Study of the Role of Residual Stress Distribution on the Mechanical Behaviour of the Metallic Glass

“Metallic glass” or “glassy metal” typically exhibits impressive properties as an engineering material: very high elastic strain limit and yield strength, high hardness, and excellent corrosion resistance. However, the widespread use of this kind of material is hindered by a lack of macroscopic room temperature plasticity. The rapid cooling required to obtain the unique microstructure of metallic glass and the way in which metallic glass is formed result in residual stress. Residual stress plays a key role which may affect the plastic deformability of metallic glass. In this project, residual stress produced during the manufacture of metallic glass and how it affects its deformation behaviour and formability will be extensively investigated. A bulk metallic glass (BMG) alloy of Zr55Al10Ni5Cu30, which exhibits exceptional glass forming ability and high thermal stability against crystallization, will be selected as a case study material. The research concerned by this project consists of four main tasks:Exploration of the manufacture of metallic glass under different cooling conditions, different levels of residual stress, and different distributions;Development of the incremental hole drilling method for in-depth residual stress distribution measurement with a small depth increment of 20 micrometers; modification of the residual stress by cyclic loading using ultrasonic SMAT (surface mechanical attrition treatment); modification of the residual stress distribution by varying the treatment parameters, including the size and composition of the treatment media, and the intensity and duration of the treatment; an FEM-based simulation approach to the formation process and residual stress distribution and their relationship with the different factors involved;Establishment of a relationship between the mechanical properties and residual stress state and the local constitutive law for different depths of material by conducting nanoindentation experiments; development of a tailor-made MBG compression test platform by considering the high strength and brittle nature of BMG;Study of the structural evolution of the material and investigation of the plastic deformation mechanism. Based on these fundamental analyses, a prestress strategy will be developed to optimise the process in order to obtain a reasonable residual stress profile to enhance the plastic deformation capability of BMG.