Even the factory titanium pipe is good, and although titanium is a great material, there really arent very many (if any at all) headers I've ever seen made of titanium, and this is part of what I do for a living.... hhmmm
I'm not sure if it's actually an alloy of some sort, but straight titanium is brittle, and can't withstand certain extreme conditions. Just like anything else, if it can be broken.
Our system uses a stainless/titanium alloy (321) which is a very expensive alloy, and most commonly used in turbo headers, but will provide the best resistance to the rapid heating and cooling cycles a sled header can see.
A factory header can be welded back together and will live for a short time, but after modifying factory headers in the design phase of our testing, I've broken several headers shortly after welding, and it always breaks next to a weld. It's too brittle to expect it to last after welding, and not many of us can replicate the factory process of welding.