Titanium combustor fires in jet engines

Above 900°C, titanium dissolves its own oxide and burns like a magnesium flare; engines have been completely burned out.

So aerospace — it's relatively lightweight. It's not as light as aluminum, but in terms of melting point and temperatures — and because it's lightweight and can be superplastically formed, we use it on the compressors of jet engines. We can make extremely complex structures. But it is very reactive. If you get above 900 degrees centigrade — it's very corrosion resistant, it has reasonably good oxidation resistance up to 900 degrees, but above 900 degrees it will dissolve its own oxide, and all of a sudden it catches fire. It's like a great big magnesium flare. They've had engines go — just a great big ball of fire — and end up with a burned-out engine. Completely burned out. Not a good day. That's why you have multiple engines. It's also why you should control the way you operate the engine, so they're not designed to get to those temperatures. But some good old Air Force pilots have been able to do it.

Compressor blades are titanium. Spike to 900°C ignites titanium like a flare, melts the engine core. ## Figures referenced

People have tried niobium alloys, but to make an oxidation-resistant niobium — if you know anything about the reactivity of niobium with oxygen, I could have told the board not to spend $18 million on that stupid project. They spent the $18 million and they got nothing. People have spent hundreds of millions of dollars trying to come up with higher-temperature alloys. But these refractory-metal alloys are very reactive. Sometimes they're called refractory alloys, sometimes they're called reactive alloys. Titanium, you get above 900 degrees and it just will go on fire like a flare. They've burned out engines because the compressor blades are made out of titanium. If you don't run the engine properly, instead of the compressors being at seven or eight hundred degrees, they can get a temperature spike up to 900 degrees, ignite the titanium, and now you just melt through your engine and end up with an empty core. It doesn't fly very well at that point.

Above 900°C, titanium consumes its own protective oxide, exposing fresh metal to hot compressed air; engine becomes its own fuel. Several Air Force incidents. Used to motivate the metallurgy of why titanium diffusion-bonds so well (the oxide *will* dissolve into the substrate at temperature). ## Figures referenced (not cases)

In titanium — our poster child for diffusion bonding — above 900° centigrade titanium will consume its own oxide. In fact that's been a problem in the titanium jet engine business. They've actually had titanium fires in the engine if they run the engine improperly and they get above 900° centigrade. The protective titanium oxide skin diffuses into the titanium. Now you expose fresh titanium to hot compressed air, and you start a fire. It's like having a flare go off in your engine, only the fuel is the engine. You end up with a hollow shell with a bunch of white powder all over everywhere — it's called titanium dioxide. This was a big concern. It happened several times. The Air Force was not happy. The commercial guys were worried it was going to happen to them, but it really was because people were pushing their engines too hard.

Why titanium can't be used in the hot section above 900°C — it ignites by dissolving its own oxide. Cited as causing "a number of fires about ten years ago in aircraft engines."

However, there are cases where this oversimplified version of cost breaks down, and that's where we get collateral weight savings. [Tom produces a jet turbine disc.] People have been trying for years to make a jet turbine disc that does not have to have the big heavy flange and the mechanical attachment. This is a nickel-base superalloy, and this one I actually have because it sort of broke. This was a land-based turbine that generates electric power. You can tell by passing this around that most of the weight is down here in the root — the mechanical attachment. This was probably only spinning at six or seven thousand rpm, but that's a lot of centrifugal force. If we had a material that was lighter weight, we'd love to use it. In the compressor section, where we only go to eight or nine hundred degrees centigrade, we do use titanium. But in the combustor section, the hot section of the engine, we don't use titanium. The reason is, titanium tends to ignite above 900 degrees centigrade — it dissolves its own oxide and no longer has the protective oxide. There have been a number of combustor fires in jet engines where the titanium ignited. It's exactly like a sparkler going off; it's exactly the same thing as a flare. We use titanium powder or aluminum powder or magnesium powder in flares to give us the big bright light. Once that fire starts, it's all over — you can't put it out. There were a number of fires about ten years ago in aircraft engines. It's a bad day for the engine when the engine ignites that way.