Titanium

In today's (2016) conventional technology Titanium is a good structural building material (light strong and corrosion resistant). But today titanium but it's rather expensive. Titanium is not at all a rare element but it is more distributed than other elements. More advanced mining techniques enabled by atomically precise technology will allow a significant drop in price.

Limits of corrosion resistance

Titanium in macroscopic form forms a stable self protecting oxide layer. If the machine parts are small enough such that oxide layer isn't much much thinner than the parts themselves (as in advanced atomically precise technology) then elemental titanium can't be used in direct contact with the atmosphere. Even in a practically perfect vacuum or in an nonreactive noble gas environment elemental titanium (as probably all metals in elemental form) may only be usable at very low temperatures where the atoms stay in place albeit the weakly directed metallic bonds.

Why titanium is one of the most useful metals

In advanced atomically precise manufacturing titanium will be very useful since titanium's oxidized forms with nonmetals (it's gemstones) usually have:

• very high hardness and
• very high melting points (See refractory material).

So they form very useful structural building materials.

Some simple titanium gemstones are: TiN TiP TiC TiSi₂ TiB₂ TiO₂ Ti₂O₃

• Aluminium the most common metal in eart's crust (thus more common than titanium) forms much viewer binary nonmetal compounds that are useful as structural building material (most useful leukosapphire Al₂O₃). sulfides nitrides and phosphides are no useful building materials here.
• Binary nonmetal compounds with the second most common element in earth's crust iron Iron are usually not very hard and metallic nontransparent (Fe₂O₃ Hematite, Fe₃O₄ Magnetite, FeS2 Pyrite).
• Binary nonmetal compounds with the common alkali metals Sodium and Potassium K are all water soluble or worse (reactive).
• Many of the binary nonmetal compounds with the common earth alkali metals Magnesium and Calcium are water soluble with a few exceptions (CaF Fluorite, MgO Periclase).

Stuff that reacted with water to a stable compound is usually too soft for structural building materials (there are exceptions).

Locations of occurrence and future usage

Titanium is especially abundant on the lunar lowlands. So there titanium might find more use than on earth. (See: [1])