Overaged metallography of alloy 909,a low coefficient of expansion superalloy

Abstract

The microstructure of alloy 909, a low coefficient of thermal expansion Fe-Co-Ni superalloy was examined in the solution treated and over aged condition, following a previous communication by the authors on the solution treated and commercially aged alloys. In the commercially aged alloy, only the Laves and the γ‚ phases were observed. With some indication, however, of alignment of the γ‚ phase along crystallograpic directions in the austenitic matrix. This alignment of the γ‚ phase has been previously reported as a precursor to the growth of the ϵ phase from the γ‚ phase. Analysis by TEM showed that the platelet phase was hexagonal (NiFeCo)₂(TiNb) with some variations in the stoichiometry about 2:1 ratio. Tensile and hardness testing indicated that the presence of the platelet shaped ϵ phase in the solution treated and overaged condition was not as effective in the strengthening of the alloys as the spherical γ‚ in the commercially aged and solution treated condition. In addition to the presence of ϵ phase, blocky A₂B Laves phase was abundant, which prevents excessive grain growth up to ∼1040°C