2, Surface Protection and Corrosion Control Lab, Central Metallurgical Research and Development Institute (CMRDI), Cairo, , Egypt
Designing of wrought TiAl alloys opens the window for a wide range of applications and it will not be exclusive to be applied as low-pressure turbine materials but also as high-pressure compressor materials. β-Ti phase in TiAl alloys allows excellent hot workability during processing and excellent mechanical properties in service temperatures. In other words, microstructure control using β-phase is a key to develop high-toughness wrought γ TiAl alloys. This can be accomplished by understanding the phase transformations involving β-phase. Effects of group Vth (V, Nb) and VIth (Cr, Mo) elements as β-stabilizers on phase equilibria of TiAl alloys above 1473 K were extensively studied by our group. It was found that V and Nb stabilize α2 against α. Also, the change in three-phase coexisting region β+α+γ that exists above 1473 K to that of β+α2+γ occurs not just by the ordering transformation α→α2 (2nd order phase transformation) but by a transition peritectoid reaction β+α→α2+γ (1st order phase transformation) at a temperature between 1453 K (the congruent temperature of α→α2) and 1400 K (the eutectoid reaction temperature of α→α2+γ) in Ti-Al binary system with decreasing temperature. Thus this phase transformation allows a unique transformation pathway for α→α2+β in the ternary systems. However, it has not been clarified yet that the addition of group VIth elements (Cr, Mo) stabilizes either α or α2. Thus, in this study, the effect of Cr addition to TiAl alloy on the phase equilibria among the four phases and phase transformation pathways within the temperature range of 1473 K~1073 K were investigated using several alloys in the composition of interest. In between 1473 K-1373 K, the slope of β/α tie-line in the three-phase coexisting region of β+α+γ remains basically unchanged where the Al content in β-phase is much lower than that in α-phase. However, this slope drastically rotates in a clockwise direction and the Al content between the two phases becomes nearly equal, 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 thermodynamically 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. In between 1173 K and 1073 K, no further shift was observed in the tie-triangle meanwhile it expands towards high Cr content in β-phase. Based on these information, a novel technique for developing a new wrought γ-TiAl with excellent workability during processing at elevated temperatures and excellent toughness in service conditions, could be proposed.