Back in the 90s I did a PhD researching the high temperature mechanical properties of titanium aluminide (4722XD), principally for LP turbine blades and stators in gas turbines.
You certainly wouldn't want to use regular titanium alloys in any of the hot parts of a gas turbine (they've been used in exhaust components for years but it's relatively cool) due to their poor creep performance and oxidation properties.
I've lost track of developments but I know back in the mid/late 90s GE were intending to use titanium aluminide in the front fan assembly of the GE90.
The advantage of titanium aluminide is that it is an intermetallic, i.e. it combines the properties of a traditional alloy with some of the properties of, in this case, a ceramic. The material comes in several different compositions, some of which were quite frankly shocking...
In one incident I was setting up a test rig and attaching a thermocouple to a specimen when the sample broke at room temperature with only a very low load on it. Not a problem except the test machine nearly caved my skull in when the swing arm came crashing down.
Other batches of titanium aluminide were a lot more consistent, the variety I focussed on 4722XD (Ti-47Al TiB2) offered good creep resistance at the typical LP temperatures of 750C under the multiaxial stress conditions found in the blade root fixing.
The key thing with titanium aluminide, and any material in the engine, is to let the temperatures rise slowly and equalise themselves. You want to minimise the temperature gradients otherwise blades end up making unscheduled appearances. I'm not sure what the state of the art materials are these days but back in the 90s you'd have found single crystal blades made from CMSX4 in the HP turbine and various nickel based alloys from the Nimonic family used elsewhere. These materials have great creep resistance but are very heavy. From memory steel has a density of about 7800 kg m-3, Nimonics have a density of about 8200 kg m-3 and titanium alloys have a density of around 4500 kg m-3.
I've written all this from memory as I finished my PhD 10 years ago yesterday so apologies if some of it seems a little vague. If anyone is interested I'll have a look through my hard drive, log books, under the bed and see if I can find out anything else.
Last edited by Mr Grimsdale; 1st Dec 2008 at 10:21.