Development of High Strength and High Ductility Micro-alloyed Gold by Inducing Gradient Nanostructures

Description

Despite a unique combination of superior physical and chemical properties, theexploitation of precious metal gold for jewelry applications is severely restricted by its lowstrength and low hardness1-3. To achieve higher strength as well as good workability withoutsignificantly changing the chemical composition, substantial research efforts have beendevoted 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 thecreation of a hard gold microstructure, for instance the solid solution and/or precipitationassociated 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 alloyswith both high strength and high ductility.Our recent research work has developed such a material. The surfaces of a 1wt% Ti-containinggold alloy (24 K) were process using Surface Mechanical Attrition Treatment(SMAT)13. Low temperature aging at 260°C resulted in an unusual precipitatednanostructure in the surface layers. The gold alloy’s surface hardness was increased by afactor of five from 28 HV to 150 HV and the total hardening thickness can exceed severalhundred micrometers (typical thickness for a 24K gold object in jewellery industry).Furthermore, preliminary molecular dynamics simulation results suggested that this could beattributed 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 andsubsequent nano-precipitation from both thermodynamic and kinetic viewpoints and theirrole in the significant enhancement of mechanical properties for microalloyed gold, theproposed research will be composed of four key tasks:(1) Study the microstructure evolutionof micro-alloyed gold alloys subjected to solid solution and/or precipitation strengtheningheat 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-alloyedgold during SMAT and clarify the mechanisms of gradation of structures related tonanocrystallization and / or nanotwinning in the treated surface layers.(3) Study themicrostructure evolution of microalloyed gold subjected to a combination of SMAT andsubsequent aging heat treatment. Taking together with results from task 1 and 2, the effect ofpreceding SMAT on the microstuctural evolution and its physical mechanism during solidsolutionand nano- precipitation will be investigated. The mechanism at the origin of nanoprecipitationin the SMATed alloy will be understood in terms of thermodynamic and kinetictheories of solid state reaction and computer simulation results.(4) Comparatively study themechanical behaviors of pure and microalloyed gold subjected to a combination of SMATand aging heat treatment. The relationship between the improved mechanical behaviours andgraded 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 generatemicroalloyed precious metals such as gold with high strength and high ductility.