Papers by Author: S.M. Lim

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.

Abstract: Multiaxial compression (MAC) is a severe plastic deformation (SPD) method that allows sequential uniaxial compression of prismatic samples to relatively large cumulative strains. The technique involves a change in loading direction (x to y to z to x…) between successive compression passes. A high-purity α-iron containing 60 mass ppm C was thus strained using passes of ε ∼ 0.4 at room temperature (0.16 Tm) and 450 °C (0.40 Tm) to total ε ranging from 1.4 to 2.9. Both optical and electron microscopy were used to characterise the deformed microstructures. Fragmentation of the initial grain structure occurs mainly in the form of a dense, homogeneous network of low angle boundaries (LAB) delimiting subgrains of about 1 3m. The original grains are easily distinguishable and maintain a relatively equiaxed appearance even at larger strains. At room temperature, high angle boundaries (HAB) are observed within some of the initial grains, and not necessarily close to the grain boundaries. These HAB may be open or closed, and tend to align themselves at approximately 45° to the orthogonal axes, suggesting the presence of microshear bands and thus a heterogeneous deformation. Such bands of localised strain criss-cross as a result of different slip systems being activated from one pass to another. When the temperature is increased to 450 °C, grain boundary migration becomes significant owing to the lack of impurities that could otherwise provide a pinning effect. The resultant subgrain structure is coarsened to about 4 3m. Besides, the enhancement of recovery at higher temperatures also appears to discourage the generation of HAB by dislocation accumulation processes.