Structure evolution and thermal stability of SMAT-derived nanograined layer on Ti–25Nb–3Mo–3Zr–2Sn alloy at elevated temperatures

Highlights: ► Stability of a nanolayer with pure β phase during thermal annealing was studied. ► The α phase precipitating along the boundaries of β grains was observed. ► The β grains exhibited good thermal stability due to the pinning effect of α phase. ► Recovery and recrystallization were observed prior to significant growth of β grains. ► Nonequilibrium GBs in the layer led to low grain growth activation energy calculated. -- Abstract: A nanocrystalline layer, which consists of pure β phase with average grain size of about 30 nm on Ti–25Nb–3Mo–3Zr–2Sn alloy, was fabricated by surface mechanical attrition treatment (SMAT). The microstructure evolution, phase transitions and thermal stability of the nanocrystalline layer during isochronal annealing at temperatures of 300–600 °C were investigated by XRD and TEM. During annealing, the nanocrystalline layer underwent recovery, recrystallization and significant grain growth. Recovery occurs at temperatures from 300 to 400 °C resulting in strain relaxation, dislocation annihilation and a very limited growth of β grains dominated by re-ordering of grain boundaries (GBs). α Precipitates start to precipitate along the β GBs at 350 °C, which exhibit equiaxed morphology with size less than 10 nm. Recrystallization proceeds at a higher temperature up to 450 °C leading to a reduction of β grain size. Due to the pinning effect of the α precipitates, β grains sizes can be maintained less than 100 nm until the annealing temperature increased to 550 °C, e.g., 0.41 Tm (melting temperature in K), above which significant grain growth occurs to overstep nanocystalline scale, and the growth activation energy of β grains is calculated as 40.8 kJ/mol. The abnormal low value is caused by the nonequilibrium GBs of β grains in the as-SMATed surface layer, which increase atomic mobility even in the presence of pinning effect of the α precipitates on the β GBs