A novel surface technique, ultrasonic nano-crystal surface modification (UNSM), was used to process BMGs (Bulk Metallic Glasses) to improve the fracture strength and strain. It has been found that the fracture strength and strain are increased by 14.6% and 15.3% respectively after UNSM. In addition, micro-defects (voids and cracks) are found at the surface layer of UNSM-treated specimen after fracture, while there are fewer micro-defects observed in the untreated sample. We attribute the improvement of fracture stress and strain to two mechanisms. First, the excess free volume generated in BMGs by UNSM helps the nucleation of shear bands and the redistribution of the localized deformation. Second, the compressive residual stress generate by UNSM can slow down crack propagation. Both mechanisms lead to the improvement of BMGs fracture strength and strain.

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