Formation and stability of refractory metal diborides in an Fe 3Al matrix

Abstract

The feasibility of forming refractory diboride particles in an Fe 3Al matrix by conventional casting techniques is examined; the microstructural stability of such particles upon subsequent exposure to elevated temperatures is discussed. Four different alloys were cast (Fe–Al–Ti–B, Fe–Al–Zr–B, Fe–Al–Nb–B, and Fe–Al–Ta–B); the nominal composition in each case was intended to yield stoichiometric Fe 3Al with 10 vol% of the stoichiometric refractory diboride. As-cast, annealed and heat treated alloys were examined using optical microscopy, X-ray diffraction, differential thermal analysis and scanning electron microscopy in conjunction with energy dispersive X-ray (EDX) spectroscopy to understand the formation and thermal stability of refractory diborides (TiB 2, ZrB 2, TaB 2, NbB 2) in Fe 3Al. In all four cases, refractory diborides formed; the Fe–Al–Ti–B and Fe–Al–Zr–B alloys yielded a two-phase microstructure of Fe 3Al and the diboride. The Ti and Zr diboride reinforcements were stable and showed minimal coarsening following high temperature exposure. The Nb- and Ta-containing quaternary alloys, in addition to the respective diborides, included a eutectic reaction at temperatures between 1150°C and 1200°C that results in the formation of an Fe-rich phase thought to be Fe 2B.