Effects Of Heat Treatment Process On Microstructure And Hardness Of Ag925 Alloys

The properties and application of silver, development of silver jewelry alloys, alloying behaviors of silver and thermodynamic treatment were summarized in the thesis. Based on the above work, the purpose and significance of the thesis are to find out proper composition and heat treatment processes of Ag925 alloys, by preparing new Ag925 alloys by adding alloying elements of Zn, Sn and In to Ag925Cu alloys, and studying the effects of heat treatment process on their microstructure and hardness, and their strengthening mechanism.Pure silver and nine Ag925 alloys were arc-melted with a non-consumable electrode and water-cooled copper crucible under an argon atmosphere, and the effect of alloying elements of Zn, In and Sn, and heat treatment process on microstructures and hardness of Ag925Cu alloys, by hardness test, differential thermal analysis (DTA),differential scanning calorimetry (DSC), optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), X-Ray diffraction analysis (XRD), and so on. These results showed:(1) Alloying elements of Zn, In and Sn significantly improved the fluidity of molten Ag925Cu alloys, so as to obtain dense as-cast microstructure, and decreased the surface tension force of molten Ag925Cu alloys so as to improve mould filling.(2) The solution strengthening effect of alloying elements of Zn, In and Sn on silver was lower than that of Cu, and their solution strengthening effects decreased in sequence. Increasing Cu content improved the solution strengthening effect of Ag925 alloys.(3) For Ag925CuZn and Ag925CuZnSn (or/and In) alloys, the higher Cu content, the better the aging strengthening effect was. Moreover, in order to get the better solution strengthening effect, aging temperature ranged between 150℃and 250℃for the solutionizing and aging process, and aging temperature ranged between 150℃and 200℃for the cold drawing and aging process.(4) Alloying elements of Zn, In and Sn coarsened grains of Ag925Cu alloys, and didn't increase stack fault energy of silver to large degree, which contributed to form lots of annealed twins. When aged above 200℃, there appeared face center cubic convex microstructure, and did not appear Cu-rich phase in these new Ag925 alloys except Ag92.5Cu2.0Zn1.0Sn4.0 alloy.