Lattice distortion in nanocrystalline Fe powder studied by positron annihilation and X-ray diffraction

X-ray diffraction and positron annihilation measurements were performed on the ball-milled nanocrystalline Fe powder over a wide range of crystallite sizes (9-90 nm). With increasing milling time, the crystallite size reduction was accompanied by a monotonic increase of the lattice parameter of Fe (i.e. lattice expansion). The positron lifetime increased significantly due to enhanced positron annihilation from the defects (excess vacancies, vacancy-clusters, etc.) generated at the Fe grain boundaries and intercrystalline regions with progressive milling up to 24 h (crystallite size similar to 12 nm). The observed lattice expansion has been successfully simulated using a theoretical model taking account of the excess free volume associated with the excess vacancies/vacancy-clusters at the grain boundaries in nanocrystalline Fe. Prolonged ball milling up to 36 h (crystallite size < 10 nm) led to an anomalous decrease of all positron lifetime parameters. The X-ray diffraction line profiles of ball-milled Fe powder exhibited anisotropic broadening due to the high density of dislocations in Fe. Milling duration >= 24 h further led to asymmetric broadening of Fe diffraction peaks indicating heterogeneous dislocation structure in the severely plastically deformed ball-milled Fe. Further analysis of asymmetrically broadened peak reflections revealed deformation induced tetragonal distortion of body centered cubic Fe lattice in the 36 h ball-milled Fe powder.