Microstructural characterization of multicomponent Nb-Ti-Si-Cr-Al-X alloys

Abstract

Recently, alloys based on the Nb-Ti-Si system have become of interest for high-temperature structural applications. In the present work, the microstructure of multicomponent Nb-30Ti-8Si-10Cr-10Al-X (in at. pct) alloys in the as-cast and heat-treated conditions was studied using X-ray diffraction, electron probe microanalysis, scanning electron microscopy and transmission electron microscopy. The effect of temperature and time on phase evolution was examined in detail. The as-cast microstructure was found to be composed of three phases:β (bcc), silicides (M₅Si₃ type), and a Cr-rich Laves phase. Theβ phase was found to display B2-type ordering. The silicides in these alloys were generally quite stable during heat treatment, whereas the Cr-rich Laves phase was observed to dissolve on solutionization at temperatures above 1300 °C. Aging of the solutionized materials between 900 °C and 1100 °C led to the precipitation of fine particles of another Laves phase in theβ matrix. In addition, theβ matrix revealed a tendency toward phase separation into Nb-rich (β₁) and Ti-rich (β₂) regions. The volume percentage and chemical composition of each phase has been determined as a function of time and temperature and the changes in microstructure have been rationalized in terms of the distribution of elements in various phases. The role of different alloying elements on the formation of these phases has also been critically examined.