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 beta 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 degrees 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 degrees C resulting in strain relaxation, dislocation annihilation and a very limited growth of beta grains dominated by re-ordering of grain boundaries (GBs). alpha Precipitates start to precipitate along the beta GBs at 350 degrees C, which exhibit equiaxed morphology with size less than 10 nm. Recrystallization proceeds at a higher temperature up to 450 degrees C leading to a reduction of beta grain size. Due to the pinning effect of the alpha precipitates, beta grains sizes can be maintained less than 100 nm until the annealing temperature increased to 550 degrees 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 beta grains is calculated as 40.8 kJ/mol. The abnormal low value is caused by the nonequilibrium GBs of beta grains in the as-SMATed surface layer, which increase atomic mobility even in the presence of pinning effect of the alpha precipitates on the beta GBs. (C) 2012 Elsevier B. V. All rights reserved.