.33%.

Fig. 7. (a) Category II clusters of C/G/K variants in columnar Grain ①, and (b) Category I clusters of J/K/L variants in equiaxed grain ④.

Fig. 8. Schmid factor (SF) distribution of prismatic slip systems in columnar and equiaxed grains. The SF of each α-lath contained in Fig. 1(a) (columnar) and Fig. 1(b) (equiaxed) (>105 α-laths in each case) is obtained using the AZtecCrystal software by applying a force along the tensile loading direction in Ref. [4]. Both the relative frequency and the accumulated frequency from high to low values of SF are calculated and plotted.

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