Microstructural characteristics and hardness of CoNiTi medium-entropy alloy coating on pure Ti substrate prepared by pulsed laser cladding

A ternary CoNiTi medium-entropy (MEA) alloy coating was successfully fabricated on pure Ti substrate by utilizing pulsed laser cladding. Phase constitutions and microstructural characteristics of the coating were analyzed by combined use of X-ray diffraction, electron channeling contrast imaging, secondary electron imaging, energy dispersive spectroscopy and electron backscatter diffraction techniques. Results show that three zones with distinct microstructures are formed after the pulsed laser cladding treatment: cladding zone (CZ), bonding zone (BZ) and heat-affected zone (HAZ). Dendritic (BCC solid-solution phase)-interdendritic (Ti2Ni type phase) structures are formed in the CZ with many nanoparticles (Ti2Co type phase) dispersed inside the interdendritic structures. The BZ consists of fine acicular grains while the HAZ below the BZ is composed of irregular-shaped bulk grains. Hardness measurements reveal that hardness of the MEA coating reaches 571 ± 46 H V, which is ∼5 times that of the substrate (114 ± 4 H V). Comprehensive analyses show that such high hardness can be attributed to the combined contributions from solid-solution hardening of the BCC phase and second-phase hardening from the Ti2Ni and Ti2Co type intermetallic compounds.