Musk tweeted in January that the rocket formerly known as BFR would be
built of stainless rather than carbon fiber.

So SpaceX is making a huge rocket out of stainless steel. This marks the first
time the material has been used in spacecraft construction since some
early, ill-fated attempts during the Atlas program in the late 1950s.
The state-of-the-art carbon fiber forming the body of the Starship rocket
(formerly known as the BFR, or Big Falcon Rocket, or Big F-other-word
Rocket) and its Super Heavy booster would be replaced by 300-series
stainless.

Elon Musk Quotes “We do have a great materials group, but initially we will
simply use high-quality 301 stainless. There’s an important other thing that
makes a big difference. For ascent you want something that’s strong at
cryogenic temperatures.”