Diffusionless FCC to BCC phase transformation in CoCrCuFeNi MPEA thin films

Arfaoui, Mohamed and Kovács-Kis, Viktória and Radnóczi, György (2021) Diffusionless FCC to BCC phase transformation in CoCrCuFeNi MPEA thin films. JOURNAL OF ALLOYS AND COMPOUNDS, 863. pp. 1-10. ISSN 0925-8388

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Abstract

The aim of the present work is growing CoCrCuFeNi HEA thin films of 40–50 nm thickness and annealing them in-situ in the electron microscope for a better understanding and controlling the early stages of phase and compositional instability of as deposited films at temperatures up to 550 °C. Carrying out in-situ electron microscopic experiments of isochronal annealing at temperatures of 450 °C and 550 °C we could follow the structural changes in thin (40–50 nm thick) CoCrCuFeNi MPEA films. We observed that at temperatures 450 and 550 °C the first change in the structure is the martensitic transformation of the FCC to BCC phase. We have shown by EDS mapping of the transformed area that no compositional changes occur during this process and, specific for martensitic transformation orientation, relationships between the two phases are present. The changes occur after about 20 min annealing time at these temperatures. Longer anneals at the same temperatures lead to separation of components. Above 40 min annealing time at 450 °C voids appear in the film, Cr will be partly bonded to oxygen (impurity in the vacuum system) in the form of a thin surface oxide layer. The other components tend to form binary like compositions of Cu-Ni and Fe–Co pairs. We also estimated the diffusion coefficient at 450 °C in this alloy to be around ~6 × 10−19 m2/s.

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