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

TL;DR

This study investigates whether nanocrystalline CoCrFeNi alloy thin films maintain a single-phase fcc structure, revealing that second phases can form at room temperature due to high nucleation rates in sputtered films.

Contribution

It demonstrates that phase evolution in nanocrystalline CoCrFeNi thin films can differ from bulk alloys, with second phases forming at room temperature due to high defect densities.

Findings

Second phases can form in thin films at room temperature.

Microstructure evolution is accelerated in sputtered thin films.

Comparison shows differences between thin film and bulk alloy phases.

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.