Simultaneous flattening and hardening of laser shock peened Zr-based metallic glass surface by nanosecond laser irradiation in a flowing nitrogen atmosphere

Abstract

Laser shock peening (LSP) is an ideal surface modification technique which has been extensively applied to improve the room-temperature plasticity of metallic glasses (MGs) by generating extra-deep compressive residual stress region and increasing the free volume content. Nevertheless, after LSP treatment, the surface quality and hardness of MGs are usually impaired simultaneously, which hinders their applications as structural and functional materials. In the current study, an attempt was made to achieve simultaneous flattening and hardening of LSP treated (LSPed) Zr-based MG by nanosecond laser irradiation in a flowing nitrogen atmosphere. The effects of laser power level on the macro- and micro-morphology, chemical composition, surface hardness, and surface roughness (Sa) of the laser-irradiated LSPed MG were comprehensively investigated. It was found that the minor surface defects of the LSPed MGs could be effectively reduced by nanosecond laser irradiation under appropriate laser irradiation parameters, and accordingly, the surface quality was improved (with a maximum reduction in surface roughness (Sa) by 86.3%). Furthermore, the nanoindentation test results indicated that nanosecond laser irradiation at a relatively high laser power of 15.6 W resulted in an impressive improvement (173.9%) in surface hardness of the LSPed MG, which could be attributed to the introduction of hard nitrided phases mainly consisting of Zr3N4. This study provides an insightful understanding of the microstructure changes and surface characteristics of the LSPed MGs after nanosecond laser irradiation in a flowing nitrogen atmosphere, which paves the way for improving the applicability of MGs as structural and functional materials.