To develop a heat-resistant cast TiAl alloy suitable for turbochargers for passenger vehicles, examinations have been conducted to determine the appropriate Nb concentration in Ti-Al-Nb ternary alloys and suitable additives for this purpose.
In addition, the material properties and the durability in a realistic environment were evaluated by using the turbine wheel materials of this alloy. The results of the studies can be summarized as follows:
- The optimum Nb concentration is about 7 at. % in general consideration of the mechanical properties, oxidation resistance, specific gravity and production costs.
- Useful additives are Cr, Si and Ni. In actual production of turbine wheels, the elements to be added to the basic Ti-Al-7 Nb alloy are selected according to its application and the size of the turbine wheel, and a decision is also made whether or not to apply HIP treatment.
- The specific high-temperature strength of the newly developed TiAl alloy turbine wheel is superior to that of conventional TiAl alloy and exceeds Inconel 713C when the HIP treatment is applied.
- Blade erosion of the conventional alloy turbine wheel during the engine endurance test was severe, while that of the newly developed alloy was negligible. This difference is thought to result from the difference in the microstructure of the blades of the two turbine wheel materials.
Tensile tests were conducted on the 0.5 Si-added and 0.5 Ni-added alloys, which showed an improvement in oxidation resistance, and the 1 Cr-added alloy, which is expected to have better mechanical properties, as well as on the base alloy. Fig. 2 shows the tensile strength and the elongation of each alloy at room temperature and 900°C.
When compared to the base alloy, the tensile strength of the 0.5 Si-added and 0.5 Ni-added alloys is superior at 900°C, and the room temperature elongation of 1 Cr-added alloy is slightly better, and that of the 0.5 Si-added alloy is lower.
The effects of each additional element on the properties of the 7 Nb alloys can be summarized as follows: Cr improves ductility at room temperature and has no effect on oxidation resistance. Mn lowers oxidation resistance. Co has no effect on oxidation resistance. Si and Ni improve oxidation resistance and high-temperature strength, but Si reduces ductility at room temperature.
The as-cast TiAl-based alloys are usually coarse in grain size. A refinement in the microstructure is therefore necessary to meet the desirable applications. Additional elements, such as W, B etc., added into the base alloy can help refine the grain size and relevant heat treatments can facilitate the development of the desirable microstructures.
In general, the fully lamellar (FL) structure has a better overall property than the other microstructures. But it has the low room-temperature tensile ductility. The duplex structure (DP) tends to have the good room-temperature tensile ductility, but its fracture toughness is low.
This dilemma has not been satisfactorily resolved recently and for the DP structure, it is hard to enhance its fracture toughness and creep resistance. On the other hand, the room-temperature ductility can be enhanced by refining the grain size of the FL structure. A fine fully lamellar microstructure has both good ductility and fracture strengths, and the fracture-toughness and creep resistance are also better than the duplex microstructure.
Figure 2: Effects of additional elements on weight gain during 950°C / 500 h oxidation test of 7Nb alloys Base 1 Cr 0.5 Si 0.5 Ni Base 1 Cr 0.5 Si 0.5 Ni : Room temperature
(a) Tensile strength (b) Elongation S
Hereby we can also consider the new TiAl alloys, containing tungsten (W) and boron (B). Using scanning-electron microscopy (SEM), the effects of W and B on the microstructural evolution of TiAl alloys, including the colony size and lamellar spacing, were analyzed.
It was found that tungsten prefers to react with boron to form borides, and disperses mainly along grain boundaries, and occasionally inside grains. With the increase of the tungsten content, the microstructure can be further refined. The addition of tungsten can restrain the grain coarsening and stabilize the microstructure up to 1,280°C by hindering the migration of grain boundaries at high temperatures.
It is also noteworthy that the β phase, a high-temperature ductile phase, forms when the tungsten content exceeds 0.4 atomic percent. The α-phase transus temperature, Tα, has been determined through differential-thermal analyses (DTA) and further proved by the investigation of the microstructural changes during various heat treatments. Different microstructures meeting desirable needs can be developed through heat treatments beyond and below the α-phase transus temperature.
1. Cellular structures and dendrites are formed in the as-cast Ti-Al-Nb-W-B alloys. This is because the addition of heavy metals, such as Nb and W, increased the under cooling of the liquid phase, which triggered the formation of the cellular structure and dendrite at the solid-liquid interface. Nb and W tend to segregate strongly at the interface of the cellular structure.
2. Porosities in the as-cast Ti-Al-Nb-W-B alloys can be generally eliminated by HIPing at 1,250°C. The cellular structures, dendrites, and macro-segregations of the alloys can also be eliminated after the HIPing (hot-isostatic pressed, at 1250°C, for 4h at a pressure of 150 MPa) and long-term homogenization at 1,2500°C.
3. A small additional amount of W can refine the grain size of the Ti-Al-Nb-W-B alloys. The lamellar spacing also decreases with increasing the W concentration. But as the amount of W exceeds 0.4 at%, its beneficial effect becomes small.
4. The addition of W promotes the formation of the β phase. When the amount of W exceeds 0.4 at%, the β phase precipitates. The β phase in the blocky form mainly distributes along the grain boundary. Its composition is close to the α2 phase, but with a lesser amount of Al.
5. From the results of microstructural changes related to different kinds of heat treatments, and differential-thermal analyses (DTA), the α-transus temperature, Tα, is estimated to be 1,290 ± 5°C.