Aluminium alloys are accepted for use in cryogenic service. 6061-T6 has the better mechanical properties. I suggest that you get access to a copy of "The Piping Handbook' by Nayyar. Contained within is a chapter on piping materials acceptable for cryogenic service. Generally speaking only stainless steels (304L & 316L) and Aluminum alloys are typically used. This is because these materials do not have a "nil-ductility transition temperature" like common carbon steels.