Designing low-thermal-expansivity, high-conductivity alloys in the Cu-Fe-Ni ternary system

Alloys that combine the high electrical and thermal conductivity of copper with the low thermal expansivity and higher strength of Invar (Fe-36Ni), are of interest for a range of applications. In suitable Cu-Fe-Ni alloys, nearly pure copper equilibriates with an Invar-rich solid solution; casting such alloys invariably produces Invar-rich dendrites in a copper-rich solid solution. Two-phase alloys that combine the properties of copper and Invar can be produced by casting followed by suitable heat treatments. The overall composition controls the relative fractions of Invar and copper and the resulting trade-off between low thermal expansivity and high electrical conductivity Analyses of phase equilibria and load transfer between ductile copper and Invar indicate that effective composites are only feasible for compositions between approximately 20 and 80 percent Invar.