An EBSD investigation of interstitial-free steel subjected to equal channel angular extrusion

The microstructural evolution of interstitial-free (IF) steel subjected to Φ = 90° Equal Channel Angular Extrusion (ECAE) for up to 4 passes via routes A, BA, C and up to 8 passes via routes BC was studied using Electron Back-Scatter Diffraction (EBSD). Routes BC and BA recorded the smallest grain size and aspect ratios and the largest average misorientation and area fraction of high-angle grain boundaries (HAGBs). During multiple passes, microstructure refinement continues until a convergence in effective subgrain and grain diameters occurs; following which the rate of HAGB formation reduces slightly. The percentage rise in the number of 3 Σ and random boundaries should be correlated with the operation of recovery mechanisms in ultrafine grained IF-steel rather than linking such special boundaries with twinning during ECAE. Compared to the scaling factor Hall-Petch (H-P) equation, the composite H-P equation indicates that although the low-angle boundaries (LAGBs) provide the maximum strengthening up to 8 passes, the contribution from HAGBs also increases with greater pass number.