Hydrogen-induced defects in Ti and their thermal stability

O. Melikhova1, J. Čížek1, P. Hruška1, F. Lukáč1,2, J. Knapp1, L. Havela1, S. Mašková1, W. Anwand3, M.O. Liedke3

PSD-17 1Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, Prague 8, CZ 18000, Czech Republic

2Institute of Plasma Physics of the Czech Academy of Sciences, Za Slovankou 1782/3, 182 00 Prague 8, Czech Republic 3Institute of Radiation Physics, Helmholtz-Zentrum Dresden-Rossendorf Bautzner Landstr. 400, 01328 Dresden, Germany

Introduction

Titanium readily absorbs hydrogen and undergoes phase transition into the hydride phase (TiHx). In the hydride phase Ti is able to absorb the hydrogen concentration as high as 1.4 wt.%. These properties make Ti and Ti-based alloys attractive for hydrogen storage applications. Hydrogen absorption in titanium matrix may introduce open volume defects since the volume of TiHx phase exceeds that of titanium matrix. Absorbed hydrogen may segregate at these defects forming defect-hydrogen complexes. In the present work positron annihilation spectroscopy was employed for characterization of hydrogen-induced defects in titanium. Defects created by hydrogen loading from the gas phase were compared with those introduced by electrochemical hydrogen charging. In general hydrogen loading introduces a high density of dislocations and vacancy clusters created by agglomeration of hydrogen-induced vacancies. Thermal stability of hydrogen absorbed in titanium and recovery of hydrogen-induced defects were studied by positron lifetime spectroscopy combined with X-ray diffraction and thermal desorption spectroscopy.

Summary

• Studied Ti samples were loaded with hydrogen electrochemically and from the H2 gas. Hydrogen loading leads to phase transition from α-Ti to δ-TiHx phase. • Hydrogenation of Ti is controlled by hydrogen diffusion through the δ-TiHx phase. As a consequence at elevated temperature (500◦C ) the sample was fully converted to the δ-TiHx phase after loading for 50 h. But at lower temperatures only a sub-surface layer was transformed into the δ-TiHx phase while bulk of the sample remained in the α-Ti phase. • Hydrogen loading of α-Ti introduced dislocations created by phase transition into the δ-TiHx phase and small vacancy clusters formed by agglomeration of H-induced vacancies. • The decomposition of TiHx phase is accompanied with introduction of additional vacancies agglomerating into vacancy clusters. • Annealing above 500oC leads to recovery of dislocations.

References

[1] F. Bečvář, J. Čížek, I. Procházka, J. Janotová, Nucl. Instrum. Methods Phys. Res. A 539 (2005) 372-385 [2] I. Procházka et al., Mater. Sci. Forum, 255-257(1997) 772-774

Samples

Bulk α-Ti samples (99.7%) annealed at 1000oC for 2 hours in vacuum. H loading: (i) electrochemical loading - at 20oC for 240 h, a constant current of 20 mA applied in an electrolyte consisting of H3PO4 and glycerine in the volume ratio 1:2; (ii) H2 gas loading - at 150oC for 100 h, H2 gas pressure of 103 bar; - at 400oC for 5 h and 45 h, H2 gas pressure of 1 bar; - at 500oC for 50 h, H2 gas pressure of 0.6 bar

Methods of characterization

• Positron lifetime (LT) measurements: - digital spectrometer with time resolution of 145 ps [1] - 22Na radioisotope with activity of 1.5 MBq deposited on 2 μm thick Mylar foil was used as a positron source. - at least 107 positron annihilation events were collected in each LT spectrum, which was subsequently decomposed into exponential components using a maximumlikelihood method [2]. The source contribution consisted of two components with lifetimes ≈368 ps and ≈1.5 ns and corresponding intensities ≈6 and ≈1%. was always subtracted from LT spectra. •Differential scanning calorimetry (DSC) and thermogravimetry (TG): - simultaneous DSC/TG apparatus Setaram Labsys Evo - the samples were heated in Al2O3 crucibles using the rate of 5 K/min. The DSC/TG measurements were performed in a protective Ar atmosphere - gases desorbed from the samples were analyzed by a quadrupole mass spectrometer attached to the DSC/TG apparatus. •X-ray diffraction (XRD) measurements - Bragg-Brentano symmetric geometry using Cu Kα (λ=1.54 Å) radiation on Xpert MRD diffractometer

Results

DSC curve of Ti, loaded from gas phase, at 400oC, 1 bar, for 5 hours (heating rate 5K/min).

