Mechanically induced phase transformation from hexagonal close-packed structure to face-centred cubic structure in hafnium

High-energy ball milling induced hexagonal close-packed (HCP) to face-centred cubic (FCC) phase transformation in pure hafnium was investigated through x-ray diffraction (XRD) and transmission electron microscopy (TEM). Both XRD analysis and TEM observation present that the HCP to FCC phase transformation was accompanied by the reduction of grain size during ball milling. The proportion of FCC phase increased with the milling time, reaching 100% after 10 h milling, while the grain size decreased with the milling time, reaching an average grain size of less than 10 nm after 10 h milling. The relationship between grain refinement and phase transformation was analyzed thermodynamically, indicating that the FCC Hf is a small size stable phase. The theoretical energy barrier for HCP to FCC phase transformation in Hf can be overcome easily by energy releasing from the system when the grain size reaches less than 10 nm.