SYNTHESIS OF HIGH-ENTROPY AlNiCoFeCrTi COATING BY COLD SPRAYING

Abstract

The cold spraying (CS) process was applied to deposit coatings using the AlCoNiFeCrTi high-entropy alloy (HEA) powder. The HEA powder was produced by short-time mechanical alloying (MA) of an equiatomic mixture of components in a planetary-ball mill followed by annealing at 1200 °С and grinding of the resultant agglomerates. X-ray diffraction and microstructural analyses were used to study the phase and structural transformations at different stages of producing the AlCoNiFeCrTi powder alloy and after it was sprayed onto a steel substrate. When the powder mixture was subjected to the MA process, a metastable nanostructured bcc solid solution formed. After annealing, the phase composition of the alloy changed and consisted of an ordered bcc solid solution (B2 phase), intermetallic s-phase (FeCr), and titanium carbide TiC. After grinding in a planetary-ball mill for 1 h, the ordered B2 phase turned into a disordered nanostructured bcc solid solution. The titanium carbide and s-phase remained in the alloy, but particles of the s-phase significantly refined and partially dissolved in the bcc solid solution. After deposition, the phase composition and nanostructured state of the starting powder alloy remained unchanged and the cold-sprayed coating consisted of a bcc solid solution, an intermetallic s-phase, and TiC carbide. The average coating thickness was 405 μm and Vickers microhardness HV was 10.0 ± 0.3 GPa. The high hardness of the coating was due to hardening effects: solid-solution and nanostructured hardening, hardening by inclusions of the intermetallic and carbide phases, and strain hardening during spraying under severe plastic deformation at supersonic speeds (~10⁵–10⁷ sec^–1) at low temperature. The HEA coating showed good adhesion to the substrate and low porosity (<1%).