A titanium aluminide based alloy consisting of 42-48 at % aluminium, 2-5 at % niobium, 3-8 at % zirconium, 0-1 at % boron, 0-0.4 at % silicon and the balance, apart from incidental impurities, is titanium. The titanium aluminize alloy composition has a satisfactory combination of high tensile strength, acceptable ductility at room temperature and low secondary creep rate at elevated temperature, so as to be suitable for use in high temperature applications for example aero-engines and automobile engines. It is suitable for compressor discs and compressor blades of aero-engines.

FIELD OF THE INVENTION 
The present invention relates to titanium aluminide based alloys. In 
particular the present invention relates to low density titanium aluminide 
based alloys which can be useful for high temperature applications such as 
in aerospace and in automobile engines. 
BACKGROUND OF THE INVENTION 
Titanium aluminide alloys, particularly gamma titanium aluminide (TiAl) 
based alloys, possess a low density combined with high strength and are 
resistant to oxidation. Gamma titanium aluminide alloys offer a 
200.degree. C. temperature advantage over conventional titanium alloys for 
use as, for example, compressor discs and blades in aero-engines and are 
only about 50% of the density of nickel-based superalloys. Many aerospace 
and automobile engine components operate at high temperatures and so a 
measurement of the strength of the alloy at room temperature, although 
important, may not be the best indication of how a component will perform 
at its operating temperature. A more useful test involves loading the 
alloy at an elevated temperature and observing its creep rate. In 
particular, the secondary (steady-state) creep rate is an important guide 
as to how the alloy will perform at elevated temperatures. In addition, 
the alloy should not be too brittle at room temperature in order to reduce 
the possibility of fracture. 
SUMMARY OF THE INVENTION 
Thus it is an object of the present invention to provide an alloy 
composition having a satisfactory combination of high tensile strength and 
acceptable ductility at room temperature and low secondary creep rate at 
elevated temperature, so as to be suitable for use in high temperature 
applications. 
The present invention resides in a titanium aluminide based alloy 
consisting of (in atomic %), 42-48 at % aluminium, 2-5 at % niobium, 3-8 
at % zirconium, 0-1 at % boron, 0-0.4 at % silicon and the balance, apart 
from incidental impurities is titanium. 
The invention also resides in an article made from the alloy defined in the 
immediately preceding paragraph. The article may be made, for example, by 
a thermomechanical process, such as forging, or by casting. 
It is to be understood that oxygen is a trace impurity, unavoidably present 
in all titanium alloys, but it is preferably maintained below 0.15 wt %. 
More preferably, the oxygen content is in the range of 0.03 to 0.15 wt %. 
It is desirable for an alloy to have a fine grained microstructure. This is 
important in limiting segregation of the alloy components. In casting 
applications, segregation can result in hot tearing as the metal shrinks 
in the mould as it solidifies. If the alloy is forged, the segregation 
results in microstructural inhomogeneity within the alloy. It has been 
found that the addition of very low levels of boron (i.e. up to 1%) 
refines the as-cast microstructure resulting in increased ductility and 
forgeability. The addition of niobium and zirconium (both beta-stabilising 
elements and zirconium is also gamma stabilizing) helps reduce or even 
eliminate the single alpha field in the phase equilibria. This allows heat 
treatments to be carried out over a wide range of temperature, whilst 
maintaining the fine-grained microstructure. This is achieved even in the 
absence of boron. The microstructure is further stabilised by the addition 
of zirconium and silicon, which results in the formation of silicide 
precipitates. 
The alloys of the present invention also exhibit excellent processing 
characteristics under hot deformation conditions. For example the alloys 
have good forgeability. 
By carefully combining the above alloying ingredients, a titanium aluminide 
alloy is; produced which has the desired strength, ductility and creep 
characteristics and a fine-grained microstructure which is retained after 
forging. 
DETAILED DESCRIPTION OF THE INVENTION 
Preferably the aluminium content of the alloy is 43-45 at %. 
Preferably the niobium content of the alloy is 3-5 at % . 
Preferably the zirconium content of the alloy is 3-5 at %. 
Preferably the boron content of the alloy is 0.2-0.5 at %. The inclusion of 
boron results in titanium boride (TiB) precipitates which at higher levels 
may segregate into clusters. This segregation has a detrimental effect on 
certain processing characteristics of the alloy and may result in 
components with poor fatigue characteristics and short operating lives. 
Such segregation is minimised at lower levels of boron inclusion. 
Inclusion of a minimum level of 0.3 at % boron results in further 
improvement of the processing characteristics of the alloy. 
Preferably the silicon content of the alloy is 0.1-0.3 at %. 
Most preferably said alloy consists of (in atomic %), 43-45 at % aluminium, 
3-5 at % nioblum, 3-5 at % zirconium, 0.2-0.5 at % boron, 0.1-0.3 at % 
silicon and the balance, apart from incidental impurities, is titanium.