What kind of metal is the Terminator T-800 model made of? Is it titanium or some other metal?
The T-800's chassis is typically said to be made of an unspecified "hyperalloy". For example:
The Terminator's an infiltration unit. Part man, part machine. Underneath, it's a hyperalloy combat chassis, microprocessor-controlled, fully armored.
Kyle Reese, script from The Terminator
Terminator: The Sarah Connor Chronicles suggests that T-800s are made out of coltan:
John Connor: That's coltan; if that's what you're made of, no wonder you're so dense.
Cameron Phillips: Not density, heat resistance. T-600 models had a titanium alloy endoskeleton, but it was vulnerable to heat. Coltan alloys have a much higher melting point.
Terminator: The Sarah Connor Chronicles, Episode 104: Heavy Metal, quoted from the Terminator Wiki
Note, however, that Cameron is a T-900 series so it's possible the T-800s are not made out of coltan and instead use the titanium alloy that the T-600s used.
The exact composition is not specified. The T-800 is made of Hyperalloy, which is a durable metallic alloy of unknown composition that provides the combat chassis and endoskeleton of several Terminator series(including the T-800).
Both Hyperalloy and Polyalloy are very interesting concepts (that could be scientifically-valid even with today's complex technology), but there is no mention of the exact molecular composition of either of them.