Title

Microstructure and phase stability of a quinternary Nb-Mo-Cr-Al-Si alloy

Date of Completion

January 2009

Keywords

Engineering, Metallurgy|Engineering, Materials Science

Degree

Ph.D.

Abstract

In this study, the goal is to give experimental phase identification and microstructure analysis of a quinternary alloy with the nominal composition Nb-27Mo-27Cr-9A1-9Si (at.%). ^ The as-cast quinternary alloy was comprised of a primary dendritic A2 solid solution surrounded by a eutectic mixture of A15 and C14 phases. The point defects are predominantly anti-site transition metal atoms for the C14 eutectic phase and constitutional vacancies for the A15 eutectic phase. Heat-treatment at 1000°C led to three types of nano-scale precipitates within the A2 dendrites: A15/C15 composite laths, C15 ellipsoids and η-carbide phase: there were no significant changes in the eutectic. Heat-treatment at 1500°C led to the formation of coarser blocky A15 and C14 precipitates within the A2 phase along with a very small amount of an unknown Al-rich phase: there was also extensive coarsening/spherodization of the eutectic mixture. It was inferred that the Laves phase in the precipitates formed at 1000°C adopts the C15 structure due to tensile coherency stresses; this transforms to the equilibrium C14 structure by synchroshear processes on {111} when the stresses are relaxed. It was proposed that a similar sequence might explain the diversity of ORs exhibited by the eutectic colonies. ^