Intermediate-temperature mechanical properties of Ni-Si alloys: oxygen embrittlement and its remedies

With good corrosion resistance, reasonable room-temperature ductility, and excellent strength up to temperatures of 700°C, Ni₃Si-based alloys show considerable potential for structural applications. The Ni-Si alloys used for acid-corrosion resistance suffer from a dynamic environmental embrittlement when tested at intermediate temperatures around 600°C. To assess these Ni-Si alloys for elevated-temperature structural application, the mechanical properties of these alloys strengthened by Ni₃Si precipitates were systematically evaluated at different temperatures in various test environments. Oxygen was identified as the embrittling species responsible for the low ductility and premature fracture of the Ni-Si alloys. A strong dependence of elongation and fracture mode on strain rate was observed. Based on the understanding of the embrittlement mechanism, some unique approaches for improving the intermediate-temperature ductility, strength and fabricability of Ni-Ni₃Si alloys were identified: Reactive element doping (such as Zr and Y) to change the grain boundary chemistry; preoxidation to form adherent oxide layers; and thermomechanical processing to tailor the grain structure/shape. Some other properties such as creep resistance and weldability of these alloys were also briefly evaluated and are discussed in this paper.