Effect of Y addition on the structural transformation and thermal stability of Ti-22Al-25Nb alloy produced by mechanical alloying

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Abstract In this study, a Ti-22Al-25Nb alloy with nanocrystalline structure was produced by high energy mechanical alloying (HEMA) and 1 at.-% yttrium was added as a thermal stabilizer. The as-milled samples were annealed at various temperatures up to 900 °C in a protective gas atmosphere, and the samples were allowed to cool to room temperature in the furnace. The phase transformations and microstructural changes as a function of the annealing temperatures and alloy compositions were studied using room- and high-temperature X-ray diffraction (XRD), focused ion beam microscopy (FIB), and scanning electron microscopy (SEM). The mechanical properties of the samples were interpreted based on the hardness results and their correlation with the microstructures. The results showed that the as-milled nanocrystalline structure of Ti-22Al-25Nb alloy increased from 3.4 nm to 350 nm after annealing at 800 °C due to the high driving force induced by the large grain boundary area. Consequently, the as-milled hardness of the Ti-22Al-25Nb alloy dropped from 7.63 ± 0.18 GPa to 5.37 ± 0.28 GPa. The grain size stability of the Ti-22Al-25Nb alloy after annealing at elevated temperature was ensured through the addition of yttrium. Thus, the grain size remained at the level of 125 nm, and the hardness value was maintained at around 6.98 ± 0.43 GPa after annealing at 800 °C.