[Paper Review] Structure and hardness of in situ synthesized nano-oxide strengthened CoCrFeNi high entropy alloy thin films

Subin Lee, D. Chatain+2 more|arXiv (Cornell University)|Feb 23, 2021

High Entropy Alloys StudiesEngineering47 references22 citations

TL;DR

This study demonstrates in situ formation of Cr₂O₃ nano-oxides in CoCrFeNi high-entropy alloy thin films via internal oxidation during annealing at 1273 K. The uniformly dispersed, incoherent Cr₂O₃ particles (1.5 vol.%, avg. radius 12.7 nm) induce Orowan-type dislocation bowing, increasing film hardness by 14% compared to the particle-free counterpart, confirming a viable route to high-strength HEAs through oxide dispersion strengthening in thin films.

ABSTRACT

In this study, we report on face-centered cubic structured CoCrFeNi high-entropy alloy thin films with finely dispersed nano-oxide particles which are formed by internal oxidation. Analytical scanning transmission electron microscopy imaging found that the particles are Cr2O3. The oxide particles contribute to the hardening of the film increasing its hardness by 14% compared to that of the film without precipitates, through the Orowan-type strengthening mechanism. Our novel approach paves the way to design medium- and high-entropy alloys with high strength by making use of oxide phases.

Motivation & Objective

• To develop a novel method for achieving nano-oxide dispersion strengthening in CoCrFeNi high-entropy alloy thin films using in situ internal oxidation.
• To investigate the microstructure and composition of the formed oxide particles and their distribution within the HEA matrix.
• To quantify the contribution of the oxide particles to the mechanical strengthening of the thin film via nanoindentation and advanced electron microscopy.
• To identify the dominant strengthening mechanism, particularly Orowan-type dislocation bowing, and evaluate its effectiveness under non-ideal particle size, shape, and distribution conditions.
• To establish a scalable thin-film-based approach for designing high-strength HEAs with enhanced hardness and potential ductility through controlled oxide phase formation.

Proposed method

• Ultrafine-grained CoCrFeNi thin films (500 nm thick) were deposited via magnetron co-sputtering on sapphire substrates under ultra-high vacuum conditions.
• Post-deposition annealing at 1273 K for 1 hour under 10⁻⁴ Pa vacuum induced internal oxidation, leading to in situ formation of Cr₂O₃ nanoparticles via rapid oxygen diffusion through columnar grain structures.
• High-resolution and analytical scanning transmission electron microscopy (STEM) with HAADF imaging, EDS, and EELS were used to characterize the crystal structure, composition, and morphology of the oxide particles.
• Nanoindentation was performed with a maximum depth of 50 nm (10% of film thickness) to minimize substrate effects and assess hardness enhancement.
• The Orowan strengthening model was applied using the Ashby-Orowan equation to estimate theoretical strength contributions, incorporating parameters such as shear modulus (84 GPa), Poisson’s ratio (0.28), Schmid factor (0.272), particle size (x = 25.4 nm average diameter), and volume fraction (1.5%).
• Stereological correction was applied to estimate the true volume fraction from projected areal fraction in STEM-HAADF images, assuming a 50 nm TEM sample thickness.

Experimental results

Research questions

• RQ1 Can internal oxidation in CoCrFeNi high-entropy alloy thin films lead to the formation of uniformly dispersed, coherent or incoherent Cr₂O₃ nanoparticles?
• RQ2 What is the dominant strengthening mechanism responsible for the observed hardness increase in the oxide-containing thin film?
• RQ3 How does the actual hardness enhancement compare to the theoretical prediction from the Orowan strengthening model under non-ideal particle size and distribution conditions?
• RQ4 To what extent do indentation size effects and shallow indentation depth influence the measured hardness and the effectiveness of the Orowan mechanism?
• RQ5 Can the in situ formation of nano-oxides in thin films serve as a scalable and effective strategy for designing high-strength HEAs with potential for high ductility?

Key findings

• Annealing the CoCrFeNi thin film at 1273 K for 1 hour under vacuum resulted in the formation of a large-grained (hundreds of micrometers), (111)-textured film with homogeneously distributed Cr₂O₃ nanoparticles.
• STEM and EDS analysis confirmed the oxide particles to be Cr₂O₃, with an average radius of 12.7 ± 7.0 nm and a volume fraction of 1.5%.
• The oxide-containing film exhibited a 14% higher hardness (measured via nanoindentation) compared to the particle-free film, indicating effective strengthening.
• The Orowan-type dislocation bowing mechanism was identified as the dominant strengthening mechanism due to the incoherent, hard (30 GPa) Cr₂O₃ particles that resist dislocation penetration.
• Theoretical Orowan strengthening prediction yielded a 240 MPa increase in yield strength (720 MPa in hardness), but the actual hardness increase was 500 MPa, suggesting partial deviation from ideal model assumptions.
• Discrepancies between theory and experiment are attributed to non-uniform particle size (2–30 nm), non-spherical shapes, and indentation size effects due to shallow indentation depth (50 nm), which may reduce Orowan effectiveness by enabling dislocation penetration through small particles.