Effect of yttrium on microstructure and mechanical properties of laser clad coatings reinforced by in situ synthesized TiB and TiC

Abstract Titanium-based composite coatings with and without Y particles were deposited by laser cladding on Ti6Al4V substrates. Solidification microstructure, phase constituents and distribution of the reinforcements with different morphologies, were investigated by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and electron probe micro analyzer (EPMA). In addition, the effects of the addition of Y on mechanical properties (in terms of microhardness and the cracking susceptibility) were also highlighted. The results showed that the coatings were composed of α-Ti cellular dendrites, coarse needle-shaped TiB phase and an eutectic in which a large number of needle-shaped TiB whiskers and a few equiaxial TiC particles were uniformly embedded. Y was not stable and was transformed into Y 2 O 3 during laser cladding. The addition of Y could refine the microstructure of the coating by hastening the spheroidization of primary phase structure. Moreover, it could also decrease the activity of carbon and prevent solute atoms from traversing the interface and moving into primary phase structure, namely, increase the fraction volume of TiC in the coating. All of there factors made the cracking susceptibility of the coating containing Y reduced on the premise that microhardness of the coating was increased. Microhardness of the coating without Y ranged from HV 875.6 to HV 659.8, the average microhardness was about HV 747.9. For the coating with Y, microhardness changed from HV 876.5 to HV 741.5 and the average michardness was about HV 795.3. Fracture toughness of the upper, middle, bottom and interface of the coating without Y were 6.33, 8.91, 11.94 and 11.93 MPa·m 1/2 . Fracture toughness of the similar positions of the coating with Y were 8.58, 12.93, 13.81, 17.11 MPa·m 1/2 , respectively. The coating with Y presented higher microhardness and fracture toughness in comparison with that without Y. Obviously, the addition of Y had a very positive effect on the microstructure and mechanical properties of the coatings.