By cryogenics I meant the techniques that are uses to cool superconducting magnets in magnetic resonance imaging machines and so on. As I understand it, it's a general term that has been hijacked by the freeze-you-when-you're-dead bunch but it just encompasses the general field of the use of very cold liquids. I'm no expert and I may be using incorrect terminology but I think I'm just repeating what I've read. Helium is used in all sorts of medical, industrial and space applications many of which use quite significant amounts.
RE Embrittlement: I'm not a materials scientist or a chemist so I can only turn to wikipedia:
The mechanism starts with lone hydrogen atoms
diffusing through the metal. At high[
clarification needed] temperatures, the elevated solubility of hydrogen allows hydrogen to diffuse into the metal (or the hydrogen can diffuse in at a low temperature, assisted by a concentration gradient). When these hydrogen atoms re-combine in minuscule voids of the metal matrix to form hydrogen molecules, they create pressure from inside the cavity they are in. This pressure can increase to levels where the metal has reduced ductility and tensile strength up to the point where it cracks open (
hydrogen induced cracking, or HIC). High-strength and low-alloy steels,
nickel and
titanium alloys are most susceptible.
High strength nickel-steel alloys and titanium are exactly what spaceships/spaceplanes tend to be made of. More specifically for Reaction Engines, their heat exchanger that cools incoming air is made of very fine tubes with walls that are thinner than a hair (makes the heat transfer efficient) and to be strong enough they are made of a nickel alloy. If you ran hydrogen through them, they'd become brittle apparently. But it's the hydrogen fuel that is the heat-sink so there has to be some other fluid to transfer heat from the incoming air to the fuel. Helium is best, apparently.
Sorry for the digression.