EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Plansee Tungsten Alloys manufactures innovative Tungsten Heavy Alloys for applications requiring resistance to extreme conditions. The improvement of these alloys requires essentially an accurate knowledge of phase equilibria in the W–Ni–Co–Fe quaternary system for a wide range of temperatures in order to avoid embrittlement from interfacial precipitation of intermetallic phases. The W–Ni–Co and W–Ni–Fe systems were experimentally studied at temperatures higher than 1000°C. Hence, the data currently available do not describe properly the stable intermetallic phases at intermediate temperatures which are of utmost importance for industrial processes.
W–Ni–Co rods with varying compositions were manufactured by powder-metallurgy (solid-state sintering). Slices from each rod were further annealed for long times at 800°C and 1000°C. The samples were quenched and subsequently analyzed in order to understand the phase changes that occur in this range of temperatures. The samples were characterized with X-ray diffraction analysis, EDS coupled SEM observations, TGA-DSC analysis, EPMA composition measurements, electronic diffraction (TEM-SAED). The isothermal sections at 800°C and 1000°C are more complex than expected with much wider composition domains involving intermetallic phases than expected from previous thermodynamic models. A new intermetallic phase appears to be stable (D0a structure - Cu3Ti prototype) at both 800°C and 1000°C and decomposition temperatures for the intermetallic phases were assessed to set a temperature limit for industrial heat-treatments. Formation enthalpies associated with the new phase were obtained through ab initio calculations for the W–Ni–Co system and support the experimental results showing an increased stability of the D0a phase with nickel addition. All the acquired data are currently used to propose a new CALPHAD modeling of the Co-Ni-W system.