Ti-H phase diagram

α-Ti hcp

β-Ti bcc

δ-TiHx fcc

T (oC)

0 200 400 600 800

lifetim

e (

ps)

0

100

200

300

400

3, vacancy clusters

2, dislocations

1, free positrons

T (oC)

0 200 400 600 800

inte

nsity (

%)

0

20

40

60

80

100

I3, vacancy clusters

I2, dislocations

I1, free positrons

(a) (b)

Results of positron lifetime investigations of hydrogen loaded Ti (H2 gas pressure 1 bar, temperature 400oC, for 5 h): the development of (a) positron lifetimes and (b) corresponding relative intensities on the annealing temperature.

Optical micrographs for: (a) annealed Ti sample, (b) the layer of δ-hydride on the surface of electrochemically loaded Ti sample (cross-section), (c) H2 gas loaded sample at 150◦C and 103 bar for 100 h partially transformed into δ-TiHx phase (cross-section) (d) H2 gas loaded sample at 400◦C and 1 bar for 5h (cross-section) (e) H2 gas loaded sample at 500◦C and 0.6 bar for 50h fully transformed into δ-TiHx (cross-section)

a b c d e

Ti, H2 gas loaded 1 bar, 400oC, 5 h

heating rate 5 K/min

T (oC)

200 400 600 800 1000

He

at

flo

w

-6

-4

-2

0

H2 p

ressu

re (

To

rr)

0

5.0x10-9

10-8

1.5x10-8

2.0x10-8

TG

(m

g)

0.00

0.01

0.02

0.03

0.04

0.05

0.06

DSC

H2 pressure

TG

exo

Ti, H2 gas loaded 1 bar, 400

oC

time (h)

0 10 20 30 40 50

concentr

ation (

wt.%

)

2

4

6

8

10

12

14

16

-TiH2 concentration

XRD patterns for Ti samples H2 gas loaded at 400oC, H2 gas pressure 1 bar: (a) for 5 h, (b) for 45 h.

Weight fraction of δ-TiHx phase determined by XRD in Ti samples, loaded from gas phase, at 400oC, 1 bar

Cu K 2 (deg)

40 60 80 100

counts

100

101

102

103

Ti, H2 gas loaded, 1 bar, 400oC, 5 h-T

i 0

02

-T

i 1

01

-T

i 1

02

-T

i 1

10

-T

i 1

03

-T

i 2

01

-T

i 0

04

-T

i 2

10

-T

i 2

11

-T

i 1

00

-T

i 1

12

-T

i 2

02

-T

i 1

04

-T

i 2

03

-T

i 2

00

-T

iH2 0

22

-T

iH2 1

31

-T

iH2 1

33

-T

iH2 0

42

-T

iH2 0

20

(a)

Cu K 2 (deg)

40 60 80 100

counts

100

101

102

Ti, H2 gas loaded, 1 bar, 400oC, 45 h

-T

i 0

02

-T

i 1

01

-T

i 1

02

-T

i 1

10

-T

i 1

03

-T

i 2

01

-T

i 0

04

-T

i 2

10

-Ti 2

11

-T

i 1

00

-T

i 1

12

-T

i 2

02

-T

i 1

04 -T

i 2

03

-T

i 2

00

-T

iH2 1

11

-T

iH2 0

20

-T

iH2 0

22

-T

iH2 1

31

-T

iH2 2

22

-T

iH2 1

33

-TiH

2 0

42

(b)

Acknowledgement

This work was supported by the Czech Science Agency (project GA17-17016S)