Ti-based amorphous alloys have received significant attention owing to their low density, high specific strength, superior corrosion resistance, and relatively low fabrication cost, which render them promising for widespread application in fields such as national defense, aerospace, sports, and medicine. However, they present some mechanical performance issues such as low plasticity. Studies have shown that introducing heterogeneous structures into metallic materials can improve their mechanical properties. Performing laser remelting on Ti-based amorphous alloys can create gradient heterogeneous structures in their depth direction, thus improving their mechanical performance. Currently, studies regarding the introduction of heterogeneous structures into amorphous alloys via laser remelting are limited. In this study, by performing microscopic analyses and numerical simulations of temperature fields, we investigate the microstructure evolution of laser-re-melted Ti-based amorphous alloys and its effect on their mechanical properties. The aim of this study is to provide experimental and theoretical support for understanding the mechanism of laser-induced heterogeneous structures and its effect on the properties of amorphous alloys.

We prepare Ti-based amorphous alloys via vacuum suction casting, followed by laser remelting. First, the microstructure of the laser-re-melted Ti-based amorphous alloys is analyzed using micro-area X-ray diffraction (XRD) and scanning electron microscope (SEM). Subsequently, a numerical simulation of the temperature field during laser remelting is conducted to analyze the thermal histories of different regions after laser remelting. Microstructure analyses and temperature-field simulations are performed to elucidate the effect of laser remelting on the structure of Ti-based amorphous alloys. Subsequently, hardness testing is performed along the depth direction after laser remelting to examine the trend of mechanical-property changes induced by the heterogeneous structure. Finally, various processes are employed for the laser remelting of Ti-based amorphous alloys, and the effects of heterogeneous structure variations on the plasticity of Ti-based amorphous alloys are revealed via uniaxial tensile tests and microstructure analyses.

Through laser remelting, a gradient heterogeneous structure comprising a melt-pool amorphous structure, heat-affected zone amorphous composite structure, and substrate amorphous structure is formed in the depth direction of the Ti-based amorphous alloy (Figs. 4 and 5). Microstructure analysis combined with a numerical simulation of the temperature field reveals the thermal history for different regions, thus elucidating the formation mechanism of the amorphous structure with a free-volume gradient distribution within the melt pool. Additionally, the effects of different residence time, heating rates, and cooling rates within the heat-affected zone on the nucleation and growth behavior of crystalline phases are analyzed. The microhardness of the laser-re-melted Ti-based amorphous alloy clarifies the effects of the pattern and mechanism of the gradient heterogeneous structure on the microhardness. Furthermore, appropriate laser-processing parameters can induce the proliferation of shear bands, thereby enhancing the plasticity of Ti-based amorphous alloys.

The effects of laser remelting on the microstructure and plasticity of Ti-based amorphous alloys are investigated via finite-element simulation, microstructural analysis, and mechanical-property analysis.

1) Finite-element simulation is performed to analyze the temperature distribution during laser remelting of Ti-based amorphous alloys. The samples undergoes different thermal histories at various depths, thus resulting in distinct regions: a melt pool, heat-affected zone, and substrate, each exhibiting different structural evolution behaviors.

2) The melt pool exhibits a high cooling rate, thus resulting in an amorphous structure. However, as the depth of the melt pool increases, the cooling rate decreases gradually, thus resulting in a gradient amorphous structure with reduced free-volume content and increased hardness. By contrast, the heat-affected zone shows reduced residence time and peak temperature with increasing depth, thus resulting in a decrease in the gradient of the crystalline-phase size but an increase in quantity. The presence of crystalline grains hinders the single propagation of shear bands, thereby resulting in the highest hardness in this region. The amorphous structure of the substrate area remains unchanged.

3) The laser remelting of Ti-based amorphous alloys results in a gradient heterostructure with various structural and performance gradients. The synergistic effects of different gradient structures are expected to suppress the expansion of single shear bands, thereby enhancing the plastic-deformation capability of amorphous alloys.

4) Different laser-processing techniques will alter the microstructure and dimensions of re-melted samples in various regions, thus inducing the corresponding changes in the gradient heterostructures. Plastic-deformation analysis based on different gradient heterostructures indicates that a rational gradient heterostructure can improve plasticity, whereas an irrational gradient structure may worsen it. Future studies shall focus on optimizing laser processing to control gradient heterostructures, thereby further enhancing material plasticity.