An Evaluation of the Mechanical Behavior of some new High Temperature Materials

Abstract

Two new high-temperature alloy systems have been discovered recently at the Indian Institute of Science, which form the basis of this thesis. The first alloy system is an Intermetallic-Intermetallic eutectic composite alloy in the nickel-rich end of the Ni-Al- Zr ternary system, first reported by Tiwary et al. These eutectic composites have very impressive room temperature yield strengths of about 2 GPa which are retained till about 700 . Coupled with 2 to 5% tensile plasticity at room temperature, excellent high-temperature oxidation resistance, good long-term microstructural stability and very low densities of about 7.35–7.95 gm/cm3, these materials are exciting candidates for high-temperature applications. The second alloy system comprises of Tungsten free Cobalt based superalloys having the classical − 0 microstructure similar to that of nickel-based superalloys as described by Makineni et al. The presence of the L12 phase in cobalt-based systems was reported by Lee et al. and Sato et al. where the addition of 25 wt.% tungsten seemed to stabilize the metastable Co3Al. This however pushed the density of the alloy to about 9–10 gm/cm3, making it too heavy for most high-temperature applications. A large body of work followed trying to reduce or eliminate the presence of tungsten in these alloys. The new alloys by Makineni et al. do not contain tungsten which reduces the density to about 8.0–8.4 gm/cm3. The high-temperature mechanical behavior of some of the alloys from these two sysi tems has been evaluated in the current thesis. This thesis is divided into six chapters.