Microstructure evolution in Ti64 subjected to laser-assisted ultrasonic nanocrystal surface modification

Surface severe plastic deformation (SSPD) can significantly improve the mechanical properties of metallic components by inducing surface nanocrystallization and beneficial compressive residual stresses. The effectiveness of the SSPD processes is significantly dependent on the plasticity of the target metals. Here, we report an innovative surface thermomechanical process called laser-assisted ultrasonic nanocrystal surface modification (LA-UNSM) that integrates localized laser heating with high strain rate plastic deformation. The laser beam locally heats the target metal and increases the local plasticity, making the SSPD treatment more effective. After LA-UNSM, a microstructure featuring a nanocrystalline layer embedded with nanoscale precipitates was achieved in Ti64, resulting in an unprecedented 75.2% increase in hardness. After LA-UNSM processing, a 25-μm severe plastic deformation layer was produced that was 2.5 times thicker than that of the room-temperature UNSM-processed material. The grains at the top surface were refined down to 37 nm, indicating a similar degree of nanocrystallization to that produced by UNSM at room temperature. Nanoscale precipitate particles with diameters in the range of 5-21 nm were non-uniformly distributed in the nanocrystalline surface layer. These precipitates were produced through laser-assisted dynamic precipitation. The extremely high surface strength obtained for the Ti64 was attributed to the composite microstructure featured by nanoscale grains embedded with nanoscale precipitates and the work-hardening.