The Winter Soldier has a metallic arm, and being a Marvel property it's unlikely to be something as simple and straight-forward as aluminium or steel.
So what metal is it made out of?
Given the Marvel Universe's propensity for creating imaginary metals, we can deduce from its properties in the movie, Captain America: Winter Soldier, what metal it is likely to be.
Created by Hydra's finest scientists sometime after 1945, likely upgrading it every time new technology became possible, we can presume by 2014 it is composed of the best technology money can buy.
The Arm gives the Winter Soldier superhuman strength on par or even greater than serum-enhanced, Captain America. The armor is finely articulated with a hardened coating protecting technology within.
The arm is super-strong and still has an articulated surface that can be removed as necessary.
given the relatively thin coating of shielding metal it needs to be quite tough.
The arm has blocked small and medium arms fire from handguns and assault rifles.
It needs to be a metal tough enough to be hardened against gunfire.
Adamantium or Secondary Adamantium: Not likely, seeing how it was able to be damaged. A prosthetic made of adamantium would not be able to be harmed by any sized, man portable weapon and would resist armor-piecing rounds as well. (unless they were composed of armor piercing adamantium). Not even sure if adamantium exists in the MCU.
Carbonadium: No. While it has excellent damage resistance and the next contender when you can't get adamantium. Carbonadium is unfortunately highly radioactive and unsafe for anyone to be around for any length of time. Since no one is wearing a hazmat suit while working on the Soldier's arm, we can assume Cabonadium is off the table.
Vibranium: While not considered particularly tough in the canon Marvel Universe, Vibranium seems quite durable in the Marvel Cinematic Universe. Captain America's shield in the MCU is not primary adamantium but a vibranium alloy. The possibility the arm is made (or coated) in vibranium, is interesting but not likely given Howard Stark's assertion that Captain America's shield was all the vibranium in the world.
Titanium alloy: Our most likely candidate: Strong, light, durable and tough, we know titanium alloys were used in the canon Marvel Universe. The android Jocasta was compose completely of hardened titanium alloys. She was as tough as any technology could make her without extraordinary materials.
Titanium alloy: This would be a good choice. It's a strong metal when used in steel alloys, able to be hardened and most importantly it would be relatively easy to secure. And it might still be a shade lighter than pure steel alloys.
Aluminum alloy: While there are aluminum alloys with comparable strength to steel, the benefit of using Aluminum versus steel would be its lower comparable weight to steel. The weakness is the lower defensive rating you would get from aluminum. It might be less likely to be an effective defense against larger ammunition rounds without deforming.
Steel Alloy: We know steel alloys are common in the Marvel Universe. Some of the earlier Iron Man armors are composed of hardened steel. Able to be made quite tough, steel is usually choice when weight isn't an object.
Given the speed and naturally-seeming movement, while steel would be the most desirable defense-wise, it would not likely be as fast as the Winter Soldier's organic arm, potentially handicapping his fighting capacity. I suspect this would have been the most common material his arm would have been made of until the late 1970s.
Also titanium is used IRL in a lot of surgeries, spinal fusion in particular, for many of the reasons already listed (light, extremely corrosive resistant, strong)
Also, since its canon that the Winter Soldier was the one who killed JFK and Magneto was attempting to save JFK, it would make sense in part why Magneto was unsuccessful- titanium is very weakly magnetic. Its also nice IRL because people who have hip replacements and have had spinal fusions don't have to worry about setting of metal detectors in an airport.
Cap's shield in the comics is made out of a steel alloy and vibranium mix under some unknown circumstance. Adamantium came later as a result of trying to duplicate the process. So, you might say Cap's shield was a mix of Proto Adamantium like Kurt Busiek once said and Vibranium. As for the Winter Soldier, I would say it is best to say it is some type of mix of the latest Russian and Hydra technology has to offer.
I think with all my brain into it...that winters arm is made of 60% of steel and 40% titanium. Because looking at the durability mainly during fights and the strength however comes with steel.... titanium is powerful and a strong metal, but when it comes to do heavy duty works most engineers opt for steel.
So winters arm is mostly of steel and titanium to 40% ratio in it. Even it was shown in iron man movies when bullet fired from assault rifle there are small piercings visible after a fight as it is mostly of titanium. Whereas Iron Monger is made of mostly steel so his suit resists more damage than Iron Man's.
The arm is probably coated in a Titanium alloy and the technology inside is probably Adamantium or Secondary Adamantium or possibly even Vibranium.
Thank you for your interest in this question.
Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
Would you like to answer one of these unanswered questions instead?