Improved plasticity and fracture toughness in metallic glasses via surface crystallization

Mechanical crystallization was induced in the monolithic bulk metallic glasses by surface mechanical attrition treatment (SMAT) to create the isolated crystallite islands in the top surface layer. Inside the isolated crystallite islands, microstructure consists of the crystallites with a gradient grain size evolution and the residual amorphous phase. Moreover, isolated crystallite islands, acting as the obstacles to restrict the highly localized deformation of shear bands/cracks, effectively limit the shear bands extension, suppress the shear bands opening, and avoid the cracks developing, which significantly enhance the overall plasticity and fracture toughness. They were suggested by the secondary shear bands in the glassy matrix and fine-shearing together with micro-cracking inside the isolated crystallite islands. Finally, the improved plasticity and fracture toughness were systematically discussed. Based on the current results, surface crystallization is proposed to optimize the mechanical properties of metallic glasses.