Microstructural refinement of an Fe–C alloy within the ferritic range via two different strain paths

One of the most popular approaches to ultrafine grain formation within bulk samples is severe plastic deformation (SPD). In this paper, grain refinement by two SPD techniques involving different strain paths, namely multiaxial compression (MAC) and torsion, has been investigated. A coarse-grained, high-purity iron–carbon alloy was deformed within the ferritic domain under cold and warm working conditions to large strains (≫1). Subsequent analyses by orientation imaging microscopy revealed that the nature and extent of refinement depend upon the processing route. In MAC, fragmentation occurs mainly by means of a dense, homogeneous network of low angle boundaries (LAB, 2 ≤ θ ≤ 15°) delimiting subgrains (∼1 μm), and only a few strain-induced high angle boundaries (HAB, θ ≥ 15°) are observed in localised regions of cold worked samples. In torsion, however, the proportion of HAB is significantly higher (37–54%), giving rise to a fragmented microstructure comprising ultrafine crystallites (1–2 μm).