Phase formation, microstructure evolution and strengthening mechanism of Au27.9Ag20.3Cu51.8 noble metal medium-entropy alloy during cold deformation and aging

Abstract

Au27.9Ag20.3Cu51.8 noble metal medium-entropy alloy (MEA) is commercially applied in the electrical contact field due to the unique advantages, such as excellent chemical stability, outstanding electrical property and good processability. However, the low strength brings the poor deformation resistance and wear resistance in service. Herein, the cold deformation and aging treatment were applied to strengthen Au27.9Ag20.3Cu51.8 noble metal MEA, and the related evolutions of the microstructure, tensile property, and electrical resistivity were experimentally investigated. The phase formation and strengthening mechanisms were theoretically discussed. The results show that Au27.9Ag20.3Cu51.8 MEA is composed of the single-phase face-centered cubic (FCC) solid solution at the initial solution treatment of 750 °C. After the aging at 400 °C for 0.5 h, lamellar discontinuous precipitation (DP), consisting of Cu-rich and Ag-rich phases, forms at the grain boundaries (GBs), and L12-type chemical short-range order (CSRO) precipitates in both the matrix and the Cu-rich phase of DP. After 20% cold deformation, the L12-type CSRO similarly emerges in the matrix. After 20% cold deformation prior to aging at 400 °C for 0.5 h, DP at the GBs and L12-type CSRO in the matrix and the Cu-rich lamellae of DP are observed. The growth kinetics of DP and L12-type CSRO are both slightly promoted by prior cold deformation. Au27.9Ag20.3Cu51.8 MEA wire with single-phase FCC sold solution achieves the yield strength of ∼422 MPa. And, 20% cold deformation prior to aging at 400 °C for 0.5 h raises the yield strength of Au27.9Ag20.3Cu51.8 MEA wire to a peak value of ∼696 MPa. The solid solution strengthening mechanism invariably offers the largest strength contribution during the cold deformation and aging, the strain and precipitation strengthening mechanisms almost equivalently contribute to the strength improvement. The electrical resistivity maintains stable with a small variation during the cold deformation and aging. The findings in this study can provide insights for strengthening the noble metal HEAs or MEAs via traditional strengthening methods, and theoretically support for tailoring the microstructure and properties of the high-performance noble metal HEAs or MEAs.