Influence of cold-rolling reduction on retained austenite texture in cold-rolled and intercritically annealed TRIP-assisted steel

The newly developed multiphase transformation-induced plasticity (TRIP) steels are of interest for industrial applications because of their excellent combination of high strength and ductility. Their performance can be successfully controlled by designing an optimum balance in the volume fractions of ferrite, bainite and retained austenite. The characteristics of the retained austenite are considered to be the main key to achieving the desired final properties. Against this background, the effects of retained austenite characteristics, such as volume fraction, carbon concentration, size and shape, on the behaviour of TRIP steels have been studied. The crystallographic orientation of the retained austenite was measured by electron backscattered diffraction (EBSD). The effect of initial cold-rolling reduction on the microtexture development of the retained austenite was studied on an aluminium-containing TRIP steel. The results show that, by increasing the cold-rolling reduction before the final austempering, the main components of the face-centred cubic phase, i.e. copper, brass and Goss, dominate the texture of the retained austenite. In contrast, the copper and Goss components of the retained austenite are absent in the texture of lightly deformed sheets. The features of the preferred orientation of the retained austenite are discussed and explained in terms of the annealing texture of the recrystallized ferrite and bainite

A novel method for severe plastic deformation at high strain rate

Severe plastic deformation (SPD) processing is defined as any method of forming under an extensive hydrostatic pressure that may be used to impart a very high strain to a bulk solid without any significant change in dimensions of the sample, producing exceptional grain refinement. Most of the SPD techniques employ very low processing speeds, however increased deformation rates are known to have a significant effect on the final microstructure. Most of the SPD processes operating at high rates do not impose hydrostatic pressures to the material and can therefore only be used for very ductile materials, while in others, the microstructural changes are limited to the surface layers of the material. To circumvent these restrictions a novel facility has been designed and developed where high hydrostatic pressures are maintained while a high shear deformation is imposed at high strain rates. The device combines the features of a high pressure torsion (HPT) unit with the principle of a torsional split Hopkinson bar (SHB) setup. A small ring-like sample, placed between two molds, is first subjected to a high, static pressure and subsequently to a high speed shear deformation upon release of torsional energy stored in a long bar. Although, the principle is rather straightforward, the design of the setup was extremely critical because of the high forces and energies involved. Tests have been performed on commercially pure aluminum. The material hardness increased in accordance with the microstructure and processing conditions; viz, annealed, only compressed and applied shear strain. Deformed grains departed from equiaxed shape and showed morphological texture in the direction of the shear even at very low strains indicating the presence of shear strains in the material. Further the material, or more specifically its mechanical properties and microstructure evolution is compared with conventional, statically deformed HPT samples

Modeling the magnetic properties of non-oriented electrical steels based on microstructural parameters

Magnetic properties of electrical steels such as magnetization behavior and electrical losses are mainly related to chemical composition, crystallographic orientation and microstructure. By now, several models have been proposed to empirically correlate magnetic properties and affecting parameters. A quantitative model based on physical understanding of the interaction between the magnetic field variables (e.g. domain structure) and local microstructural variables (e.g. grain orientation and misorientation, grain boundary plane inclination) is still missing. To obtain a better understanding of the interaction between grain boundaries and domain walls, the magnitude of free pole density at grain boundaries was taken into account. Experimental results from 3-dimentional EBSD experiments were employed to measure the grain boundary orientation for several samples with different chemical composition and grain size. The free pole density was calculated using the relative misorientation between adjacent grains, and was included in a model together with grain size, magnetocrystalline anisotropy energy and silicon equivalent. By comparison with the experimental results of the magnetic induction measured at low, medium and high magnetic fields, is shown that the magnetization behavior can be more accurately predicted when the above mentioned phenomena are taken into account

Measuring plasticity with orientation contrast microscopy in aluminium 6061-T4

Orientation contrast microscopy (i.e., electron backscattered diffraction, EBSD) was employed to monitor the plastic strain in loaded tensile samples of aluminium alloy Al6061 in T4 condition. The kernel average misorientation (KAM) is known to be an appropriate parameter in orientation contrast microscopy which has the potential to characterise the plastic strain by monitoring the local misorientations. This technique was applied here to gauge the extent of the plastic zone around a fatigue crack. To establish the magnitude of strain (which can be identified by the KAM parameter), a series of tensile samples were strained in the range of 1% to 25%. KAM maps were compared, and the average misorientations were related to the tensile strain values. The KAM distribution functions for all the strained samples were derived from a large scanned area. In addition, Vickers microhardness tests were conducted for these series of samples. This allowed the comparison of the mesoscopic plastic strain measured by Vickers microhardness with the micro plastic strain locally obtained by KAM. Noise was observed in the average KAM values up to a plastic strain of 1.5%. For the plastic strain exceeding 1.5%, noise no longer dominates the KAM map, and a positive-though not linear-correspondence between plastic strain and KAM was observed. The observed plastic zone at the tip of the fatigue crack by micro-Vickers hardness measurements was about an order of magnitude higher than the plastic zone observed on the KAM maps. In view of the calibration of KAM values on the tensile samples, it could be concluded that in the larger area of the plastic zone, the strain did not exceed the critical value of 1.5%

Dynamic High Pressure Torsion (DHPT) : a novel method for high strain rate severe plastic deformation

Metals with a fine-grained microstructure have exceptional mechanical properties. Severe plastic deformation (SPD) is one of the most successful ways to fabricate ultrafine-grained (UFG) and nanostructured (NC) materials. Most of the SPD techniques employ very low processing speeds. However, the lowest steady-state grain size which can be obtained by SPD is considered to be inversely proportional with the strain rate at which the severe deformation is imposed. In order to overcome this limitation, methods operating at higher rates have been envisaged and used to study the fragmentation process and the properties of the obtained materials. However, almost none of these methods, employ hydrostatic pressures which are needed to prevent the material from failing at high deformation strains. As such, their applicability is limited to materials with a high intrinsic ductility. Additionally, in some methods the microstructural changes are limited to the surface layers of the material. To circumvent these restrictions, a novel facility has been designed and developed which deforms the material at high strain rate under high hydrostatic pressures. Using the facility, commercially pure aluminum was processed and analysis of the deformed material was performed. The microstructure evolution in this material was compared with that observed in static high pressure torsion (HPT) processed material