Microstructural characteristics and properties of CoCrFeNiNbx high-entropy alloy coatings on pure titanium substrate by pulsed laser cladding

CoCrFeNiNbx (x = 0 or 1 at.%) high-entropy alloy (HEA) coatings with good metallurgical bonding were successfully prepared on surfaces of pure titanium sheets by pulsed laser cladding. Phase constitutions and microstructural characteristics of the HEA coatings were characterized and analyzed by combined use of X-ray diffraction, electron channeling contrast imaging, energy dispersive spectroscopy and electron backscatter diffraction techniques. Results show that the HEA coating without Nb is consisted of BCC solid-solution phase with equiaxed bulk grain morphology and Cr2Ti Laves phase (C14-type hexagonal structure) with fine interdendritic lamellar morphology. After adding 1 at.% Nb, in addition to the BCC solid-solution phase and the Cr2Ti Laves phase, Cr2Nb Laves phase with C15-type cubic structure also appears in the interdendritic region in the HEA coating. Hardness tests reveal that the CoCrFeNiNbx HEA coatings are significantly harder than the pure titanium substrate (similar to 122 HV). The hardness value of the HEA coating with 1 at.% Nb reaches 1008 HV, which is similar to 8.3 times that of the substrate and considerably higher than that of bulk HEAs with similar compositions. Such high hardness can be attributed to the combined contribution from solid-solution hardening, and hardening from grain refinement and Laves phases with fine lamellar morphologies.