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

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 gamma' phases were observed. With some indication, however, of alignment of the gamma' phase along crystallograpic directions in the austenitic matrix. This alignment of the gamma' phase has been previously reported as a precursor to the growth of the e phase from the gamma' 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 epsilon phase in the solution treated and overaged condition was not as effective in the strengthening of the alloys as the spherical gamma' in the commercially aged and solution treated condition. In addition to the presence of e phase, blocky A(2)B Laves phase was abundant, which prevents excessive grain growth up to similar to 1040 degrees C