On the influence of N on residual microstrain in cryomilled Ni

The factors that influence the development of residual microstrain during milling in a liquid nitrogen atmosphere, defined hereafter as cryomilling, are investigated. The residual microstrains in cryomilled Ni, processed under various cryomilling conditions, were examined by X-ray diffraction (XRD) and analyzed through the single line approximation (SLA) method. The average residual microstrains are determined to be in the range of 2×10⁻³ to 6×10⁻³. The residual microstrain on the (200) plane is higher than those on the other planes by 33 pct. The residual microstrain and its anisotropy in Ni are reduced after heat treatment at 800 °C for 1 hour. The measured microstrain is proposed to evolve from the presence of N and O as impurity atoms in the Ni lattice. Both N and O are introduced from the environment and then their solubility in Ni is enriched via the generation of defects that occurs during cryomilling. The stable site for N and O atoms in Ni is the octahedral site, and the sizes of N and O atoms exceed those of the octahedral site of Ni by 48 and 16 pct respectively. Accordingly, a lattice strain field is expected around interstitial N atoms that are located at octahedral sites. By comparing the crystal structure around the octahedral site, the stable site for impurity N atoms, in the Ni lattice with that of Ni₃N structure, the lattice strains are estimated to be in the range of 5 to 15 pct. The result shows that the (200) plane has strains that are 2 times higher than those in other planes, and this is argued to be the reason for the measured anisotropy of residual strain in Ni after cryomilling.