Mukashev K.M., Umarov F.F., Shadinova K.S.

National Pedagogical University after Abai, Kazakhstan

Kazakh British Technical University, Almaty, Kazakhstan

POSITRON ANNIHILATION STUDIES OF HYDROGEN BEHAVIOR IN METALS WITH STRUCTURE DEFECTS

Measurements of angular distribution of annihilation photons are applied to investigations of hydrogen behavior in annealed, plastically deformed titanium alloys. It is shown that hydrogen interaction with materials becomes more intensive in presence of structural imperfections of deformational origin. Probable mechanisms of hydrogen interaction with structure defects are discussed.

Key words: positron annihilation, angular distribution of annihilation photons, titanium alloys, hydrogenation.

1. Introduction

Together with important merits, titanium and its alloys have certain shortcomings, the most serious of which is their hydrogen brittleness. Hydrogen behavior in metals was a subject of comprehensive studies [1,2], and the problem of titanium interaction with hydrogen had arisen from the very beginning of its industrial production. This problem is especially vital for titanium since it casts doubt on expediency of titanium application as a structural material. It was stated that any amount of hydrogen depreciates its mechanical properties. The hydrogen brittleness becomes apparent at very low hydrogen concentrations being accompanied with extreme titanium activity in respect to hydrogen absorption. Therefore, successful application of articles made of titanium and its alloys and prediction of their viability require thorough and detailed studies of hydrogen behavior in conditions characteristic for operation of nuclear power facilities.

The most important processes that accompany interaction of hydrogen with metals are surface adsorption, chemisorption, diffusion, absorption, and formation of metal-containing chemical compounds. The rate of hydrogen dissolution in metal grows abruptly due to allotropic transformations and transitions from solid to liquid state. However, there are specific features of hydrogen interaction with titanium and some other similar metals.

It is generally accepted that hydrogen in such metals is usually neither completely ionized to proton nor it forms a negative ion. More characteristic for titanium are intermediate processes when an atom exists partially in excited and partially in ionized state. Hence, a size of a hydrogen atom in a metal is much bigger than the proton size. Being located in an interstitial position, such an atom causes certain lattice distortion and alters the lattice constant when hydrogen concentration is high enough. The latter just gives a reason to assert that hydrogen atoms in titanium form an interstitial solid solution. Titanium and similar metals can absorb considerable amount of hydrogen at low temperature. This process is exothermal; the accumulated hydrogen easily evolves at heightened temperatures (T>1000⁰C) and can be completely extracted from a crystal at temperatures higher than 1400⁰C.

The equilibrium diagram of Ti - H system [3] compiled on the base of numerous experimental data demonstrates that hydrogen expands the -phase region and simultaneously makes the region of -phase more narrow. At the temperature 335⁰C and hydrogen concentration 35 at. % eutectoid decomposition of the -phase to - and - phases is observed in the system, and - phases are hydrogen interstitial solid solutions in - and - Ti, respectively [2,4].

Existing theoretical concepts and scanty experimental data are insufficient for general conclusions on hydrogen behavior in metals. Nevertheless, they show that together with variation of such operational factors as temperature, pressure or ionizing radiation, hydrogenation can be considered as one of the universal methods for controlled reversible structural and chemical metal modifications.

In this paper ADAP measurements are used to study effect of deformation and radiation processing on hydrogen accumulation in titanium alloys of different composition.

2. Experimental

The objects of experimental studies were iodide titanium, an alloy Ti-1.4 at.%V and a system of Ti-Al alloys. Original materials were annealed at 900⁰C, deformed by 50% by rolling. Hydrogenation was conducted at temperature 200⁰C during three hours, hydrogen pressure was (4.9-5.9)10⁵ Pa. Hydrogen was produced by desorption of hydrides LaNiH_x and TiH_x. Before hydrogenation samples were kept at the room temperature in vacuum of 0.13-1.33 Pa during 10 hours directly in the reactor, where hydrogen was admitted after preliminary processing. Pressure was measured by a standard manometer with accuracy of 300 Pa, the volume of the reactor system being 400 cm³.

3. Results and Discussion

Hydrogen Interaction with Deformational Defects. An amount of hydrogen absorbed by the samples of the alloys was determined from differences of hydrogen pressure in the reactor system and additionally by weighing of the samples with accuracy of 10^-6 g. No weight differences were detected after hydrogenation at 200⁰C for the samples with different initial state.

At the temperature of 500⁰C hydrogenation proceeds more intensely as a sequence of enhanced hydrogen diffusion, weight of the samples noticeably grows. The results obtained can be analyzed using annihilation parameters in Table 1.

ADAP spectra are represented by parabolic and gauss components that fill the areas S_p and S_g , respectively. The relation of these areas [5]

F = S_p / S_g

defines redistribution of positron annihilation probability in respect to free electrons and electron of the ion framework; F is the relative alteration of this probability in respect to the initial state of a material; is the cut-off angle of the parabolic part of an ADAP spectrum.

Table 1. Annihilation parameters of hydrogenated titanium alloys

Material state	Titanium			Ti – 5.2 at.% Al			Ti – 1.4 at.% Al
	F	, Mrad	%	F	, Mrad	%	F	, Mrad	%
Annealed Annealed+H(200^0C) Annealed+H(500^0C) =50%+H(200^0C)	0.26 0.29 0.37 0.32	5.82 5.85 5.92 6.05	- 15 41 22	0.34 0.33 0.46 0.46	5.61 5.83 5.92 5.82	- 0 36 35	0.27 0.29 0.33 0.38	6.12 5.92 5.91 6.22	- 9 23 43
Ti pressed powder TiHx pressed powder	0.40 0.50	5.25 5.00	54 90	Error			0.01	0.05	1.0

Hydrogen introduction into crystal lattice of the alloys studied leads to considerable increase in the maximum of the ADAP spectrum, contracting it at the same time. Hydrogenation efficiency depends on the nature of the doping element, conditions of preliminary processing and hydrogenation of the original material.

