The Effect of Electroplating Nickel on the Mechanical Properties of Brittle Mg-Based Bulk Metallic Glasses

Abstract

Magnesium-based bulk metallic glasses (BMGs) are typical intrinsic brittle lightweight BMG alloys, and their improvement in plasticity has attracted widespread attention in the field of BMGs. We used the electroplating method to modify the surface of Mg59.5Cu22.9Ag11Gd6.6 BMGs and investigated the geometric confinement effect of the Ni coating on the mechanical properties of the BMG. The results show that under the plating conditions of adding 1 g/L nano Al2O3 to the plating solution, adjusting the plating temperature to 50 °C, and plating time to 3 h, a smooth and dense nickel coating with a thickness of about 150 μm can be formed on the surface of the Mg-based BMG. The uniaxial compression tests showed that the average fracture strength of the BMG was increased from 565 MPa to 598 MPa by a 50 μm Ni coating, and the fluctuation range of strength was decreased from 429 MPa to 265 MPa, a reduction of 36%. The Weibull analysis showed that the Weibull modulus m was increased from 4.3 to 4.8 by the coating, and the safety stress was increased from 54 MPa to 235 MPa, indicating that electroplating nickel could improve the reliability of the Mg-based BMG alloy. However, no significant improvement of the compression plasticity was found, which indicated that improving the room temperature plasticity of brittle Mg-based BMG alloys by the geometric confinement of electroplating Ni was limited. The influence of the thickness of the Ni coating on the maximum stress level and stress distribution in the BMG samples was analyzed by ANSYS finite element simulation. It was found that when the thickness of the coating was 30% of the radius of the cylindrical compressed sample, the stress distribution caused by the Ni coating was the most uniform, and the maximum stress level was relatively reduced, which is beneficial for improving the geometric confinement effect. As a result, the Mg-based BMG sample coated with a Ni coating of 150 μm thickness exhibited ~0.3% macroscopic compressive plasticity. This is of great significance for understanding the plastic deformation mechanism of brittle BMGs improved by geometric confinement.