X-ray diffraction study on a nanostructured 18Ni maraging steel prepared by equal-channel angular pressing

Starting from a hierarchically substructured, heavily dislocated, and highly alloyed martensitic structure, an 18Ni maraging steel was deformed by four passes of equal-channel angular pressing at ambient temperature. X-ray diffraction peak profile analyses according to the modified Williamson–Hall and Warren–Averbach methods were used for determination of apparent grain size, dislocation density, and character of the prevailing dislocations, aided by supplemental transmission electron microscopy. A mean grain size of about 60 nm was determined, corresponding reasonably to the mean dislocation cell size illustrated by means of transmission electron microscopy. Furthermore, a dislocation density of 1.3 9 1016 m-2 along with an about 5:1 ratio of screw to edge type dislocations were identified. A dislocation arrangement parameter larger than unity was determined for the present deformed structure, representing a weak dipole character of the dislocation structure and weak screening action of the strain fields of multiple dislocations.