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, vacancyclusters, etc.) generated at the Fe grain boundaries and intercrystalline regions with progressive milling up to 24 h (crystallite size ∼ 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.