Strain mapping in a deformation-twinned nanocrystalline Pd grain

Strain in a deformation-twinned nanocrystalline Pd sample of about 24 nm grain size was mapped by means of geometric phase analysis based on an individual high-resolution transmission electron microscopy image. The in-plane components of the strain tensor were calculated and charted. Strains with magnitudes of about 0.8% were found in the grain interior. Twins and matrix were significantly distorted relative to each other (by about 3° on average) and showed a strong rotation gradient from top to bottom, revealing that the whole grain is bent. An estimate of the strain energy stored in the Pd grain yielded a value of Estrain=4.157Jg-1. Based on the strain distribution observed, a temporal deformation scenario has been developed. In our judgement, deformation twins had formed first and subsequently dislocations were activated, most likely by the misfit strain/stress concentrations generated by the twins themselves. The interaction of the dislocations with the twin boundaries left behind Shockley partials and this accounted for the strain concentrations finally observed along the twin boundaries. It is concluded that in the temporal evolution of deformation in a nanocrystalline material, twinning must be by far the fastest deformation mode, which accounts for the abundance of deformation twins, especially at high strain rates.