The order-disorder transformation and microstructure evolution in nickel-aluminium intermetallics after heavy-ion irradiation

Nickel-aluminium intermetallics constitute interesting systems for the study of irradiation-induced phase transformations (order → disorder, amorphization) and production of defects. Irradiation of ordered intermetallics may induce disorder or amorphization depending on the projectile characteristics, irradiation conditions and chemical and physical properties of the target. The behavior of the various intermetallic compounds of the nickel-aluminium system with respect to electron and ion irradiation has been extensively studied in the recent past. In more recent years, however, significant interest has driven the motivation for modelling these processes using molecular dynamics computer simulations. These results have demonstrated the feasibility of simulating cascade damage in pure metals and intermetallics and led to the partial corroboration of phenomenological data which has been compiled from a set of experiments. In this frame, and in spite of the numerous efforts previously done, little information has been collected in order to perform a comparative study on the behavior of NiAl and Ni3Al under heavy-ion irradiation in a consistent approach. The present study reports on: the synthesis of stoichiometric and ordered intermetallic thin films as proper irradiation media, which has evolved into a subject of substantial complexity. It is remarkable, nevertheless, that very few systems have been studied regarding the microstructure and growth modes of intermetallics on metallic substrates. In this context, the granular-heteroepitaxial inverse Nishiyama-Wassermann (211)[211]B2∥ (110)[110]fccand the heteroepitaxial (110)[110]L12∥ (110)[110]fcc relationships have been lately demonstrated for NiAl and Ni3Al grown onto nickel single-crystals at high temperature, respectively. Inherent to the deposition technique, and at the low substrate temperature regime, is the formation of defect structures and polycrystalline materials which are not the most convenient for post-irradiation microstructure assessment. Therefore, special attention has been given to bulk single-crystals which have been irradiated and prepared in cross-section for transmission electron microscopy observations; the numerical link between previous molecular dynamics results and current transmission electron microscopy image simulation, in order to establish under cascade damage conditions which type of defects are created and how do NiAl and Ni3Al intermetallics compare regarding the effect of primary ion energy and damage accumulation. The former has been seen to undergo an amorphous transformation upon a 15 keV-nickel recoil, which has not been registered for the latter. These results have been valuable in order to understand structural differences under cascade damage production and to validate molecular dynamics computer simulations from a microscopic point of view; the microstructure investigation using transmission electron microscopy of heavy-ion irradiated NiAl and Ni3Al samples, thus establishing a set of physical parameters (defect visibility, disordered zone and defect densities, and respective mean sizes) to be directly compared in a self-consistent approach to the results obtained through image simulation of damaged intermetallics. A direct crystalline-to-amorphous transformation has been seen not to occur either at liquid nitrogen or room temperature for the former intermetallic, as predicted from molecular dynamics results. Instead, a defect accumulation driven mechanism should be active. Microstructural evolution under heavy-ion irradiation must be thought on the basis of the disordered zone to defect cluster densities ratio, ρ d /ρ c , in order to compare both intermetallics in a self-consistent approach due to different fluences (or doses), and primary ion energies. Being ρ d /ρ c an estimator of irradiation-induced disordering efficiency, it is now arguable that at the high energy regime this ratio decreases with increasing primary ion energy. Aronin's exponential law S = S0exp(–εΦ), which is characterized by the initial long-range order value S0, the dose in dpa Φ, and the disordering efficiency ε, does not explicit any knowledge on the nature of ε. This parameter has been measured for 6 MeV-nickel primary ions in Ni3Al, yielding a value in the range of 10.0±0.7 /dpa. Further research must be undertaken in order to establish an experimental value for equivalent irradiation conditions in NiAl. The disordering efficiency ε is certainly a function of the disordered zone to defect cluster densities ratio, ε = f (ρ d /ρ c ). Further validation of such a relation would be of paramount importance in order to understand irradiation-induced disordering at high primary ion energies and to establish the fact that a less effective disordering process takes place in a regime of dominant subcascade formation.