The rear end of the high-pressure compressor in an aero-engine is in a temperature environment set by the overall pressure ratio chosen for the engine cycle (figure 1). Since the 1950’s, this temperature level has risen by about 300°C. Titanium alloys have progressively improved in temperature capability up to 630°C, figure 2. This would allow most compressors to be designed completely in titanium. However, practice in the United States has been to switch at approximately 520°C to nickel alloys and incur a weight penalty.
Figure 1. Schematic of a gas turbine engine.
Figure 2. Progressive improvement of the temperature capability of titanium alloys has reached 630°C with IMI834
The development of IM1834 is a good example of the metallurgist's response to the needs of the designer. The requirements were for higher tensile and fatigue strength and enhanced creep performance. These were met by optimising the structural balance between primary alpha content and the transformed beta phase in the titanium alloy.
Producing integrally bladed discs, or blisks, is a natural progression in that the blade attachment features are deleted, resulting in significant weight and cost savings. For small engines the most economic manufacturing method is to machine both disc and aerofoils from a single forging. There may be a penalty to pay in that the material strength of the aerofoil may be reduced compared to that of a forged blade. Attention to the forging method and to the manufacturing processes can overcome this.
Metal Matrix Composites
Titanium metal matrix composites can be applied to both aerofoils and discs. The use of silicon carbide fibre offers about 50% more strength and twice the stiffness of the high temperature titanium alloys, combined with reduced density. Aerofoil design will benefit from the increased stiffness due to selective reinforcement, providing the ability to control vibration modes and blade untwist. Further exploitation of this technique will be with integrally bladed rings which are expected to provide a 70% weight saving relative to a conventional geometry in titanium.
Another material development project is the use of intermetallics. Compounds of nickel/aluminium and titanium/aluminium have been investigated with current emphasis on the latter. Most intermetallic compounds are brittle at room temperature. The first applications are therefore likely to be in small components such static and rotating compressor aerofoils where the advantages over titanium include higher specific strength and stiffness as well as improved temperature and fire resistance. The use of these materials could extend to more critical components. One possible application is as an alternative matrix to the titanium alloy in a metal matrix composite, although such an application will require alternative fibres, to minimise any thermal expansion mismatch, and novel processing technology.
Eventually, operating temperatures up to about 800°C will be possible, and intermetallics could offer a very attractive weight saving of around 50% compared with nickel-based alloys.