Deformation characteristics of ultrafine grained and nanocrystalline iron and pearlitic steel - An in situ synchrotron investigation

The deformation characteristics of ultrafine grained (UFG) iron, ultrafine lamellar (UFL) pearlite and nanolamellar (NL) pearlite were investigated using in situ synchrotron compression tests. The direct measurement of the peak position and peak width of the transmitted diffraction rings provided insights into the correlation between the strain hardening and the evolution of coherent domain size and dislocation density. The results showed that the hard cementite supported the initial hardening in UFL pearlite, while the dislocation density in the ferrite lamellae gradually increased. At higher stresses, the cementite lamellae deformed plastically or fractured, thus transferring the hardening process to the ferrite lamellae. On the other hand, in UFG iron and NL pearlite an initial high strain hardening was observed due to strain path change and redistribution of dislocations. The nanolamellar structure of pearlite further introduced a strong anisotropy in yield behavior, without affecting the hardening characteristics of ferrite lamellae. The physical origin of the anisotropy in flow behavior of NL pearlite was identified as the importance of shear stress on dislocation motion within the ferrite nanolamellae. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.