Grain Refinement in High-Purity α-Iron Base Alloys during Thermomechanical Processing

Abstract

Thermomechanical processing involving severe plastic deformation (SPD) is a popular approach to ultrafine grain formation in bulk samples. In the present study, two grades of highpurity α-iron were deformed within the ferritic domain in cold and warm torsion to large strains (>> 1). Examination of the deformed samples using orientation imaging microscopy revealed a highly fragmented, lamellar structure aligned almost parallel to the direction of shear. Between 37 and 54 % of boundaries detected are high angle ones (HAB). Some of these HAB are associated with the original grain boundaries. However, a good number are believed to originate from dislocation accumulation processes, during which the misorientation angle across certain strain-induced low angle boundaries (LAB) rises with increasing strain. The resultant microstructure is composed of ultrafine crystallites on the order of 1 – 2 1m. In fact, localised regions of equiaxed grains on the micron scale were observed within samples deformed between room temperature and 300 °C. Nonetheless, other areas remain relatively unfragmented despite persistent straining until failure. At higher temperatures, the microstructure is more homogeneous, but the average grain size is coarsened. Optimal grain refinement thus appears to be a compromise between several competing factors: large strains at relatively low temperatures for high dislocation density, higher temperatures to enable sufficient dynamic recovery, and low grain boundary mobility that is aided by low temperatures and/or pinning by solute atoms or second phase particles. Furthermore, the development of a torsion texture composed of a single ideal orientation at large strains is unfavourable towards the generation of HAB.