INCOLOY 908, a low coefficient of expansion alloy for high-Strength cryogenic applications: Part I. Physical metallurgy

Abstract

INCOLOY 908 is a low coefficient of thermal expansion (COE) iron-nickel base superalloy that was developed jointly by The Massachusetts Institute of Technology and the International Nickel Company for cryogenic service. The alloy is stable against phase transformation during prolonged thermal treatments and has a COE compatible with that of Nb3Sn. These properties make the material ideal for use as a structural component in superconducting magnets using Nb3Sn. The evolution of microstructure has been studied as a function of time at temperature over the temperature range of 650 °C to 900 °C for times between 50 and 200 hours. A detailed analysis of precipitated phases has been conducted using X-ray diffraction (XRD), transmission electron microscopy (TEM), and analytical scanning and scanning transmission electron microscopy (STEM) techniques. The primary strengthening phase has been found to be γ’, Ni3(Al, Ti). INCOLOY 908 is stable against overaging, which is defined as the transformation of γ’ to η, Ni3Ti, for times to 100 hours at temperatures up to 750 °C. Upon overaging, the strengthening phase transforms to η. A new phase,Hx, has been identified and characterized.