Microstructure evolution and grain growth kinetics in annealed nanocrystalline chromium

The kinetics of thermal evolution of the microstructure of nanocrystalline chromium (nano-Cr) has been studied by time-resolved synchrotron radiation techniques: high-resolution powder diffraction and small-angle X-ray scattering (SAXS). The as-prepared electrodeposited nano-Cr with average grain size of 27 nm shows the same bcc structure as alpha-Cr. The nano-Cr cubic lattice parameter thermal expansion is the same as that of reference polycrystalline alpha-Cr. Annealing of nano-Cr at temperatures above 400 degrees C leads to a grain growth process with the final grain size not exceeding 125 nm even at a temperature of 700 degrees C. The single power-law behavior is observed by SAXS in as-prepared nano-Cr changes during annealing above 400 degrees C. In nano-Cr samples annealed at temperatures between 400 and 700 degrees C, the low-q part of the SAXS signal shows a Porod-type behavior while the high-q part shows a power-law Q(-alpha) with the exponent alpha < 4. This effect is probably due to changes of the grain surface roughness during grain growth.