Influences of Laser Surface Alloying with Cr on Microstructural Characteristics and Hardness of Pure Ti

A high-purity Ti sheet was treated by laser surface alloying (LSA) with Cr at two different powers (100 and 200 W), with microstructural features in various laser-modified zones characterized by energy-dispersive spectrometry, electron channeling contrast imaging, and electron backscatter diffraction techniques. Hardness variation induced by the LSA was also examined and correlated with these microstructural features. Results show that at both laser powers there are two modification zones with distinct microstructural characteristics: (i) melted zone (MZ) near the surface, composed of martensitic α plates supersaturated with Cr and dense nanotwins inside them; (ii) heat-affected zone (HAZ) beneath the MZ, featured by irregular-shaped grains and substructures with varied sizes and little Cr in their interiors. Hardness measurements show that remarkable hardness increase (~ 2.5 times that of the matrix) could occur in the MZ after the LSA treatments. This can be ascribed to combined contribution from grain refinement, the presence of abundant nanotwins, and solid solution of Cr. The subgrains in the HAZ, produced by dislocation recovery, have only marginal contribution to hardness increase. The hardness of the Ti(Cr)-200W specimen is slightly lower than that of the Ti(Cr)-100W specimen, which is related to the dilution of Cr and the reduced cooling rate associated with enlarged modification zone at higher power. After comparing with laser surface treatments without alloying, it is confirmed that the LSA with Cr in the present work is much more effective in hardening the surface of pure Ti.