Atomistic origin of microstrain broadening in diffraction data of nanocrystalline solids

The origin of microstrain broadening in X-ray diffraction patterns of nanocrystalline metals is investigated by comparing data obtained from virtual diffractograms and from direct analysis of computer-generated samples. A new method is introduced that allows the local deformation gradient to be calculated for each lattice site in the microstructure from atomic coordinates obtained by molecular dynamics simulations. Our results reveal that microstrain broadening in undeformed samples cannot be attributed to lattice dislocations or strain fields near grain boundaries. The broadening arises, instead, from long-range correlated displacement fields that extend throughout the grains. The microstrain therefore provides a quantitative measure for distortions far from grain boundaries. This suggests that diffraction-based strategies for inferring the dislocation density in ultrafine-grained metals do not necessarily apply to nanocrystalline materials.