Effect of ingot up-scaling on microstructure and mechanical properties of an advanced intermetallic γ-TiAl based alloy

The effect of ingot up-scaling on microstructure and mechanical properties of an advanced β-solidifying γ-TiAl alloy with a composition Ti-44Al-6(Nb, Zr, Hf)-0.15B (at.%) (designated as TNZ γ-TiAl alloy) has been investigated. A comparison of the microstructure and mechanical properties was performed for the alloy manufactured as a small laboratory ingot and a large ingot with a diameter of 300 mm produced at an industrial enterprise. It was shown that ingot up-scaling led to the same solidification and solid-state transformation pathway as in the case of the laboratory-scale ingot: L → L + β → β → β + α → α + βm → α + γ + βm → α + γ + β0m → α2 + γ + β0m, where βm / β0m is the remaining metastable β / β0 phase. However, the as-cast microstructure of the industrial ingot was found to be coarser than that of the laboratory-scale ingot, which was ascribed to a slower cooling rate of the large ingot. Near the same processing route was applied for the laboratory ingot and a large workpiece cut from the large ingot. These included unidirectional forging to a strain of ε ≈ 25 – 30 % followed by two-stage heat treatment using furnace and air cooling after first annealing. Homogeneous refined duplex type structures with different lamellar spacing in the lamellar constituents (conditions 1 and 2) were reached in the case of the small ingot in contrast to the industrial ingot, in which coarser and more inhomogeneous near duplex structures with different lamellar spacing in the lamellar constituents (conditions 3 and 4) were obtained. Tensile and creep tests were performed for the obtained microstructural conditions. It was revealed that the ingot up-scaling resulted in some decrease in ductility / strength and an increase in creep resistance. This was explained by the coarser microstructures obtained in conditions 3 and 4 compared to conditions 1 and 2, which is associated with a coarse cast microstructure in the case of the large-sized ingot. The possible methods for reducing the ingot up-scaling effect are proposed.