Dimensional Stability

i have worked in machinery for years with atomotive, you can not use the same two kind of gears they will wear them selfs out. example timming gears, one castiron and the other steel.

Despite what the first poster wrote, and I am somewhat confused by what he wrote, you certainly can use similar materials in gear trains. It is done all the time. However, if the materials are not hardened or naturally hard, they will wear each other out quickly. Hard materials also need good lubrication and should be polished for a good surface finish, as they tend to be somewhat abrasive. On the other hand, a good practical alternative it to allow one meshing part to be a softer, sacrificial material, knowing that it will wear out faster and need to be replaced often. This is acceptable as long as replacement cost is low and it can be done quickly and easily. This is usually taken into condsideration in the overall design.

That being said, however, I don't think that your original question was properly addressed. Have you considered titanium, or do you not have the ability to form and work it properly? This is just about the ultimate in metallic material performance, which is why it is popular in aerospace parts. But it is difficult to machine, requiring special tooling and processing.

Some "stainless" metals that could hold up well, especially if heat is a factor, include Monel (Ni-Cu-Fe), Inconel (Ni-Cr-Fe), and other nickel-iron alloys, including several stainless steels.

Hardening of conventional ferrous materials often does create distortion and usually requires a finishing operation, such as grinding, to obtain final precision dimensions.

You have not described your application in any detail, so it is difficult to prescribe a more specific material selection.