Development of High Strength and High Ductility Micro-alloyed Gold by Inducing Gradient Nanostructures
DescriptionDespite a unique combination of superior physical and chemical properties, the exploitation of precious metal gold for jewelry applications is severely restricted by its low strength and low hardness1-3. To achieve higher strength as well as good workability without significantly changing the chemical composition, substantial research efforts have been devoted over past decades to investigate the role of gold’s microstructure in its yielding, plastic deformation and failure behavior. This research has led to several methods for the creation of a hard gold microstructure, for instance the solid solution and/or precipitation associated with microalloying in addition to strain-hardening and grain refinement.1,4-12However, few approaches have been reported so far for pure gold or microalloyed gold alloys with both high strength and high ductility.Our recent research work has developed such a material. The surfaces of a 1wt% Ti-containing gold alloy (24 K) were process using Surface Mechanical Attrition Treatment (SMAT)13. Low temperature aging at 260°C resulted in an unusual precipitated nanostructure in the surface layers. The gold alloy’s surface hardness was increased by a factor of five from 28 HV to 150 HV and the total hardening thickness can exceed several hundred micrometers (typical thickness for a 24K gold object in jewellery industry). Furthermore, preliminary molecular dynamics simulation results suggested that this could be attributed to nano-precipitation associated with the diffusion of alloying element, i.e. Ti.To improve understanding of the mechanisms for the formation of graded nanostructures and subsequent nano-precipitation from both thermodynamic and kinetic viewpoints and their role in the significant enhancement of mechanical properties for microalloyed gold, the proposed research will be composed of four key tasks:(1) Study the microstructure evolution of micro-alloyed gold alloys subjected to solid solution and/or precipitation strengthening heat treatment and its kinetics, revealing its dependence on the chemical characteristics, content of alloying element and the heat treatment parameters (including aging temperature, atmosphere and time).(2) Study the microstructure evolution in surface layers of micro-alloyed gold during SMAT and clarify the mechanisms of gradation of structures related to nanocrystallization and / or nanotwinning in the treated surface layers.(3) Study the microstructure evolution of microalloyed gold subjected to a combination of SMAT and subsequent aging heat treatment. Taking together with results from task 1 and 2, the effect of preceding SMAT on the microstuctural evolution and its physical mechanism during solidsolution and nano- precipitation will be investigated. The mechanism at the origin of nanoprecipitation in the SMATed alloy will be understood in terms of thermodynamic and kinetic theories of solid state reaction and computer simulation results.(4) Comparatively study the mechanical behaviors of pure and microalloyed gold subjected to a combination of SMAT and aging heat treatment. The relationship between the improved mechanical behaviours and graded nanostructures to the composition of the microalloyed gold will be established.The outcome of the present work will pave a novel and effective way to generate microalloyed precious metals such as gold with high strength and high ductility.
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- high strength , high ductility , microalloyed gold , graded nanostructures , SMAT