Nanocrystalline equiatomic CoCrFeNi alloy thin films: Are they single phase fcc?

Abstract

The bulk quaternary equiatomic CoCrFeNi alloy is studied extensively in literature. Under experimental conditions, it shows a single-phase fcc structure and its physical and mechanical properties are similar to those of the quinary equiatomic CoCrFeMnNi alloy. Many studies in literature have focused on the mechanical properties of bulk nanocrystalline high entropy alloys or compositionally complex alloys, and their microstructure evolution upon annealing. The thin film processing route offers an excellent alternative to form nanocrystalline alloys. Due to the high nucleation rate and high density of defects in thin films synthesized by sputtering, the kinetics of microstructure evolution is often accelerated compared to those taking place in the bulk. Here, thin films are used to study the phase evolution in nanocrystalline CoCrFeNi deposited on Si/SiO 2 and c-sapphire substrates by magnetron cosputtering from elemental sources. The phases and microstructure of the films are discussed in comparison to the bulk alloy. The main conclusion is that second phases can form even at room temperature provided there are sufficient nucleation sites.

Exploring stability of a nanoscale complex solid solution thin film by in situ heating transmission electron microscopy

“…The thermal stability of CrMnFeCoNi thin films was reported by several authors. 9,12,16 However, these investigations covered films with a thickness of more than >100 nm. Thus, our aim is to compare a thin film with an average thickness of ~10 nm with those having thicknesses more than 100 nm to better understand the thermal stability of nanoscale CSS.…”

“…Our main aim is to understand how such a nanoscale thin film (~10 nm) behaves under thermal load compared with thicker (> 100 nm) CSS thin films, which have been already reported. 9,12,16 Moreover, the annealing treatment and the additional local irradiation of the nanoscale thin film by the electron beam (e-beam) of the TEM can lead to interesting effects and might change (e.g., its morphology and composition). The limited sampling volume of CSS islands leads to noisy EDS and EELS spectra, therefore multivariate statistical analysis 17 was carried out to extract quantitative information on elemental composition.…”

Thermal Stability of Medium- and High-Entropy Alloys of 3d-Transition Metals

“…Nanocrystalline CoCrFeNi thin films deposited on Si/SiO2 and c-sapphire substrates by magnetron co-sputtering have recently been investigated 35 . In addition to the FCC phase, the films contained an additional σ phase even at room temperature and in as-deposited films due to a large number of nucleation sites and accelerated kinetics as compared to bulk systems.…”

Catalytically active and corrosion resistant cobalt-based thin films

“…Three different zones have been identified: zone I where Ts<<Tm and the formation of narrow grains due to low diffusion, zone II where Ts and Tm are closer to each other and large grains can form; and an intermediate zone, zone T in between 1 and 2 where Ts<Tm and competitive growth can occur. For CoCrFeNi thin films, columnar growth is recurrent in literature [45][46][47][48]61,62,66], and some have identified zone T at low temperatures (T<500°C) [45,47] and zone II for higher temperatures (T=900°C) [45].…”

“…Altering the formation of these initial layers can be done by several means, the most common ones are based on how to increase or decrease surface diffusion of adatoms. Increasing the substrate temperature usually increases the surface diffusion of adatoms and they can form larger clusters which results into larger grains, for CoCrFeNi commonly from 50 nm to 100 nm in width [45,61,62]. Crystal orientation and texture also influences the morphology.…”