Ultimate Marvel's changes to the company's classic costumed catalog ranged from the massive to the inconsequential, but some of the most intriguing differences were ones rarely brought up in the canon at all. While the metallurgical composition of Captain America's shield is a subject of much focus and discussion in the mainstream Marvel Universe, the same question is far more of a mystery when it comes to the 1610 Ultimate Marvel continuity.
Steve Rogers' iconic shield is not only symbolic of the nation whose flag adorns it, but of the hero, the team he fights with, and the heroes of Marvel as a whole. As such, it is only natural for it to become the focal point for so much analysis, and no matter the setting it always proves a powerful weapon that flies in the face of physics. It can withstand almost any impact, ricochet numerous times seemingly without losing momentum, and it even serves as a powerful melee weapon in the right hands. The Ultimate shield is no exception.
Where the Ultimate shield does diverge is that its actual composition never explicitly comes up. While there is rampant misinformation about the shield online about it being made of either vibranium or adamantium, at no point is such a thing outright stated. In fact, Ultimate Captain America cycles through 4 shields that only have slight deviations from one another and none of them have their material components explained.
Given that the shield is featured in numerous stories about Captain America, there are plenty of stories to examine. In World War II, Captain America fought with a shield that was shaped differently from any other, more triangular than circular. Nevertheless, the shield is as bulletproof as any other, embeds into stone, and seems remarkable enough that a Soviet program to reproduce their own super-soldier could not recreate the shield. Certainly, something seemed special about it, but that specific special quality remains a mystery.
Leaving plenty of room for pure speculation, all subsequent shields are as much of a question. When Captain America returns to service in the modern-day he begins using his standard circular shield which behaves much the same way as his previous one. Its origins or the fate of his original shield are never discussed, and it is destroyed when the magically-altered Valkyrie breaks the shield in half with her sword. Eventually, Thor provides Captain America with a replacement "forged by dwarven blacksmiths," but that shield is destroyed soon afterwards by Howard Stark wielding the magical weapon Mjolnir against it. Without explanation, Steve begins using a replacement thereafter, but there weren't many more details given out about that shield either.
For such an important figure as Marvel's noble leader, you would think the question would come up more often, but instead various sites proliferate misinformation about the shield that is not grounded in the comics themselves. Ultimate adamantium is remarkably weaker than its mainstream counterpart while vibranium almost never comes up in the 1610 universe, and yet somehow both materials are baselessly proposed as the metal behind the marvelous feats of Ultimate Captain America's shield
Given that the U.S. sources seem capable of reproducing the materials behind the shield, there is certainly something unique going into its design. At the same time, it is replaced easily enough (and with little narrative attention) that the shield is not truly one-of-a-kind. It may just be one of the Ultimate Universe's most intriguing lingering mysteries, but until a future writer picks it up a mystery may just be all that it remains.