Jelena Horky

Bimodal Grain Size Distribution Enhances Strength and Ductility Simultaneously in a Low-Carbon Low-Alloy Steel

Low-carbon low-alloy steel specimens were quenched, then cold rolled, and finally annealed. Electron backscatter diffraction (EBSD) micrographs revealed a bimodal grain structure where ultra-fine grain structures with low-angle grain boundaries are alternating with regions of larger grains. The average total dislocation density was measured by X-ray line profile analysis, whereas the geometrically necessary dislocation density was obtained from the analysis of EBSD data. Using the combination of the Hall–Petch and Taylor equations, a good correlation was found between the total dislocation density and the measured flow stress in the different states of the alloy. The difference in evolutions of the total and the geometrically necessary component of the dislocation densities is discussed in terms of the successive processes of quenching, rolling, and annealing of the alloy.

Mechanical properties, structural and texture evolution of biocompatible Ti-45Nb alloy processed by severe plastic deformation

Biocompatible β Ti-45Nb (wt%) alloys were subjected to different methods of severe plastic deformation (SPD) in order to increase the mechanical strength without increasing the low Young׳s modulus thus avoiding the stress shielding effect. The mechanical properties, microstructural changes and texture evolution were investigated, by means of tensile, microhardness and nanoindentation tests, as well as TEM and XRD. Significant increases of hardness and ultimate tensile strength up to a factor 1.6 and 2, respectively, could be achieved depending on the SPD method applied (hydrostatic extrusion - HE, high pressure torsion - HPT, and rolling and folding - R&F), while maintaining the considerable ductility. Due to the high content of β-stabilizing Nb, the initial lattice structure turned out to be stable upon all of the SPD methods applied. This explains why with all SPD methods the apparent Young׳s modulus measured by nanoindentation did not exceed that of the non-processed material. For its variations below that level, they could be quantitatively related to changes in the SPD-induced texture, by means of calculations of the Young׳s modulus on basis of the texture data which were carefully measured for all different SPD techniques and strains. This is especially true for the significant decrease of Young׳s modulus for increasing R&F processing which is thus identified as a texture effect. Considering the mechanical biocompatibility (percentage of hardness over Young׳s modulus), a value of 3-4% is achieved with all the SPD routes applied which recommends them for enhancing β Ti-alloys for biomedical applications.

Changes of Thermoelectric Properties and Hardness After HPT Processing of Micro- and Nanostructured Skutterudites

In this paper the influence of the starting material on the physical properties after severe plastic deformation (SPD) will be discussed. A bulk p-type skutterudite DD0.44Fe2.1Co1.9Sb12(DD stands for didymium which consists of 4.76% Pr and 95.24% Nd) was (1) hand milled and hot pressed, resulting in crystallite sizes in the μm range and (2) ball milled and hot-pressed, reducing the crystallite size to about 100nm, and afterwards deformed using high pressure torsion (HPT). It could be shown that in both cases the lattice parameters were slightly higher after HPT processing, the difference of the electrical resistivity values between heating and cooling was much larger for the skutterudite, which stems from the microstructured alloy. The thermopower data for both alloys are slightly higher, resulting in power factors at 800K almost like (originally nanosample) or even higher (originally microsample) than before HPT. As the thermal conductivity is always lower after SPD, a much higher ZT can be expected. After deformation hardness measurements showed a much higher increase for the nanosample compared to that of the microstructured one.

Influence of deformation temperature on texture evolution in HPT deformed NiAl

NiAl is an intermetallic compound with a brittle-to-ductile transition temperature at about 300°C and ambient pressure. At standard conditions, it is very difficult to deform, but fracture stress and fracture strain are increased under high hydrostatic pressure. On account of this, deformation at low temperatures is only possible at high hydrostatic pressure, as for instance used in high pressure torsion. In order to study the influence of temperature on texture evolution, small discs of polycrystalline NiAl were deformed by high pressure torsion at temperatures ranging from room temperature to 500°C. At room temperature, a typical shear texture of body centred cubic metals is found, while at 500°C a strong oblique cube component dominates. These textures can be well simulated with the viscoplastic self-consistent polycrystal deformation model using the primary and secondary slip systems activated at low and high temperatures. The oblique cube component is a dynamic recrystallization component.

Nanocrystallization and Dissolution of Immiscible Powder Alloys Using High Pressure Torsion

Precompacts out of immiscible systems CuCr (75/25 wt%) and WCu (80/20 wt%), respectively, were made by pressing mixed powders and sintering. By applying different strains and hydrostatic pressures of HPT at room temperature, disc-shaped samples with a diameter of 8 mm were produced. They were investigated by Light Microscopy, Scanning-Electron Microscopy using Back-Scattered Electrons, and X-ray Line Profile Analysis. In addition, Vickers microhardness data were collected. Both systems showed the highest microhardness at a shear strain of about γ = 170. The density (for the case of Cu25Cr) of the consolidated material could be increased to the theoretical value. Microhardness and grain sizes were studied individually for each of the phases, too.

Influence of hydrostatic pressure on texture evolution in HPT deformed NiAl

NiAl is an intermetallic compound with a brittle-to-ductile transition temperature of about 300°C at ambient pressure. At standard conditions, it is very difficult to deform, but fracture stress and fracture strain are increased under hydrostatic pressure (HP). On account of this, deformation at low temperatures is only possible at high HP, as for instance used in high pressure torsion (HPT). In order to study the influence of HP on texture evolution, small discs of polycrystalline NiAl were deformed by HPT at different temperatures ranging from room temperature to 500°C and different HPs. The influence of HP is presented for deformation at room temperature and 500°C. It is found that HP affects the formability of the samples as well as texture and microstructure.