Phase Equilibria Among β/α/α2/γ Phases and Phase Transformations in Ti-Al-Cr System at Elevated Temperatures

Effects of Cr addition to TiAl alloy on the phase equilibria among β/α/α2/γ Phases and phase transformation pathways within the temperature range of 1523 K∼1073 K were investigated using several alloys in the composition of interest. In between 1473 K and 1373 K, The slope of β/α tie-line in the three-phase coexisting region of β+α+γ remains basically unchanged. However, this slope drastically rotates in a clockwise direction, in between 1373 K and 1273 K and below the eutectoid reaction temperature in the binary system (1400 K). This is a strong indication that α-phase exists even below the 1400 K, i.e. addition of Cr stabilizes α against α2 and the three-phase coexisting region of β+α2+γ at lower temperatures is formed through a ternary eutectoid reaction (α → β+α2+γ) with decreasing temperature. This three-phase tie-triangle moves towards lower Al content in phase diagram. This suggests that Cr addition results in increase of the volume fraction of γ-phase with decreasing temperature even in alloys with low Al content.