Interpretation of the experimental data should take into account both phenomena associated with hydrogen dissolution and positron interaction with the hydrogenated material.

Dissolving in ideal crystals with a perfect structure, hydrogen takes interstitial sites in the lattice and probably displaces atoms from their equilibrium positions. It causes lattice distortions that can act as centers of positron localization. In real crystals hydrogen segregates at various lattice defects, that leads to reduction in the probability of positron trapping. Indeed, plastic deformation enhances hydrogen solubility, while the redundant hydrogen is accumulated near dislocations in the fields of elastic strains. In the case of micropore formation as a result of vacancy coagulation, they can be filled in with hydrogen.

These alterations in the form of ADAP spectra after material hydrogenation testify to the appropriate redistribution in the probability of positron annihilation in respect to conduction and inner-shell electrons. However, hydrogenation of a material from the annealed state at the temperature of 200⁰C practically does not affect positron annihilation parameters (Ti-5.2%at.Al), or these alterations are very small. This fact can be associated both with the level of hydrogenation and the degree of its effect on annihilation characteristics.

Increase in hydrogenation temperature to 500⁰C intensifies hydrogen absorption and, hence, material hydrogenation, as a result the probability parameter F increases from 23 to 41%. However, the most significant are not so the observed alterations in the annihilation probability after hydrogenation as the fact that the angle of the Fermi momentum remains constant in the range of experimental error. This result shows that hydrogenation is not accompanied by formation of new structural defects in metals and the electron subsystem is not subjected to considerable rearrangement. To some extent, it can be considered as an argument in favor of anion model of hydrogenation or, at least, as confirmation of partial ionization of hydrogen atoms in a metal.

Plastically deformed metals can accumulate considerable amount of hydrogen. The high deformation degree (=50%) of the metals studied creates such high concentrations of vacancy and dislocation defects that causes saturation of the annihilation parameter F. Subsequent hydrogenation at comparatively low temperature of 200⁰C is obviously accompanied by trapping of atomic hydrogen by structural imperfections. As a result new complex defects appear in crystal lattice, that depreciates trapping efficiency of the centers of positron localization previously introduced by plastic deformation. This leads to appropriate alterations in the probability parameter F which makes 22% for titanium and 43% for the alloy Ti-1,4 at.% V, while the same parameters after plastic deformation were in the range 45-80%.

The rise in hydrogenation temperature up to 500⁰C causes material transfer to the hydride state. As a result the compact cast metal samples became scattered into powders and ADAP spectra couldn’t be measured. Therefore, in addition to compact cast metals, two samples for positron annihilation measurements were pressed of industrial titanium powder and its hydride, the average particle size in powders was about 100. Values of the annihilation parameter F for these materials were F=0.40 and F=0.50, respectively.

Such a considerable increase in the F parameter in the first case (up to 54%) can be explained by high defect concentration characteristic for pressed materials [8]. In the second case increase in F- value by 90% is caused by high concentration of hydrogen in titanium in the form of its hydride TiH_x. Thus, disintegration of the compact material after its hydrogenation at 500⁰C can be provoked by deformational cracks that appear in crystal in the course of hydride formation.

When penetrating into crystal lattice hydrogen segregates at dislocations, vacancies and other lattice defects not only as separate neutral atoms but also in the form of ionized protons (or negative ions) characterized by high mobility, the latter can transform to neutral atoms of lower mobility. Interaction of simple defects at high temperature leads to their coagulation and transformation to complex defects of a crack type. The introduced hydrogen can dissociate at the inner surfaces of these defects and form molecules of still lower mobility [2]. As hydrogen is supplied from the external source, such molecules expand in volume and provoke disintegration of a sample. This is only one of many other mechanisms of metal destruction. Hydrogen exists in metals not only in the forms of neutral atoms and protons but also negative ions. The latter can segregate at dislocations and form chemical bonds with a metal. Titanium hydrides that appear as a result of this process have lower density compared with the metal matrix and provoke its disintegration as the volume occupied by hydride molecules grows.

4. Conclusion

Thus, increase in hydrogenation temperature to 500⁰C leads to the complete annealing of radiation defects that results in the drop of the annihilation probability parameter F and increase in up to the level corresponding to hydrogenation of annealed materials at 500⁰C. This observation leads to conclusion that annealing of radiation defects does not affect the processes of hydrogen accumulation in a metal. Moreover, absence of radiation defects prevents hydride formation and accumulation, and materials maintain their origin form without destruction. However, hydrogenation temperature of 500⁰C is not sufficient for hydrogen extraction from a metal.

References

1. Smith D. Hydrogen in metals - Chicago University Press, 1948, 246p.

2. Sliyan O.D. Hydrogen Distribution in the Zone of Deformational Cracks // Zhurnal Fizicheskoi Khimii (Journal of Physical Chemistry), vol. 54, No. 11, p. 2913-2917.

3. Livanov V.A., Bukhanova A.A., Kolachev B.A. Hydrogen in Titanium. – Moscow, Gostekhizdat, 1962, 245 p.

4. Rosenfeld B., Chabik S., Pietrzak R. et al. Investigations on the Influence of Deformation and Hydrogen Inclusions on Changes in Shape of Angular Correlation Curves on Annihilation Quanta in Titanium. // Bul.Acad. Pol. Sci., Ser. Sci. Math., Astron. Et Phys., 1977, vol.25, No.6, p. 597-601.

5. Mukashev K.M. Low positrons physics and positron spectroscopy. – Almaty, 2009. 507 pp.