Raúl A. Enrique

Simulations of dynamical stabilization of Ag–Cu nanocomposites by ion-beam processing

Recent theoretical results indicate that ion-beam mixing can be used to synthesize nanocomposite structures from immiscible elements, relying on a self-organization phenomenon at steady state under irradiation. According to this modeling, self organization requires that the range of the forced atomic relocations in displacement cascades exceeds a critical value. Experimental evidence supporting the formation of nanocomposites by this mechanism has been found in the immiscible system Ag‐Cu irradiated with 1 MeV Kr ions. To address this experimentally relevant model system, and to test the theoretical predictions, we study, by molecular dynamics ~MD!, the characteristics of irradiation mixing in a Ag‐Cu alloy subjected to bombardment with 62 keV He, 270 keV Ne, 500 keV Ar, and 1 MeV Kr ions. The distribution of atomic relocations measured by MD is then used to perform lattice kinetic Monte Carlo ~KMC! simulations of phase evolution, during which both thermal decomposition and irradiation mixing operate simultaneously. The KMC results show that, in the framework of this self-organization mechanism, a nanocomposite structure can be stabilized at steady state by irradiation with heavy ions ~Ne, Ar, and Kr!, but not with He ions. As the characteristic relocation range for He ions is half of that measured for the heavy ions, these results support the theoretical prediction of the existence of a critical relocation range for compositional patterning to take place under irradiation.

Self-organized Cu-Ag nanocomposites synthesized by intermediate temperature ion-beam mixing

We perform an investigation of ion-beam mixing in the immiscible system Ag–Cu based on cross-sectional transmission electron microscopy. Multilayered samples consisting of ten periods of (6.7 nm Cu/11.2 nm Ag) are irradiated at temperatures ranging from 25 to 225 °C with normally incident 1 MeV Kr ions to doses in the range 1–2×1016 ion/cm2, enough to reach a nonequilibrium dynamical steady state. Regardless of the irradiation temperature, extensive grain growth takes place. At intermediate temperatures, competition between thermal decomposition and irradiation mixing results in a nanometer-scale phase separation. This spontaneous decomposition demonstrates that ion-beam mixing can be used as a processing tool to synthesize nanocomposite materials.

Nonequilibrium self-organization in alloys under irradiation leading to the formation of nanocomposites

Abstract Alloys under irradiation are continuously driven away from equilibrium: Every time an external particle interacts with the atoms in the solid, a perturbation very localized in space and time is produced. Under this external forcing, phase and microstructural evolution depends ultimately on the dynamical interaction between the external perturbation and the internal recovery kinetics of the alloy. We consider the nonequilibrium steady state of an immiscible binary alloy subject to mixing by heavy-ion irradiation. It has been found that the range of the forced atomic relocations taking place during collision cascades plays an important role on the final microstructure: when this range is large enough, it can lead to the spontaneous formation of compositional patterns at the nanometer scale. These results were rationalized in the framework of a continuum model solved by deriving a nonequilibrium thermodynamic potential. Here we derive the nonequilibrium structure factor by including the role of fluctuations. In order to consider an experimentally relevant situation, we perform kinetic Monte Carlo simulations of temperature-controlled heavy-ion irradiation in the immiscible model system Ag–Cu, where we obtain specific irradiation data by molecular dynamics simulations. We find that irradiation with 1 MeV Kr ions can indeed induce the spontaneous formation of a nanocomposite, in agreement with recent experimental results. Our results suggest, on the other hand, that this type of microstructure could not be obtained by irradiation with lighter particles, such as He ions. The simulation results predict, as well, the dependency of the structure factor on the type of irradiation particle used and are in qualitative agreement with the formula derived analytically. We propose that these statements can be tested by small angle scattering experiments.

Solute embrittlement of SiC

The energies and stresses associated with the decohesion of β-SiC in the presence of mobile Pd and Ag impurities are studied from first principles. Density functional theory calculations are parameterized with a generalized cohesive zone model and are analyzed within a thermodynamic framework that accounts for realistic boundary conditions in the presence of mobile impurities. We find that Pd impurities will embrittle SiC when Pd is in equilibrium with metallic Pd precipitates. Our thermodynamic analysis predicts that Pd embrittles SiC by substantially reducing the maximum stress of decohesion as a result of a phase transition between decohering planes involving an influx of Pd atoms. The methods presented in this work can be applied to study the thermodynamics of decohesion of SiC in other aggressive environments containing oxygen and water, for example, and yield environment dependent cohesive zone models for use in continuum approaches to study crack propagation and fracture.

Compositional and order patterning in driven alloys: the role of external-forcing characteristic lengths

Abstract This paper discusses the role played by the characteristic lengths introduced by an external forcing. Modeling and simulations show that these lengths, when large enough, can trigger patterning of the composition and of the degree of chemical order. Two practical situations are discussed: alloys subjected to irradiation and alloys subjected to sustained shearing.

Traction curves for the decohesion of covalent crystals

We study, by first principles, the energy versus separation curves for the cleavage of a family of covalent crystals with the diamond and zincblende structure. We find that there is universality in the curves for different materials which is chemistry independent but specific to the geometry of the particular cleavage plane. Since these curves do not strictly follow the universal binding energy relationship (UBER), we present a derivation of an extension to this relationship that includes non-linear force terms. This extended form of UBER allows for a flexible and practical mathematical description of decohesion curves that can be applied to the quantification of cohesive zone models.

Spontaneous formation of nanometer-scale self-organized structures in Ag-Cu alloys under irradiation

Ion-beam irradiation can be used as a processing tool to synthesize metastable materials. A particular case is the preparation of solid solutions from immiscible alloys, which have been achieved for a whole range of systems. In this process, enhanced solute concentration is obtained through the local mixing induced by each irradiation event, which if occurring at a high enough frequency, can outweigh demixing by thermal diffusion. The resulting microstructure forms in far from equilibrium conditions, and theoretical results for these kind of driven alloys have shown that novel microstructures exhibiting self-organization can develop. To test these predictions, we prepare Ag-Cu multilayered thin films that we subject to 1 MeV Kr + -ion irradiation at temperatures ranging from room temperature to 225 °C, and characterize the specimens by x-ray diffraction, TEM and STEM. We observe two different phenomena occurring at different length scales: On the one hand, regardless of the irradiation temperature, grains grow under irradiation until reaching a size limited by film thickness (~200 nm). On the other hand, the distribution of species inside the grains is greatly affected by the irradiation temperature. At intermediate temperatures, a semi-coherent decomposition is observed at a nanometer scale. This nanometer-scale decomposition phenomenon appears as an evidence of patterning, and thus confirms on the possibility of using ion-beam irradiation as a route to synthesize nanostructured materials with novel magnetic and optical properties.

Effective Interactions Approach to Phase Stability in Alloys Under Irradiation

Phase stability in alloys under irradiation is studied considering effective thermodynamic potentials. A simple kinetic model of a binary alloy with phase separation is investigated. Time evolution in the alloy results from two competing dynamics: thermal diffusion, and irradiation induced ballistic exchanges. The dynamical (steady state) phase diagram is evaluated exactly performing Kinetic Monte Carlo simulations. The solution is then com- pared to two theoretical frameworks: the effective quasi-interactions model as proposed by Vaks and Kamishenko, and the effective free energy model as proposed by Martin. New developments of these models are proposed to allow for quantitative comparisons. Both theoretical frameworks yield fairly good approximations to the dynamical phase diagram.