Answer
At $1000^{\circ}F$, Inconel X-750 is the superior choice for wave springs due to its high nickel content (70% min) and the formation of a stable $Cr_2O_3$ protective oxide layer. It maintains its spring properties and resists creep through $\gamma'$ precipitation. A286 (an iron-base superalloy) is also suitable up to $1000^{\circ}F$ but is more prone to oxidation and loss of modulus at the upper end of that range. The modulus of elasticity $E$ for Inconel X-750 drops from $31 imes 10^6$ psi at room temperature to approximately $25 imes 10^6$ psi at $1000^{\circ}F$, a factor that must be included in the rate calculation ($k \propto E$). A286 is often chosen for its cost-effectiveness when the extreme corrosion resistance of Inconel is not required.
