JOURNAL OF MATERIALS SCIENCE LETTERS 7 (1988) 108-110 Trapping of hydrogen in helium-implanted metals E. ABRAMOV*, D. ELIEZER Ben-Gurion University of the Negev, *and also Nuclear Research Centre Negev, Beer-Sheva, Israel Helium is formed in metals as a secondary product of fission/fusion energy technology. At high tempera- ture, helium atoms are produced from the transmuta- tion of l°B or from a two-step process with 58Ni in amounts as low as a few parts per million. Early experiments show that helium is strongly trapped at radiation-produced defects in metals [1]. Atomistic calculations using pair potential interactions verified these findings [2]. It was initially thought that the helium embrittlement in metals was due to the trapping and subsequent bubble formation at radiation-induced defects. It has been shown, how- ever, that helium may be trapped in metals even in the absence of radiation damage [3]. As there are many sources from which hydrogen can be introduced to the pre-damaged metal (e.g. gaseous hydrogen or its isotopes, aqueous solutions, and hydrogen-ion bombardment), the synergetic effects caused by the presence of both helium bubbles and hydrogen atoms in the lattice, should be considered. It is well established that hydrogen may be trapped at particular defect sites in metals; direct evidence such as that obtained by auto-radiography techniques [4, 5], and indirect evidence as in permeation experi- ments [6] or deuterium depth profile measurements [7, 8], are numerous. These results have even allowed the drawing of various classifications of possible trap types in steel [9-11]. The purpose of the present study is to suggest a mechanism for trapping of hydrogen around or near helium bubbles. Because hydrogen trapping controls the distribution of hydrogen around the bubbles, understanding of the trapping mechanism can lead to failure prediction [12]. Many studies [13-17] have been recently conducted to characterize the shape and dimension of the bubbles as well as to determine the helium density inside the bubbles. Transmission electron microscopy (TEM) observations [13-16] show a microstructure which contains many small spherical bubbles in the range of 1 to 2nm. Other experimental works [15, 17] esti- mate the density of helium inside the bubbles to be 2 × 1023atoms cm ~3. Using the equation of state of helium to very high pressure [18], we have found the pressure inside the bubbles to be about 3.5 × 101°Nm -2 (350kbar), for which helium should be solid at room temperature. Hydrogen trapping near or around helium bubbles was reported by various workers [13, 16, 19]. Linear ramp thermal desorption measurements and other experimental techniques, shows that the interaction energy between hydrogen atoms and helium bubbles is in the range of 0.7 to 0.9eV/atom. It should be TABLE I Trapping energy of hydrogen atoms in various de- fects in iron Trapping energy Defects (eV) 0.03-0.10 interstitials 0.25- 0.31 dislocation 0.40-0.50 vacancies 0.60-0.70 clusters 0.90-1.00 inclusions (TIC) 0.70-0.90 helium bubbles mentioned that these values are the extreme values measured and actually there is a spatial distribution of binding energies. This trapping energy measured is very strong relative to other defects such as vacancies, clusters, etc. (Table I). Some workers [13, 19] attempted to explain the trapping of hydrogen around helium bubbles and relate it to.a chemisorption-like interaction at the walls of the bubbles or to the strains surrounding the isolated bubbles. However, a detailed and qualititative model which can help to understand this trapping and to estimate the binding energy has not yet been established. In this letter we suggest a preliminary model for hydrogen trapping around helium bubbles. According to this model hydrogen atoms are attracted towards the bubble due to positive stresses (tensile stresses) which exist around the bubble. These high stresses resulted from the very high pressure (350 kbar) which exists inside the bubble. In order to calculate the trapping energy of the hydrogen atoms around the helium bubbles we must consider the stress field situation around the over- pressurized bubbles. The growth of helium bubbles is controlled by a self-trapping mechanism in which helium atoms are attracted towards the bubble and metal atoms are rejected by the bubble. In this case, which differs from the case of growth caused by pressure rise, a simple plastic theory for the calcula- tion of the stress field near the bubble surface cannot be used. We suggest a simplified, but consistent, analysis of this complex problem by using some assumptions. First, we will examine the hydrostatic component of the stress tensor which is defined as ah = 1/3 X aii = 1/3 (axx + ayy ~- tTzz) (1) where ai~ is the three principal stresses (axx, ayy, azz) at a given point. From equilibrium consideration the hydrostatic stress, ah, in the bubble surface or actually in the material-free surface which bounds the bubbles, is equal to the pressure which exists inside the bubble, 1 08 0261-8028/88 $03.00 + .12 © 1988 Chapman and Hall Ltd.