The Possible Importance of Pressure in Metastable Precipitate Formation in Ion Implanted Metals
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THE POSSIBLE IMPORTANCE OF PRESSURE IN METASTABLE PRECIPITATE FORMATION IN ION IMPLANTED METALS.
JOHN H. EVANS Materials Development Division, Harwell Laboratory, Oxon, OXll ORA, U.K. ABSTRACT Prompted by the recent discovery that the heavier inert gas atoms implanted into metals precipitate in
the solid phase,
indicative of very high pressures
(,>,I GPa), the present paper discusses the conditions under which such pressures might be expected. The metal/inert gas results are briefly described and then used as a model to show that the two essential features apart from low or moderate metal temperatures, are the insolubility of the implanted species in the host matrix and its precipitation on a very fine scale. This combination suppresses the bias-driven cavity swelling that would otherwise control vacancy acquisition in an irradiation environment.
The extrapolation to other combinations of implanted ion and metal will be discussed. Where the implanted ion is insoluble and precipitates on a scale similar to the inert gas atoms, exact analogy suggests that the precipitates will again be under high pressure.
The formation of high pressure phases might
not be unexpected and could be a factor in explaining the presence of phases previously thought to be metastable. INTRODUCTION In
1984 two groups independently discovered
that two of the heavier inert
gases (Ar and Xe) implanted into aluminium precipitated in a solid form [1,2]. The result was subsequently extended to other inert gases in other metals [eg 3-7] and demonstrated in a rather direct way that inert gas bubbles in metals formed by implantation at ambient temperatures are under rather high pressures, of the order of several GPa. There appears to be general agreement that the results are consistent with a pressure-driven cavity growth process such as loop punching [8] in which a cavity can gain vacancies at the expense of punching out interstitial loops into the matrix. The present paper has two main aims. The first is to understand how a pressure-driven cavity growth mechanism can operate in an environment where the implantation-induced displacement damage might be expected to provide an adequate supply of vacancies, particularly for the heavier inert gas atoms. As in a previous approach to this question [9], it is important to examine the behaviour of the three main components in the system, ie vacancies, selfinterstitials and gas atoms, and the way in which these components are partitioned to the various defect sinks. It emerges that the net supply of irradiation produced vacancies to cavities will be limited by the well known bias-driven swelling mechanism. In the following we look in turn at each of the growth mechanisms and then compare them quantitatively. The effect of additional parameters such as specimen temperature will also be covered together with the role of the near-surface. Other depth information, clearly important in ion implantation, is included implicitly by always considering local values of gas deposition and displacement damage.
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