What kind of material is the Robert Downey Jr. Iron Man's armor supposed to be made from? I know he crafted the prototype from the things he had on hand, but once he was able to build the next one(s) in his own lab, what hi-tech material did he use for the movie armor?
Cinematic Iron Man (Earth-19999) likely needs to use a variety of materials for his armors. His first suit was certainly made of scrap iron, but his later suits would need to be far lighter and stronger to be able to survive the stresses the were subjected to. He will need materials that are heat resistant, strong, resilient and stress tolerant but not all in the same measures.
The chest and dorsal regions, where he will take the bulk of damage because it has the widest surface area, will need to be the toughest part of the armor. He will be using titanium-nickel alloys with a touch of vibranium if he can get it. (to radically reduce impact damage). His flight surfaces (seen whenever he is maneuvering) will need to be even stronger since they will have to bear great stresses whenever he turns at high speeds and will likely be made of crystalline titanium (see below).
Since the suit has to be layered, inner layers will likely contain padding, ballistic mesh and other like materials to offer some level of cushion for the passenger. It is also theorized there must be some sort of antigravity and force field technology to reduce damage dealt to the suit. Otherwise, Stark should suffer far greater contusions from impacts and explosions. Though it is never mentioned it must be inferred since ancient armors had to pad the user to protect them from concussive force.
Areas requiring heat resistance will likely use carbon nano-tubes to channel heat away from the wearer. The bootjets, the repulsor areas and the chest unibeam cavity all likely use carbon-nanotubes materials. The material is brittle so it may end up being part of a coating to reduce heat buildup.
Likely the rest of the suit, the arms and legs will use crystallized titanium common to high velocity aircraft engines. Durable but expensive, capable of taking variances in temperature and surviving extreme stresses. Seeing how important limbs are, Stark would likely want the most protection possible without adding too much weight.
All the materials mentioned are known to exist on modern Earth as we know it. It is also possible materials and techniques as yet unknown are used in the design of the Iron Man armor since it is currently beyond our technical capability as yet to build or design.
Over time, subsequent armors will have surely been made from different materials and composites depending on the purpose and required properties of the armor, but the Mark 2 and Mark 3 armors are at least primarily a gold titanium alloy.
From the first movie:
Tony Stark: [reading the newspaper] Iron Man. That's kind of catchy. It's got a nice ring to it. I mean it's not technically accurate. The suit's a gold titanium alloy, but it's kind of provocative, the imagery anyway.
I found the following information on this site: http://marvel.com/news/story/12004/marvel_science_iron_mans_armor
According to it, Iron Man's armor is made out of some combination of alloys, the most traditional solution being nitinol, titanium and nickel. It is extremely strong, has a high resistance to heat, and it even "heals" itself (if heated until it bends, it can be heated back into position).
That's the "real life and traditional" scenario.
Look up Iron Man Armor in wikipedia. It has an extensive list of all the suits of Iron Man with the materials and purposes.
However, it only deals with the comic-book armor unless I missed something. I'm going to assume that you're talking about the armor from Iron Man and Iron Man 2? (Don't ask about 3, I haven't seen it yet)
I do believe that his suit is made out of a combination of the stuff that is used in Captain America's shield. It's not stated but some Marvel universe metal is used, that I know for sure.
The latter part was a bit speculative on my part and apologize for any errors or mistakes.