Answer
A286 (iron-base superalloy) is chosen for wave springs operating between $700^{\circ}F$ and $1000^{\circ}F$ due to its resistance to stress relaxation. Stress relaxation is the time-dependent decrease in load under a constant deflection, primarily driven by dislocation climb and grain boundary sliding. For A286, the precipitation of $\gamma'$ ($Ni_3Ti$) particles during the aging process ($1325^{\circ}F$ for 16 hours) pins dislocations, reducing the relaxation rate. If a wave spring is designed for a $100$ lb preload, exposure to $900^{\circ}F$ for 1000 hours might result in a 5-10% loss in load. Engineers must over-design the initial preload using the Arrhenius equation to predict the 'end-of-life' load, ensuring the system remains functional despite the inevitable loss of spring force over time.
