Criteria for the amorphisation of intermetallic compounds under electron irradiation

MeV electron irradiation can stimulate solid-state amorphization in some intermetallic compounds. The irradiation induced amorphization phenomenon facilitates a clearer observation of the composite microstructure of the compounds. MeV electron irradiation is applied to a composite structure containing intermetallic compound “A,” which undergoes solid-state amorphization and crystalline phase “B,” which does not undergo amorphization. The composite structure transforms into a mixture of amorphous and crystalline phases by the irradiation. The distribution of A and B in the structure can hence be easily determined. High-voltage electron microscopy (HVEM) offers a unique microstructure observation technique that uses the difference between the sensitivities of compounds to undergo solid-state amorphization when MeV electron irradiation is applied to them.

Electron-irradiation-induced solid-state amorphization (SSA) in C16-Zr₂Ni was investigated by molecular dynamics (MD) simulations by focusing on the atomic-scale mechanism. C16-Zr₂Ni is a typical intermetallic compound that undergoes SSA under MeV electron irradiation. Electron-irradiation-induced SSA was achieved through the accumulation of lattice defects by MD calculations using an embedded-atom (EAM) potential. The Frenkel pairs introduced in the intermetallic compounds could not be removed because of the difficulties in converting the defective crystal back into the original crystal through thermal recovery of the lattice defects; this caused chemical and topological disorder in the crystal. Thermal relaxation (thermal recovery of lattice defects) increased topological and chemical disorder in the local areas of the crystal. The increase in the disordered local area in turn induced SSA. Changes caused by thermal relaxation of the lattice defects in the atomic configuration induced SSA in intermetallic compounds.

By the interface migration-charge transfer (IMCT) atomistic model the nucleation of amorphous or crystalline phases is modelled in binary compound films, both metallic and non-metallic, where dense collision cascades form upon irradiation. Non-equilibrium compositional and electronic density profiles develop at the interface between each cascade and the surrounding crystalline matrix due to interface enrichment of one of the film constituents. By local charge transfer reactions (CTR), system relaxation is mimicked via formation of dimers of an effective compound. We calculate the energy cost to produce one such dimer, the difference of formation enthalpy between each effective compound and the corresponding initial compound and the local strain associated to a CTR for representative sets of metallic and non-metallic ion bombarded compounds. Qualitative differences are found in the parameter trends between the two groups of compounds amorphized and respectively, crystalline under ion beam irradiation.

By the segregation-charge transfer (SCT) atomistic model, nucleation of crystalline or amorphous phases is modelled in binary alloy films, irradiated under conditions suitable to the formation of dense collision cascades. Off-equilibrium compositional and electronic density profiles develop at the interface between each cascade and the surrounding unperturbed crystalline matrix, as a consequence of interface enrichment of one of film constituents. Local charge transfer reactions (CTR), each involving a pair of dissimilar atoms of the initial alloy, are introduced to mimic system relaxation via formation of dimers of an effective compound. The energy cost to produce one such dimer, the difference of formation enthalpy between each effective compound and the corresponding initial alloy and the local strain associated with a CTR are calculated for a set of seventeen ion-bombarded alloys, out of which ten are vitrified and seven remain crystalline. Qualitative differences are found in the parameter trends between the two groups of alloys.

It has long been observed that a crystalline-to-amorphous (c-a) transition occurs in silicon carbide (SiC) irradiated at low temperature. However, the microscopic mechanisms leading to the transition are not well understood. We report in this paper a molecular dynamics (MD) simulation of low-energy (100 eV) recoil accumulation at cryogenic temperature (20 K), up to ≈1 dpa, in which the irradiated computational sample becomes amorphous and is subsequently annealed at high temperature (2320 K). The simulation suggests that, at least for low-mass impinging particles, provided that no direct impact amorphization (DIA) takes place, the driving force for the c-a transition in this material is the accumulation of Frenkel pairs up to a critical concentration ($\text{≈1.9×10}{}^{22}\text{cm}{}^{\mathrm{-3}}$). The role of antisites in the process is negligible. In fact, antisite formation during the annealing could be the bottleneck for complete recovery. A simple and intuitive analytical model based on the concepts of recombination barriers and interstitial migration is also proposed, to describe the temperature dependence of the critical dose for amorphization.

Phase nucleation in irradiated metallic alloys is studied by the atomistic Segregation-Charge Transfer (SCT) model, taking into account that when bombardment conditions favour the development of dense collision cascades the system explores kinetic and thermodynamic regions conventionally not accessible. Locally non-equilibrium compositional and charge density profiles evolve at the interface between a cascade and the crystalline matrix, driven by the interface enrichment of one of the film constituents. Charge relaxation to (meta)stable equilibrium is simulated by Charge Transfer Reactions (CTR), each involving a couple of dissimilar atoms of the initial alloy. Both the amount of energy needed in a single CTR and the local strain undergone by the target as a consequence of ion formation are calculated. Considering a set of nineteen alloys which are reported to be amorphized and seventeen ones which remain crystalline under ion bombardment, for both structure stability parameters threshold values are found, which allow to separate crystalline from vitrified alloys.