Study on the evolution of phase relations in the Ti–Al–Nb/Cr systems at 0–50 at.% Al region

Abstract

β-Solidifying γ-TiAl based alloys have gained scientific attention and showed potential applications as structure materials. When adding the β-stabilizing elements (e.g., Nb and Cr), ordered B2 and ωo phases are often formed during the production process of the TiAl based alloys. The information about the evolution of these phases as the temperature changes is necessary and important for design and preparation of desired TiAl based alloys with excellent properties. In this work, phase relationships and evolvement of Ti–Al–Nb/Cr systems have been investigated in the service temperature ranging from 950 to 700 °C. In the Ti–Al–Nb ternary system, B2 phase will directly transform into ωo phase at the temperature range of 913.6–921.1 °C, which results in an increase of the microhardness (i.e. 460 HV to 530 HV). At 700 °C, ωo phase has the composition of 15.8–20.4 at.% Nb, 29.8–33.3 at.% Al, and 46.3–54.4 at.% Ti. Equilibrium composition of ωo is defined as Ti3Al2Nb. The ternary compound O_Ti2AlNb has been found to be formed via β(Ti)+Ti3Al + Nb2Al→O at 981.5 ± 6.5 °C. As for the Ti–Al–Cr ternary system, when temperature decreases from 950 to 900 °C, the center section of the β(Ti) phase region shrinks and separates, resulting in the formation of independent B2 phase region. The island-like B2 region can remain stable at 700 °C. According to the evolution of phase relationship, one solid transition reaction Ti3Al + C14→B2+C15 at 900–800 °C has been deduced to occur in the Ti–Al–Cr ternary system. Present work provides an important guide for fabricating Ti–Al–Nb/Cr alloys with different ratio and distribution of plastic/brittle phases.