For places that get hot, like the engine, firewalls and ducts, titanium is used. Titanium does not bend at high temperatures like steel and aluminum.

Another material used in airplanes recently is "composite material". Composite materials are layers of fibers glued together. The fibers are made of boron or graphite. When glued together they are very, very strong and light in weight.

The price or cost of composite materials is very high. One reason is because they are very difficult to make. The people making them must have a lot of experience. As more and more airplane parts are made from composite materials the cost will go down.