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A kinetic model has been designed to study substitutional solute segregation during irradiation in facecentered-cubic metals. The model includes a (100) split interstitial binding to impurities to second-neighbor distances, vacancy binding to impurities to first-neighbor distances, and the possibility of migration of the bound complexes. Also taken into account are the efFects of vacancy and interstitial diffusional encounters with impurities and spatially independent reaction terms. The resultant rate equations have been solved numerically for a thin-foil geometry as a function of time for different temperatures, defect-production rates, internal sink concentrations, foil thicknesses, defect-impurity binding energies, and initial impurity concentrations. Using parameters appropriate for Zn in Ag, significant solute segregation is found in the temperature range from 0.2 T to 0.6 T (T is the melting point). The temperature for maximum segregation is appreciably higher for heavy-ion bombardment or high-voltage-electron-microscope irradiation rates than for fast-reactor irradiation rates. It is also found that interstitials make a major contribution in the transport of solute during irradiation. The present calculations are intended to indicate the general pattern of segregation behavior and would be useful in areas of high-voltage electron microscopy, void formation, radiation-enhanced diffusion, and advanced materials development for nuclear-reactor applications.

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The crystalline-to-amorphous (c-a) phase transformation can be induced by a variet of solid-state processes ranging from energetic particle irradiation, interface inter diffusion reactions, hydrogen charging and mechanical deformation to the application of high pressures. During the past decade, such transformations have become the focus of considerable research not only because of their potential technological applications, but also because of strong scientific interest in the relationship between the c-a transition and the melting process.A common feature underlies all solid state amorphization processes: The atomic disorder created in the crystalline lattice in the form of static atomic displacement can induce volume change and elastic softening of the lattice. A particularly striking example of the softening effect is shown in Figure 1 for the case of radiation-induced amorphization of the intermetallic compound Zr3Al. The compound, which has the Ll2 (Cu3Au)-type superlattice structure, was irradiated with energetic ion at room temperature in a high-voltage electron microscope interfaced to a tandem ion accelerator. The rapid decrease in the intensities of both fundamental and superlattice reflections show that irradiation introduces antisite defects (chemical disorder) as well as static atomic displacements. The disordering of the long-range ordered structure, which occurs prior to the onset of amorphization, is accompanied by a volume expansion of about 2.5% and a ~25% decrease in the average velocity of sound. This decrease in sound velocity corresponds to a ~50% decrease in the average shear modulus, which is comparable to that observed for many metals during heating to melting. The volume dependence of this disorder-induced elastic softening is also similar to that associated with heating. In both cases, the shear modulus is a linearly decreasing function of volume expansion. However, for a given amount of expansion, the softening associated with static atomic displacements is nearly twice as large as that associated with increasing anharmonic lattice vibrations.

The effect of S segregation to grain boundaries on the intergranular embrittlement of Ni has been studied at room temperature using Auger electron spectroscopy and slow strain rate tensile tests. The grain-boundary S concentration was varied by time-controlled annealing of dilute Ni–S alloy specimens at 625 °C. The ductile-to-brittle transition in Ni, as determined from percent integranular fracture and reduction-in-area measurements, occurred over a narrow range of S concentrations centered on 15.5±3.4 at. % S. This critical S concentration for 50% intergranular fracture of polycrystalline Ni is similar to the 14.2±3.3 at. % S required to induce 50% amorphization of single-crystal Ni by S+-ion implantation. This suggests that segregation-induced intergranular fracture, like implantation-induced amorphization, may be a disorder-induced polymorphous melting process. In agreement with experimental observations, the polymorphous melting curve for the Ni–S solid solution on the phase diagram drops rapidly to zero as the alloy composition approaches ∼18 at. % S. The critical grain-boundary concentration for intergranular fracture, while slightly less, is within experimental error of the concentration predicted for polymorphous melting as well as that measured for ion-implantation-induced amorphization.