Nanoscale compositional changes along fast ion tracks in equilibrium solid solutions: A computer simulation of ultra-fas
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Nanoscale compositional changes along fast ion tracks in equilibrium solid solutions: A computer simulation of ultra-fast solidification and thermomigration Edmundo M. Lopasso, Alfredo Caro, Eduardo Ogando Arregui1 and Magdalena Caro Centro Atómico Bariloche - Instituto Balseiro, 8400 Bariloche, Argentina. 1 Dep. de Electricidad y Electrónica, UPV-EHU apdo. 644, 48080 Bilbao, España. ABSTRACT Starting from two equilibrium solid solutions in the Au-Ni system, we analyze the change in composition due to a 400 eV/Å fast ion track simulated by molecular dynamics in the Embedded Atom approximation. We aim at determining the influence of the thermodynamic forces derived from the large thermal gradients and the rapid solidification across the solidus and liquidus on the motion of solute atoms. One dimensional gradients as well as analytic models are used to quantitatively determine the domains of influence of these forces. Evidence shows that the liquidus and solidus equilibrium solidification predicted by the phase diagram is not reached during the track. The solute concentration is mainly determined by the combined diffusion and thermomigration mechanisms in the liquid stage. INTRODUCTION In the usual interpretation of radiation effects in alloys, equilibrium solid solutions are not expected to experience modification of solute distribution as a consequence of energetic collision cascades. However, the presence of huge thermal gradients and the rapid quenching across the two phase field delimited by the solidus and liquidus lines provide thermodynamic forces that may give rise to solute redistribution. In the short initial ballistic stage the atoms are displaced from their equilibrium positions, creating interstitials, vacancies and ion mixing near the cascade. The thermal stage that follows involves a liquid-like system, followed by a temperature drop that drives the system towards re-solidification; in these stages precipitation, dissolution, amorphisation, and diffusion (besides other possible transformations), influences the solute redistribution. The diffusion mechanisms in the liquid phase are influenced by the thermomigration of solutes. The governing equation in this case is the diffusion equation with the heat of transport contribution [1]: ∂C s Q*C s = ∇ ⋅ D ∇C s + ∇ T 2 ∂t kT
(1)
in which Cs is the solute concentration, t is the time, D is the diffusion coefficient, Q* is the heat of transport, k is the Boltzmann constant and T is the temperature. Under a thermal gradient, the thermomigration effect would drive the solute to the cold region of the sample if Q* is positive, and to the hot region of the sample if Q* is negative. Additional effects could appear as system cools down from the liquid, and the characteristics of the phase diagram affect the final solute distribution in the solid phase. In systems with liquidus and solidus lines with negative slopes, the solute is pushed towards the liquid as the solidification interface moves on cooling. If solid diffusion is neglected due
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