| The alloying behavior of gold and silver, corrosion behavior of gold, silver and their alloys, anti-tarnish measurements of silver and its alloys, and development of 990Au and 925Ag alloys have been summarized in the dissertation. Based on the previous work, two 990Au alloys based on Au-Ti(-Y) system were prepared by medium frequency induction melting and iron mould casting under an argon atmosphere, and fourteen 925Ag alloys based on Ag-Cu(-Zn, Sn, In, Ge, Y, Nd, Si) system were prepared by arc-melting with a non-consumable electrode and water-cooled copper crucible under an argon atmosphere.Strengthening mechanisms of 990Au alloys based on Au-Ti(-Y) system, and microstructure, strengthening mechanisms, corrosion behavior in artificial sweat at 37℃and H2S atmosphere at room temperature of 925Ag alloys based on Ag-Cu(-Zn, Sn, In, Ge, Y, Nd, Si) system have been systematically investigated, in order to supply some theoretic references for the development of high strength 990Au alloys and good overall-properties 925Ag alloys. Strengthening mechanisms of 990Au alloys based on Au-Ti(-Y) system are investigated, by means of hardness test, tensile test, optical microscopy (OM) and transmission electron microscopy (TEM). These results show that strengthening modes of 990Au alloys based on Au-Ti(-Y) system are mainly solid solution strengthening, grain refining strengthening, work hardening and precipitation strengthening. For 990Au alloys based on Au-Ti(-Y) system, Ti has good grain refining strengthening and aging strengthening effects, and high Ti content improves their precipitation strengthening effect. In addition, Y has good grain refining strengthening and solid solution strengthening effects. Due to their high hardness and tensile strength, and good ductility, 990Au alloys based on Au-Ti(-Y) system are suitable for manufacturing jewelry items with complex fashion and for insetting gem, which were ever made of 18 carat and other low carat gold alloys.Microstructure and strengthening mechanisms of 925Ag alloys based on Ag-Cu(-Zn, Sn, In, Ge, Y, Nd, Si) system are investigated, by means of hardness test, differential thermal analysis (DTA), differential scanning calorimetry (DSC), X-Ray diffraction analysis (XRD), OM, scanning electron microscopy (SEM), TEM and energy dispersive spectrometry (EDS). These results show that microstructure of 925Ag alloys based on Ag-Cu(-Zn, Sn, In, Ge, Y, Nd, Si) system depends on their composition and heat treatment processes. For Ag-7.5Cu alloy, its melt fluidity is poor so as to lead to as-cast microstructure with some macropores, and there appearsα-Cu phase but doesn't appear convex microstructure after solutionization and further aging. For Ag-3.0~5.0Cu-2.5~4.5Zn, Ag-4.0Cu-2.5Zn-1.0Sn, Ag-4.0Cu-1.5~3.1Zn -0.4~2.0In, Ag-4.0Cu-2.1Zn-1.0Sn-0.4In, Ag-4.0Cu-3.0Cu-0.5Ge, Ag-4.0Cu-3.0Zn-0.5Y, Ag-4.0Cu-3.0Zn-0.5Nd and Ag-4.0Cu-0.6Zn-1.5Sn-1.0In-0.2Ge-0.1Y-0.1Nd alloys, their melt fluidity are good so as to lead to dense as-cast microstructure, and there appears convex microstructure along grain boundaries after solutionization and further aging, but the fraction of convex microstructure depends on their composition, aging temperature and aging time. In addition, there appears Cu-rich and Nd-rich phase in Ag-4.0Cu-3.0Zn-0.5Nd alloy in any heat-treated state, and there appears Cu-rich and Si-rich phase in Ag-4.0Cu-3.4Zn-0.1Si alloy in any heat-treated state. For Ag-4.0Cu-2.5Zn-1.0Ge alloy, its melt fluidity is low so as to lead to as-cast microstructure with some macropores, and there doesn't appear convex microstructure along grain boundaries after solutionization and further aging. Moreover, its grain boundries widen and there appears grain boundary segregation of Cu and Ge in any heat-treated state. Strengthening modes of 925Ag alloys based on Ag-Cu(-Zn, Sn, In, Ge, Y, Nd, Si) system are mainly solid solution strengthening, work hardening and aging strengthening. For 925Ag alloys based on Ag-Cu-Zn system, Cu content