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.