Laser-clad Al-Ti-Zr and Cr-Ti-Zr coatings on Zr alloy: composition-induced microstructural and hardness differences

In this study, Al-Ti-Zr and Cr-Ti-Zr metallic coatings with thichness of 250-280 mu m were fabricated successfully on Zr alloy using pulsed laser cladding. By jointly employing multiple characterization techniques, microstructural characteristics of the coatings were probed with their formation reasons explored. The Al-Ti-Zr coating consists of a single BCC solid-solution phase (average size 8.6 +/- 6.3 mu m) with hardness of 405.5 +/- 16.6 HV that is more than twice that of the substrate (198.7 +/- 2.0 HV). Such marked hardening is mainly related to the contribution from the solid-solution strengthening produced by Al and Ti. The Cr-Ti-Zr coating consists of cellular BCC phase (average size 0.9 +/- 0.4 mu m) and C14-type Cr-2(Ti,Zr) Laves phase with nanoscale lamellar structures. Its average hardness is 584.8 +/- 33.6 HV, about three times of that of the substrate, which can be ascribed to synergistic effects of solid-solution, grain refinement and second phase strengthening. The formation of the single BCC solid-solution phase in the Al-Ti-Zr coating is mainly due to the relatively small differences in atomic size and electronegativity between Al, Ti and Zr. For the Cr-Ti-Zr coating, however, due to large atomic size and electronegativity differences between Cr and Zr (as well as Cr and Ti), Cr-2(Ti,Zr) Laves phase was formed in addition to the BCC phase.