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

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.