A Quantitative Study on the Effect of Heat Treatment on the Microstructure and Orientation of Titanium Hydride in Electrochemically Hydrogenated Pure Titanium

doi:10.1007/s40195-022-01423-5

Abstract

Abstract:

The electrochemical hydrogen charging of pure titanium and its alloys has been investigated previously, while how a subsequent annealing treatment affects the type of hydride and its orientation relationship with matrix is not clear. In the present study, a quantitative study on the microstructure and orientation of titanium hydrides during electrochemical hydrogen charging and subsequent annealing treatment was carried out using scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The results show that δ-hydride is the main in both the electrochemically hydrogenated sample and the subsequent annealing treated sample. After electrochemical hydrogen charging for 48 h, the surface is mainly composed of dense δ-hydride with a thickness of approximately 42 μm, the orientation relationship between α-matrix and δ-hydride follows only the orientation relationship of OR2, {0001}_α//{1$\overline{1}$1}_δ, $\langle 1\overline{2}10\rangle_{\alpha }$//$\langle 110\rangle_{\delta }$ and an interface plane $\{ 10\overline{1}3\}_{\alpha }$//$\{ 1\overline{1}0\}_{\delta }$. Besides OR2, a part of hydrides show an orientation relationship of OR1 with the matrix after annealing, {0001}_α//{001}_δ, $\langle 1\overline{2}10\rangle_{\alpha }$//$\langle 110\rangle_{\delta }$ and an interface plane of $\{ 10\overline{1}0\}_{\alpha }$//$\{ 1\overline{1}0\}_{\delta }$. It is further found that the relative frequency of OR1and OR2 is closely related to annealing duration. Under an argon atmosphere at 450 °C, the frequencies of OR1 and OR2 are nearly balance with an annealing time of 12 h, while OR1 becomes to be the predominant one with a relative frequency of 96.5% after annealing for 96 h. The mechanism for the evolution of orientation relationship of hydrides with annealing time was discussed.

Key words: Titanium alloys, Titanium hydride, Heat treatment, Microstructure, Orientation relationship

Changzheng Li, Yunchang Xin, Guangjie Huang, Qing Liu. A Quantitative Study on the Effect of Heat Treatment on the Microstructure and Orientation of Titanium Hydride in Electrochemically Hydrogenated Pure Titanium[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(12): 2057-2068.

Figures/Tables 16

Table 1 Four orientation relationships of hydride transition

	Orientation relationship	Interface plane
OR1	{0001}//{001}, 〈1$\overline{2}$10〉//〈110〉	{10$\overline{1}$0}//{1$\overline{1}0$}
OR2	{0001}//{1$\overline{1}$1}, 〈1$\overline{2}$10〉//〈110〉	{10$\overline{1}3$}//{1$\overline{1}$0}
OR3	{10$\overline{1}$0}//{1$\overline{1}$1}, 〈1$\overline{2}$10〉//〈110〉	{0001}//{1$\overline{1}$2}
OR4	{$\overline{1}$011}//{001}, 〈1$\overline{2}$10〉//〈110〉	{10$\overline{1}$1}//{1$\overline{1}\overline{1}$}

Fig. 1 Inverse pole figure map of as-received T40 after annealing at 700 °C for 1 h

Fig. 2 BSE images of cross-sectional microstructure of hydride layer after electrochemical hydrogen charging for a 48 h; b 120 h

Fig. 3 a Inverse pole figure map; b phase map; c pole figures of α-Ti and δ-hydride in T40 after electrochemical hydrogen charging

Fig. 4 BSE images of sample HT-12 h (a, b), sample HT-96 h (c, d)

