The Refinement of Grain Structure in a High-Purity α-Iron Base Alloy under Multiaxial Compression

Publication suite au congrès : Int. Conf. on Processing & Manufacturing of Advanced Materials (THERMEC'2006)International audienceMultiaxial 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 µm. 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 µm. Besides, the enhancement of recovery at higher temperatures also appears to discourage the generation of HAB by dislocation accumulation processes