Effect of Laser Energy Density on Phase Evolution in Laser Surface Melted As-Cast Zr-Cu Alloy

The study investigates the role of different laser energy densities on the micro-structural evolution in the laser surface re-melted as-cast Zr-8 wt% Cu alloy. In the current study, different values of laser energy density was achieved by varying the scan speeds. The micro-structural investigation of the laser re-melted regions were carried out using Optical microscope, FESEM (Field Emission Scanning Electron Microscope) in back-scattered mode and TEM (Transmission Electron Microscope). The compositional evolution in the laser re-melted regions was carried out using EDS (Energy Dispersive Detector) attached to FESEM and TEM. The mechanical property of the re-melted regions was estimated using nano-indentation technique. The micro-structural examination showed that the laser re-melting of as-cast dendritic morphology leads to the development of < 200 nm Zr and Cu rich regions. The TEM-SAD analysis revealed the formation of amorphous Zr-Cu metallic glass in the laser re-melted regions. It was also observed that the fraction of Cu in the amorphous phase reduces with decrease in laser energy density which is attributed to enhanced solute trapping of Cu in beta-Zr phase at higher scan speeds which was shown using Scheil solute trapping model of Thermocalc. The laser re-melted regions developed at high laser energy density shows superior values of nano-hardness and elastic modulus as compared to regions developed at lower laser energy density. This is attributed to higher concentration of Cu in Zr-Cu amorphous phase in laser re-melted regions developed at high laser energy density.