Phase transformations/separation via spinodal decomposition enhancing the structure & properties of MA+CS Al 0.1-0.5(Mn) CoCrCuFeNi HEA coatings on Mg vacuum annealed at 600oC

Abstract

Novel HEA coatings in the Al0.1-0.5 (Mn)CoCrCuFeNi multi-material systems on Mg substrate were prepared from mechanically alloyed (MA) HEA powder feedstocks and by cold spray (CS) coating deposition at 650, 750 & 850oC, respectively, followed by vacuum annealing at 600oC under 0.05 bar pressure for 1h. The effect of not only vacuum annealing on microstructure and quality of HEA coatings (porosity, surface passive films, hardness, surface roughness) on Mg substrate but also that of CS process gas (N2) temperature and the composition of HEA coatings on annealed microstructures and properties were carried out by macro photography, LM, SEM+EDS, XRD, XPS, porosity, roughness measurements and microhardness testing. First time direct SEM-SE imaging of annealed HEA coatings/Mg-substrate interface confirmed its rough nature leading to good bonding between them. Combined LM, SEM BSE+SE imaging with EDAX analyses and XRD work showed that the microstructure of the 600oC vacuum annealed HEA coatings had a multiphase mixture of not only solid solutions of d. o. A1(FCC1), d.o. A2 (BCC), d. o. B2 (BCC), and Cu-rich regions (FCC2), but also I. M. Cs of σ, o. L12 (FCC) phase in Al-HEAs and L1o (tetragonal) in Mn-HEA. Signs of spinodal decomposition especially in Al 0.5 HEA composition occurring at 600oC annealing via phase separation of A2/B2 phases were observed with SEM+XRD investigation due to its lowest ΔHmix and convenient VEC values among other studied HEA coating compositions. SEM and XRD work confirmed that the amount of A1 phase was increasing while those for A2/B2 phases were decreasing with 600oC annealing in HEA coatings. The reverse trend was observed in samples cold sprayed at 850oC compared to 650oC samples. The lattice parameter increases in the phases of annealed samples compared to CSed ones indicated lattice expansion and stress relief with annealing. Al and Mn additions to HEA alloys resulted in an increase in lattice parameter values leading to lattice distortion and lattice strain. However, in Al 0.5-HEA composition, ordering reaction during annealing lead to lattice contraction when CS process gas temperature (N2) was 850oC. Al and Cu additions are observed during annealing processing to promote L12 formation in Al-HEAs and L1o formation in Mn-HEA and to suppress σ-phase formation leading to a better combination of mechanical properties. Surface oxide film content of the annealed HEA coatings didn’t increase compared with that of the MA+CS HEA feedstock powder but there were changes in its chemical composition: α and γ-Al oxides, Al(OH)3, CoO, Co3O4, Co(OH)2 formations were observed together with other oxides as seen in CS HEA coatings, However, a decrease in the Cr and Ni oxides were observed in the surface films. Low porosity, dense HEA coatings with annealing were produced. Porosity and microhardness of annealed HEA coatings were lower than those observed in CSed samples whereas surface roughness values were more or less the same. Highest porosity values were observed in Mn-HEA coating samples.

Keywords: Vacuum annealing, HEA coatings, characterization, phase stability