Characterization of thermal-induced distorted face-centered cubic structure in pure titanium

After 900 degrees C high-vacuum annealing and air cooling, a special band was observed in pure titanium. Energy-dispersive spectrum showed that the composition of the band and the matrix is the same. By tilting the band to different zone axes and taking selected-area electron diffraction (SAED) patterns, we found that the structure of the band is distorted face-centered cubic (fcc) with the lattice constants a, b, and c deviating slightly from each other on the basis of the analyses of the SAED patterns together with the tilting angles. The lattice constants a, b, and c are calculated based on the measurement of interplanar spacing according to high-resolution transmission electron microscopy (HRTEM) images. The orientation relation between the hexagonal-close-packed (hcp) matrix and the distorted fcc band is [112< according to SAED analysis. The phase boundary between the matrix and the band was observed to demonstrate serrated configuration composed of two kinds of micro-steps: on the basis of Cs-corrected HRTEM characterization. The distorted fcc band is deduced to be a thermal-induced metastable phase transformed from the high-temperature body-centered cubic (bcc) structure during the allotropic transformation from bcc to hcp based on orientation relation analysis.