Directionally solidified in situ metal matrix composites. Part 2. Final report. [Co--Al--Cr, Co--Si--Nb, Co--Cr--Zr]

This work constitutes Part II of an alloy screening program to identify new eutectics based on Ni, Co and Fe suitable for the manufacture of directionally solidified turbine blades operating at up to 1150 C. During this Part II, thirteen alloy systems, selected for screening during Part I, have been experimentally investigated, leading to the identification of three eutectics of potential interest: Co-Al-Cr, Co-Si-Nb and Co-Cr-Zr. As a result of the overall program, Part I and Part II, a total of 9 eutectics have thus been found to meet the objectives of a melting point above 1200 C, a specific gravity below 9 g/cc and a reasonably low parabolic oxidation rate. These eutectics thus have, before any development, an oxidation resistance comparable or superior to the chromium free gamma/gamma-delta eutectic superalloy Ni-2.55 A1-21.75 Nb (wt %). It is therefore expected that an alloy optimization program would improve their oxidation behavior, as compared to the gamma/gamma-delta alloy with 6% chromium, presently the most advanced eutectic of this class of materials.