Patent Number: 054901871
Section: description

DETAILED DESCRIPTION OF THE INVENTION Experimental studies by Applicants have led to in situ observations by hot-stage transmission electron microscopy (TEM) of anomalously rapid helium bubble diffusion in aluminum with a low concentration of lead, and in aluminum with a low concentration of indium, at annealing temperatures above the melting point of the impurity species. In these experimental studies, samples of 99.999% pure aluminum alloyed with 200 ppm by weight of lead, and separately, with 1000 ppm by weight of indium, were thinned to electron transparency and subsequently irradiated with 50-keV helium ions to produce a helium concentration of approximately 20 atomic ppm. The implanted samples were then annealed at 723-743 K for several minutes, during which time video recordings were made of spherical helium bubbles undergoing Brownian motion and eventually coalescing or disappearing at a foil surface. The participation of the impurity, which is not visibly apparent at the annealing temperature, was verified in each case by cycling the sample temperature between room temperature and the annealing temperature. It was observed that solid precipitates attached to helium bubble facets appeared during slow cooling to room temperature but did not appear during a rapid quench. In the latter case, the precipitate subsequently formed at approximately 520 K, and disappeared again above the melting temperature of the impurity, as the sample temperature was increased. This is shown in FIG. 1 which is a bright-field electron micrograph, taken after slow cooling the aluminum matrix to room temperature. FIG. 1 shows solid indium precipitates (dark spots) attached to preferred helium bubbles (white spheres) in the aluminum matrix. Also apparent in FIG. 1 as collections of dark spots (indium) superimposed on the light spherical region are remnants of helium bubbles that have reached a foil surface. FIG. 1 also illustrates the insolubility of the indium with the host aluminum metal. Video images were used to obtain bubble diffusion coefficients D.sub.b at the annealing temperatures in the following manner. Spatial displacements transverse to the electron beam were measured for each of several lead- and indium-coated bubbles during successive 1 second time intervals. The collection of N measurements for each bubble must possess (in the limit of an infinite number of measurements) a Gaussian probability distribution, since the N measurements can be regarded as single measurements for N noninteracting, identical bubbles, all initially located together on a two-dimensional plane at r=0 and at time t=0. Thus the radial distribution of measurements r at time t can be expressed by the following equation: ##EQU1## so that integrating p over the entire plane produces the total N measurements. The number of displacements r between r.sub.i and r.sub.j (with r.sub.i &lt;r.sub.j) is then approximately EQU n.sub.ij =N exp [-r.sub.i.sup.2 /4D.sub.b t]-N exp [-r.sub.j.sup.2 /4D.sub.b t] This expression provides the areas N.sub.i,i+1 for Gaussian histograms that are compared to the histograms of measured bubble displacements. The bubble diffusion coefficient D.sub.b that provides the best match, for each of the monitored helium bubbles, is presented in the following Table 1. TABLE 1 ______________________________________ R (nm) D.sub.b (nm.sup.2 /s) D.sub.s (.mu.m.sup.2 /s) ______________________________________ Pb/He 5.15 2.0 0.70 6.0 1.2 0.77 In/He 7.9 2.0 3.85 16.0 7.0 227.0 26.5 0.8 195.0 ______________________________________ The rapid bubble diffusion shown in Table 1 results from enhanced diffusion of aluminum atoms at the bubble/matrix interface. If the latter is assumed independent of bubble size, the corresponding surface diffusion coefficients D.sub.s may be calculated from the following standard relationship: EQU D.sub.b =(3.OMEGA..sup.4/3 /2.pi.R.sup.4)D.sub.s where R is the gas bubble radius and .OMEGA. is the volume of a matrix atom. These values are given in Table 1 and are also presented in FIG. 2 together with D.sub.s taken from other experiments showing helium bubble growth in pure aluminum at various annealing temperatures. FIG. 2 shows the calculated surface diffusion coefficients D.sub.s plotted against the scaled annealing temperature T/T.sub.m for helium bubble diffusion in aluminum. In FIG. 2, T.sub.m is the melting temperature (933 K) of the aluminum matrix. The open circles and the crosses shown in FIG. 2 indicate values of D.sub.s for bubbles with attached liquid lead and indium precipitates, respectively, determined from direct observation of their Brownian motion at 723-743 K. (The numerical values are shown in Table 1). The open and solid diamonds in FIG. 2 indicate the D.sub.s (m.sup.2 s.sup.-1) values for helium bubbles with and without attached lead precipitates, respectively, at 823 K, derived from experimental studies by Applicants. The solid circle shows the average value of D.sub.s determined from measurements of coarsening of helium bubbles in neutron-irradiated, helium-implanted pure aluminum, during annealing at 0.96 T.sub.m. The bars associated with that point indicate the range of five values. The solid curve shown in FIG. 2 is obtained from the expression EQU D.sub.s =0.086 exp [-(2.1 eV)/kT]m.sup.2 s.sup.-1 which is a fit to the data point with a 2.1 eV activation energy for surface diffusion. The bars at 0.83T.sub.m (823 K) indicate the estimate of D.sub.s derived from similar measurements of bubble growth in helium-implanted pure aluminum. The mechanism by which atomic diffusion at the bubble surface is increased is unclear. However, it is important to note that the binary phase diagrams for these aluminum alloys show negligible solubility of the impurity in the matrix material so that the impurity will segregate to the free surfaces provided by the gas bubbles; an impurity melting temperature lower than that of the matrix and of the annealing temperature; and some solubility of the matrix atoms in the liquid impurity. These characteristics suggest a liquid dissolution process, whereby a liquid layer of impurity atoms at the bubble surface acts as a conduit for rapid transport of dissolved aluminum atoms. Two liquid dissolution mechanisms for bubble diffusion, that rely on the properties of the attached liquid precipitates have been considered by Applicants. Volume diffusion of an equilibrium concentration of aluminum atoms through a thin layer of liquid impurity at the bubble surface produces bubble diffusion coefficients in good agreement with those derived from observations. Alternatively, the liquid coating may instead simply remove the bubble facets. In a preferred embodiment of the present invention, the direction of bubble or void migration is biased by the application of a temperature gradient across the host metal or alloy which contains small amounts of impurities, such as lead or indium. For example, those skilled in the art of nuclear reactor technology know that during the fission process the internal temperature of a fuel rod will be greater than the external or cladding temperature, thereby constituting a temperature gradient across the fuel rod cladding. The same is true for nuclear reactor containment structural materials, wherein the internal surface temperature of a containment vessel will be greater than the external surface temperature, thereby creating a temperature gradient across the containment structural material. The temperature gradient to be applied should be such that the higher temperature is greater that the melting point of the impurity metal particles, but lower than the melting point of the host metal or metal alloy. The diffusing surface atoms will tend to move from the hotter side of the bubble or void toward the colder side, thus producing a net movement of the bubble or void up the temperature gradient and out of the metal/alloy material. Controlling the migration of the bubbles and voids is particularly advantageous in fusion and fission reactors and results in preventing the known long-term deleterious effects of inert gases in the reactor cladding or containment structural materials. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments described explain the principles of the invention and practical application and enable others skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.