Fig. 5 XRD patterns of sample HT-12 h

Table 2 Area fraction of δ-hydride in sample HT-12 h and sample HT-96 h

Sample	Area fraction of δ-hydride
HT-12 h	12% ± 0.5%
HT-96 h	5% ± 0.5%

Fig. 6 EBSD images of samples HT-12 h and HT-96 h: a phase map of HT-12 h; b Euler angles map of α-matrix in HT-12 h; c Euler angles map of FCC δ-hydride in HT-12 h; d phase map of HT-96 h; e Euler angles map of α-matrix in HT-96 h; f Euler angles map of FCC δ-hydride in HT-96 h

Fig. 7 Pole figures of α-Ti and δ-hydride in samples a HT-12 h, b HT-96 h

Fig. 8 STEM images of hydrides in sample HT-96 h: a low magnification view; b, c high magnification views

Fig. 9 Orientation relationships between matrix and δ-hydrides in grains A-D in Fig. 3a. orientations: Grain A (178.5°, 33.3°, 12.1°), Grain B (158.6°, 31.8°, 10.7°), Grain C (137.8°, 50.0°, 10.4°), Grain D (175.0°, 49.2°, 11.3°), Hydride 1 (74.3°, 34.8°, 3.4°), Hydride 2 (292.9°, 26.5°, 75.8°), Hydride 3 (49.1°, 36.1°, 69.3°) and Hydride 4 (96.3°, 36.6°, 60.3°)

Table 3 Interface planes of δ-hydride variants

Variant	Variant1	Variant 2	Variant 3	Variant 4	Variant 5	Variant 6
Interface	{10$\overline{1}3$}//{1$\overline{1}$0}	{01$\overline{1}3$}//{1$\overline{1}$0}	{$\overline{1}103$}//{1$\overline{1}$0}	{$\overline{1}013$}//{1$\overline{1}$0}	{0$\overline{1}13$}//{1$\overline{1}$0}	{1$\overline{1}03$}//{1$\overline{1}$0}

Fig. 10 Stereographic projections of the orientation relationship planes, orientation relationship directions and interface planes of the α-Ti (HCP) and the δ-hydride in grains A, B, E, G and H in Fig. 6d. The coincidence of projections of the α-Ti and δ-hydride are indicated by black dashed frames, the black dotted lines outline the surface plane traces of the δ-hydrides and the solid lines connect the center of the pole figures with the poles corresponding to the surface planes and perpendicular to the surface plane traces

Fig. 11 a Euler map of δ-hydride across a grain boundary (Grain C and Grain D in Fig. 6d); b stereographic projections of the orientation relationship planes, orientation relationship directions and interface planes of δ-hydride and the two grains; c Euler map of two adjacent δ-hydrides with different orientations in Grain F; d orientation relationship plane, orientation relationship direction and interface plane of two orientation relationships between the two δ-hydrides and their matrix

Table 4 Fractions of grains containing δ-hydride with OR1and OR2 in samples HT-12 h and HT-96 h

Sample	Orientation relationship (OR)	Interface plane	Fraction of grains (%)
HT-12 h	OR1{0001}_α//{001}_δ 〈11$\overline{2}$0〉_α//〈1$\overline{1}$0〉_δ	{10$\overline{1}$0}_α//{1$\overline{1}$0}_δ	91
HT-12 h	OR2 {0001}_α//{1$\overline{1}$1}_δ 〈11$\overline{2}$0〉_α//〈1$\overline{1}$0〉_δ	{10$\overline{1}$ 3}_α//{1$\overline{1}$0}_δ	89
HT-96 h	OR1{0001}_α//{001}_δ 〈11$\overline{2}$0〉_α//〈1$\overline{1}$0〉_δ	{10$\overline{1}$0}_α//{1$\overline{1}$0}_δ	84
HT-96 h	OR2 {0001}_α//{1$\overline{1}$1}_δ 〈11$\overline{2}$0〉_α//〈1$\overline{1}$0〉_δ	{10$\overline{1}$ 3}_α//{1$\overline{1}$0}_δ	3

Fig. 12 TEM analyses of two types of δ-hydrides in sample HT-96 h: a δ-hydride with OR1; b δ-hydride with OR2

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