Titanium-X

Titanium-X (チタン-X) is a lightweight alloy used in X's construction. Nothing else is known about it.

Mega Man X
The intro of Mega Man X states that X's body skin is made of "Lightweight Titanium-X alloy".

Trivia

 * Titanium-X is probably an improved version of Ceratanium, considering that Titanium-X appears to be lighter than Ceratanium, since X weighs much less than Mega Man.