Study On Microstructure And Properties Of TiAl Based Alloys Containing Yttrium

Ingots of Ti-45Al-5Nb, Ti-45Al-5Nb-0.3Y and Ti-43Al-9V-0.3Y (at%) were prepared by induction skull melting (ISM) in a water cooler copper crucible. Pancakes and sheets of TiAl alloys were obtained by canned forging and pack rolling, respectively. This research involves the solidification, forging and rolling of the alloys. Microstructures and properties are also emphasized and systemically discussed. Some results are shown as the following.The calculation of thermodynamics and microstructrue observation were conducted to determine the existence status of element Y in Ti-45Al-5Nb alloy and formation mechanism of the Y-rich phase. The formation heat value of Ti-Y is positive, so is that of Nb-Y. On the contrary, the formation heat value of Al-Y is negative. At the same time, the activity coefficient of element Y in the melt is the highest. These facts indicate that the formation of Al-Y compounds is possible. Microstructures of Ti-45Al-5Nb-0.3Y alloy reveal YAl2 phases with bcc structure. The phases are enriched along the grain boundaries. There are slight precipitations within the lamellar colonies as the fine particles.Y addition into as-cast Ti-45Al-5Nb alloy leads to the refined microstructures. The average grain size decreases from 600μm to 100μm. The average lamellar spacing decreases from 470nm to 200nm. Both strength and ductility of as-cast Ti-45Al-5Nb alloy are increased by Y addition at room temperature. But the two alloys present brittle and cleavage fractures. Y addition has slight effect on the phase transformation of Ti-45Al-Nb alloy. Cooling rate has drastic effect on the continuous cooling transformation of as-cast Ti-45Al-5Nb and Ti-45Al-5Nb-0.3Y alloys. Furnace cooling (FC) leads to the fully lamellar microstructure. Increasing cooling rate to air cooling the lamellar transformation is suppressed byα→γm reaction. Oil cooling leads to the extremely fine lamellar microstructure. At higher cooling rate (water cooling),α2 phases are predominant.The hot deformability of Ti-45Al-5Nb is improved by Y addition. As-forged Ti-45Al-5Nb-0.3Y alloy is comprised of a large number of dynamic recrystallizaion (DRX)γgrains, curved and broken lamellae, a small amount of remnant lamellae. The DRXγgrain size reaches 1²Î¼m. As-forged Ti-45Al-5Nb- 0.3Y alloy presents better tensile properties in comparison to as-cast material at room temperature. The ultimate tensile strength of as-forged alloy is about 708.1MPa and the elongation is about 0.95%. The fine duplex (DP), near lamellar (NL) and fully lamellar (FL) microstructures are obtained by further heat treatment. These microstructures have better room temperature ductility. The DP microstructure has the highest ductility of about 1.9% the ultimate tensile strength is about 658.9MPa. The NL micrsotructure has the ultimate tensile strength of about 658.9MPa and elongation of about 1.75%. The FL microstructure has the highest ultimate tensile strength of about 715.1MPa and elongation is about 1.51%.As-cast Ti-43Al-9V-0.3Y alloy is mainly comprised ofγphase, besides minorα2, B2 and YAl2. The alloy has the fine-grained near lamellar microstructure with colony size of about 80μm. There exist some B2 precipitations within lamellar colonies. As-forged alloy has the streamline microstructure and is drastically refined. As-rolled alloy has the fine near gamma microstructure withγgrains of about 20μm. The streamline structure vanishes. The B2 phases with size of 8μm are distributed alongγgrains. After forging and rolling, the tensile properties are greatly improved. At room temperature, the strength of as-forged and as-rolled material increases about 89¹32MPa, and the elongation increases from 0.5% to 1% and 1.2%, respectively. At 700℃, the strength of as-rolled material increases about 63⁷2MPa and the elongation increases from 5.7% to 7.9%.Ti-45Al-5Nb alloy presents the excellent oxidation resistance at high temperature, so does Ti-45Al-5Nb-0.3Y alloy. Effect of Y addition is negligible. The excellent oxidation resistance is involved in the Nb5+ doped effect, a continuous Al₂O₃ scale formation and a Ti(Nb)-rich layer near the substract. Ti-43Al-9V-0.3Y alloy has relatively lower oxidation resistance. The decrease of oxidation resistance is due to the formation of a low melting point oxide, V₂O₅, which worsens the protection effect of the scale. Moreover, a ternary oxide, AlVO4, forms by a reaction between V₂O₅ and Al₂O₃. Al₂O₃ is depleted by this reaction, which further weakens the protection effect of the scale.