Considerations on the selection of alloys for use in pressure cells at low temperatures

Issues surrounding the choice of metals subject to large tensile stresses in a cryogenic environment are discussed. Problems with the use of materials under such conditions often arise from their loss of ductility and toughness. The focus of the present article is therefore mainly on these properties; their dependency on temperature, crystal structure, grain size and purity level; and their application to the quantitative prediction of fracture behaviour. It also discusses the selection of alloys (on the basis of their toughness, ductility and freedom from flaws) that can be used safely under ambient and cryogenic conditions. A recent proposal to employ a particular titanium alloy for the above purpose is considered, in light of the tendency of metals with a bcc crystal structure (including β-phase titanium alloys) to become brittle at low temperatures. Other materials, which are known to be (or are likely to be) satisfactory under such conditions are examined. For example, certain platinum alloys may possess very high cryogenic tensile strengths and ductilities, and low magnetic susceptibilities. Nanocrystalline metals have the potential to provide these characteristics, as well as low specific heats and high thermal conductivities. Such properties could make nanocrystalline materials very useful in (for example) work at microkelvin temperatures, or more generally where very accurate cryogenic temperature control is needed. Sources of cryogenic mechanical property information on a wide range of materials are also presented.