Official Answer
Answer
Beryllium Copper (Alloy 25) is selected for spiral retaining rings when non-magnetic properties or high electrical conductivity are required. It also maintains its ductility and strength at cryogenic temperatures, unlike many carbon steels that become brittle. CuBe2 is heat-treated to an 'AT' or 'HT' temper to achieve tensile strengths up to $200$ ksi. Its lower Modulus of Elasticity ($E \approx 18.5 \times 10^6$ PSI) compared to steel ($30 \times 10^6$ PSI) means the ring will have less 'cling' force for a given deflection, which must be compensated for by increasing the wire thickness or the initial interference fit during design.
