Phase constitution, microstructure and properties of pulsed laser-clad ternary CrNiTi medium-entropy alloy coating on pure titanium

Abstract

A ternary CrNiTi medium-entropy alloy (MEA) coating with excellent surface performance (hardness and wear resistance) was successfully prepared on pure Ti sheet by pulsed laser cladding. Microstructural characteristics of the MEA coating were probed by combined use of multiple characterization techniques and reasons for the formation mechanisms of various phases in the coating were well explored. Results show that fine cellular grains are formed in the MEA coating during the ultrafast non-equilibrium solidification process induced by pulsed laser cladding. These grains have an average size less than 1 μm and correspond to a BCC solid-solution phase. There appears irregular-shaped Cr2Ti Laves phase (C14-type) inside most of the cellular grains, while intergranular structures are demonstrated to be NiTi intermetallics. Hardness tests reveal that the CrNiTi MEA coating has a hardness of 940 ± 35 HV which is ~8 times that of the pure Ti substrate (119 ± 9 HV). Also compared to the pure Ti substrate, a much lower wear rate is noted for the coating demonstrating greatly improved wear resistance. Comprehensive analyses show that the excellent surface performance of the CrNiTi MEA coating can be ascribed to combined contributions from the solid-solution hardening and grain refinement hardening of the BCC phase, as well as second phase hardening produced by Cr2Ti Laves phase and NiTi intermetallics.