must be higher than 3.0% in order to realize high hardness after solutionization and further aging. For most of 925Ag alloys based on Ag-Cu-Zn(-Sn, In, Ge, Y, Nd, Si) system containing 4.0% Cu, the proper aging temperature is between 200℃and 250℃after solutionization, and the proper aging temperature is between 150℃and 200℃after 70% cold rolling.Corrosion behavior, the formation rate and the degree of coverage of corrosion products and tarnishing resistance of 925Ag alloys based on Ag-Cu(-Zn, Sn, In, Ge, Y, Nd, Si) system are investigated in artificial sweat at 37℃, by means of static immersion experiment, potentiodynamic polarization measurement, XRD, SEM, EDS and X-Ray photoelectrical spectrometry (XPS). These results show that corrosion behavior of 925Ag alloys based on Ag-Cu(-Zn, Sn, In, Ge, Y, Nd, Si) system in artificial sweat at 37℃depends on their composition and heat-treated states, and that their corrosion modes are mainly pitting corrosion and stress corrosion. In these four heat-treated states, that is, the solutionized state at 700℃for 0.5h, the aged state at 250℃for 4h, the 70% cold rolled state and the 70% cold rolled and aged state at 200℃for 4h, corrosion rate of 925Ag alloys based on Ag-Cu(-Zn, Sn, In, Ge, Y, Nd, Si) system is high in the 70% cold rolled state, and is low in the solutionized state at 700℃for 0.5h or the aged state at 250℃for 4h, but their corrosion behavior depends on their composition in various heat-treated states. The formation rate and the degree of coverage of corrosion products and tarnish resistance of 925Ag alloys based on Ag-Cu(-Zn, Sn, In, Ge, Y, Nd, Si) system in artificial sweat at 37℃depends also on their composition and heat-treated states. In the other three heat-treated states except the 70% cold rolled state in the above content, for Ag-4.0Cu-3.5Zn, Ag-4.0Cu-2.5Zn-1.0Sn, Ag-4.0Cu -1.5~3.1Zn-0.4~2.0In and Ag-4.0Cu-2.1Zn-1.0Sn-0.4In alloys, the formation of black corrosion products consisting of AgCl and Ag2O is effectively retarded in various heat-treated states so as to keep good tarnish resistance, however, for Ag-7.5Cu, Ag-4.0Cu-2.5~3.0Zn -0.5~1.0Ge, Ag-4.0Cu-3.0Zn-0.5Y, Ag-4.0Cu-3.0Zn-0.5Nd and Ag-4.0Cu-0.6Zn-1.5Sn -1.0In-0.2Ge-0.1Y-0.1Nd alloys, black corrosion products films form easily in some heat-treated states so as to decrease tarnish resistance.Corrosion behavior and tarnish resistance of 925Ag alloys based on Ag-Cu(-Zn, Sn, In, Ge, Y, Nd, Si) system in H2S atmosphere at room temperature are investigated in the aged state at 250℃for 4h and the 70% cold rolled and aged state at 200℃for 4h, by means of static exposure experiment, SEM, EDS and XPS. These results show that corrosion behavior and tarnish resistance of 925Ag alloys based on Ag-Cu(-Zn, Sn, In, Ge, Y, Nd, Si) system depend mainly on their composition in H2S atmosphere at room temperature. For Ag-4.0Cu -3.5Zn alloy, white ZnS first forms so as to retard the formation of black Ag2S and Cu2S, thus its tarnish resistance is obviously better than that of Ag-7.5Cu alloy in H2S atmosphere at room temperature. For Ag-4.0Cu-1.5Zn-2.0In, Ag-4.0Cu-2.1Zn-1.0Sn-0.4In, Ag-4.0Cu -2.5~3.0Zn-0.5~1.0Ge and Ag-4.0Cu-0.6Zn-1.5Sn-1.0In-0.2Ge-0.1Y-0.1Nd alloys, white ZnS, yellow SnS2, bright yellow In2S3 or grey GeS2 first form so as to effectively retard the formation of black Ag2S and Cu2S, thus their tarnish resistance is better than that of Ag-4.0Cu-3.5Zn alloy in H2S atmosphere at room temperature.According to strengthening mechanisms and corrosion behavior in artificial sweat at 37℃and H2S atmosphere at room temperature of 925Ag alloys based on Ag-Cu(-Zn, Sn, In, Ge, Y, Nd, Si) system in the above content, Ag-4.0Cu-1.5Zn-2.0In and Ag-4.0Cu-2.1Zn -1.0Sn-0.4In alloys are of good overall properties, for example, good melt fluidity, high hardness, the difficulty to form black corrosion products in artificial sweat at 37℃, and good tarnish resistance in H2S atmosphere at room temperature. |
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