Composition Rule of High Hardness and Electrical Conductivity Cu-Ni-Si Alloys

Cu Ni Si alloys are among the most widely used electrical conductive (>30%IACS) alloys with quite high strength level (>500 MPa), so they are especially suitable for lead frames and connector joints. However, these two properties are quite composition sensitive, apart from their tight connection with processing. Moreover, their compositions fall within quite broad ranges that poses difficulties for the industries. For instance, typical C7025 alloy has a specified composition (mass fraction, %) range of Ni 2.2 similar to 4.2, Si 0.25 similar to 1.2, Mg 0.05 similar to 0.3, plus less than 0.5 of other impurity elements. Obviously the composition ranges of the elements are far from even their absolute contents. The present work focuses on understanding the composition rule of Cu-Ni-Si via a new structural tool, the cluster-plus-glue-atom model. In this model, any solid solution is described by a nearest neighbor coordination polyhedron plus a one-to-six glue atoms. Specifically for Cu-based alloys, the cluster is cubooctahedron. The composition formula for solute-rich Cu-Ni-Si alloys and pure Cu are established, respectively [(Ni2/3Si1/3)-Cu-12]Cu-1(similar to 6), and [Cu-Cu-12]Cu-3. A series of Cu-Ni-Si alloys were designed on the basis of the cluster-plus-glue-atom model. In the concentrated solute region with Cu content less than 95%, the alloys were designed using the single cluster model [(Ni2/3Si1/3)-Cu-12]Cu-1(similar to 6). In the dilute solute region where Cu content is larger than or equal to 95%, the alloys were designed using the double cluster model {[(Ni2/3Si1/3)-Cu-12]Cu-3}(A)+{[Cu-Cu-12]Cu-3}(B). The alloys were arc-melted into ingots under Ar atmosphere and were subjected to a solution treatment at 950 degrees C for 1 h plus water quenching, and then to an ageing at 450 C for 4 h plus water quenching. The microstructure and properties of the alloys were characterized and tested by XRD, OM, TEM, Vickers hardness tester and digital metal conductivity instrument. The composition rule of the designed Cu-Ni-Si alloy was obtained by experiments.The results shown a special range of Cu content in 95.0%-95.8% as a composition sensitive region, in which, in addition to ageing precipitation strengthening, the alloys also have amplitude modulated decomposition strengthening, resulting in a sudden increase in Vickers hardness and a decrease in electrical conductivity. Vickers hardness and electrical conductivity change with composition variations in an irregular manner. In the concentrated and dilute solute region before and after the composition sensitive region, Vickers hardness (H) is linearly related to the Cu content (C-Cu) by H= -12.6C(Cu)+1362.7 and H=-26.2C(Cu)+2777.3, and the corresponding electrical conductivity (sigma) is also linearly related to the C-Cu by sigma=0.2C(Cu)-28.6 and sigma=5.2C(Cu)-466.