Comparative Study of Mechanical and Functional Properties of Age-Hardened Superelastic Ti–Zr–Nb Alloy with Different Initial Microstructures

The study examined the effects of aging on the mechanical and functional properties of thermomechanically treated superelastic Ti–18Zr–15Nb (at.%) bar stock with different initial microstructures. The objective was to enhance straightness and maintain excellent superelasticity. Aging was conducted over a wide range of temperatures (200–350 °C) and times (10–1200 min). Aging at 250–350 °C led to the emergence and growth of X-ray diffraction lines corresponding to the isothermal omega (ω_iso) phase. TEM analysis showed that the average size of ω_iso particles was approximately 3.5–4 × 7–9 nm, regardless of the initial treatment. The bar stock with a mixed statically polygonized and recrystallized structure (grain size of 2.9 µm) exhibited the highest strength, attributed to the increased concentration of ω_iso particles along the subboundaries. Young's modulus (E) increased significantly by 15%–30% after aging above 250 °C. The superelastic recovery strain deteriorated significantly after aging at 200 °C for 60 min, decreasing from 2.5%–3.1% to 0.6%–1.1% across all alloy conditions. The optimal aging condition, where superelastic behavior and Young's modulus remained unchanged while the apparent yield stress (σ_0.2) increased, was achieved at a temperature of 250 °C and an aging time of 10 min.