F. Wetscher

Cyclic high-pressure torsion of nickel and Armco iron

Cyclic high-pressure torsion, a modified version of high-pressure torsion, is applied to Armco iron and nickel. The results in terms of microstructure and flow stress are compared to samples deformed by conventional high-pressure torsion. For both processes and both materials, a saturation in the decrease of the structure size and the increase in the flow stress is observed. The minimum size of the structural elements which is obtainable is smallest for the conventionally high-pressure torsion deformed samples and increases with decreasing strain per cycle in cyclic high-pressure torsion.

High Pressure Torsion of Rail Steels

To study the influence of shear deformation on the evolution of the microstructure and the mechanical strength in rail steels, three steels with different microstructure (two pearlitic, one bainitic) were deformed by High Pressure Torsion (HPT). In order to evaluate in addition the effect of the strain path, a cyclic form of HPT was applied. The mechanical strength was determined by means of in-situ measurement of the flow stress and microhardness measurements. The differences of the mechanical strengths between the monotonic and cyclic deformed samples clearly indicate that a monotonic deformation promotes higher dislocation densities and leads to the assumption that dissolution of the cementite takes place more pronounced.

Differences in Structural Evolution in Single- and Dual-Phase Materials during Severe Plastic Deformation

Severe plastic deformation (SPD) has been applied to two classes of metallic materials, single phase and dual phase materials. The applied shear strain has been varied between 1 and 1000 and the homologous temperature between 0.08 and 0.4. The deformation experiments are performed by high pressure torsion (HPT). The resulting microstructures were investigated by backscattered electron imaging, orientation image microscopy, and in selected cases by transmission electron microscopy. It will be shown that the behavior of single phase material is relatively uniform. With increasing strain, the size of the structural elements decreases and reaches a saturation between a shear strain of 10 to 100. The temperature and the alloying are the main parameters, which controls the saturation size of the structural elements (grains). The behavior in the dual phase materials is more complex, it varies from simple homogenisation, fragmentation of one phase, to desintegration and supersaturation of the phases.

Fatigue and Fracture Processes in Severe Plastic Deformed Rail Steels

On the surface of rails a severely deformed layer is formed during the application due to the permanent loading of a rail-wheel contact [1]. In this deformed layer cracks are formed and may grow to a critical length. Therefore, it is of great importance to determine the fracture properties of these plastically deformed regions to enhance finite element calculations for live time predictions and define service intervals.

Properties of Nanoscaled Multiphase Structures and Non-Equilibrium Solid Solutions Obtained by Severe Plastic Deformation [Abstract + Supplemental Data]

Grain size reduction induced by severe plastic deformation (SPD) and the resulting mechanical properties have been widely investigated for pure metals but less is known and reported about multi-phase materials. To study the grain size reduction mechanisms in multiphase structure subjected to SPD, two copper based composites (Cu-10%Fe and Cu-43%Cr) were severely deformed by torsion under high pressure. The grain size achieved with these composite materials is much smaller than in pure metals. It is for example in a range of 10 to 20 nm for the Cu-43%Cr composite, e.g. one order of magnitude lower than in pure Cu processed by SPD. Three dimensional atom probe data show also the formation of non equilibrium supersaturated solid solutions. The mechanisms of the deformation induced intermixing are discussed together with its influence on the mechanical properties.