The influence of oxygen concentration and the phase composition on various properties of Ti–42Al–5Mn (at%) forged alloy was evaluated, with the objective of expanding the utilization of this material. As a result, it was established that reduced oxygen concentration and increased β-phase fraction are effective for increased manufacturability (hot forgeability and machinability), and that increased oxygen concentration, reduced β-phase fraction, and increased lamellar structure fraction are effective for increased high-temperature strength. Accordingly, it is difficult to achieve optimization of all characteristics simultaneously. Nevertheless, at a high oxygen concentration of 0.16 wt% obtained via ceramic crucible melting, relatively favorable levels of all properties can exist simultaneously.

With the objective of achieving high strength and low cost for hot-forged TiAl based alloy containing β phase, the applicability of oxide-crucible-melted ingot was considered for compositions centered on Ti–28.7Al–7.0Mn (wt%). First, as a result of evaluating the influence of crucible contaminants on the hot-forgeability of ingots, it was confirmed that, even at the extremely high oxygen content of 0.35 wt%, hot-forgeability could be maintained if the amount of β phase was recovered at the forging temperature by reducing the Al content. Next, as a result of verifying the industrial practicality of large ingot, it was found that hot-forging under industrially required conditions was indeed possible for an yttria-crucible-melted ingot corresponding to the above-mentioned center composition with an oxygen content of 0.16 wt%; and in terms of machinability and Charpy impact properties of hot-forged and heat treated material, this ingot was superior to triple-melted VAR ingot having an oxygen content of 0.055 wt%, as well as having substantially better creep rupture strength.

In order to promote the development of low-cost TiAl precision cast turbine wheels, consideration was given to the influence of reduced purity. In the research reported here, the contamination behavior of Ti–46 at%Al cast material from various oxide crucibles was examined, and the influence of impurities and inclusions on material properties of Ti–46 at%Al cast material was evaluated. As a result, it was found that oxygen concentration had the greatest effect on properties, although it was observed that a relatively high concentration of up to approx. 0.4 wt% did not reduce room temperature tensile strength. Next, upon attempting the precision casting of TiAl turbine wheels, it was confirmed that items produced from an yttria crucible and having oxygen concentration of 0.17 wt% had greater room temperature tensile strength than turbine wheels from a conventional water-cooled copper crucible, together with comparable toughness required in practical use. Accordingly, the upper limit for oxygen concentration in TiAl castings, formerly considered to be 0.1 wt%, can be revised upward, which is extremely beneficial from the standpoint of reducing production costs.Graphical abstractExternal appearances of the TiAl turbine wheels produced in the precision casting test using various crucibles. (a) 100% yttria, (b) yttria + 15 wt% zirconia, (c) water-cooled copper.Highlights► Cost reduction of TiAl precision castings by reduced purity was considered. ► Oxygen concentration can be raised more than 0.1 wt% of present specifications. ► Feasibility of a turbine wheel made by ceramic crucible melting became clear.