Papers by Author: Jenő Gubicza

Abstract: High-Pressure Torsion (HPT) is one of the most effective severe plastic deformation techniques in grain refinement. The goal of this study was to investigate the influence of HPT on the microstructure and hardness of a Ti-rich High-Entropy Alloy (HEA). The evolution of the grain size due to 1 turn of HPT was studied by transmission electron microscopy. Besides the refinement of the microstructure, a phase transition also occurred during HPT, as revealed by X-ray diffraction. The initial bcc structure transformed into a martensitic phase throughout the material. The features of this phase transformation were studied on a sample compressed to low strain values. The hardness as a function of the distance from the center in the HPT-processed disk was measured and correlated to the microstructure.

Abstract: The evolution of phase composition, microstructure and hardness in 316L austenitic stainless steel processed by high-pressure torsion (HPT) was studied up to 20 turns. It was revealed that simultaneous grain refinement and phase transformation occur during HPT-processing. The γ-austenite in the initial material transformed gradually to ɛ-and α’-martensites due to deformation. After 20 turns of HPT the main phase was α’-martensite. The initial grain size of ~42 μm was refined to ~48 nm while the dislocation density increased to ~143 × 10¹⁴ m^-2 in the α’-martensite phase at the disk periphery processed by 20 turns. The microstructure and hardness along the disk radius became more homogeneous with increasing numbers of turns. An approximately homogeneous hardness distribution with a saturation value of ~6140 MPa was achieved in 20 turns.

Abstract: The deformation mechanisms in ultrafine-grained hexagonal close packed Zn were investigated at different strain rates and temperatures. The influence of grain size on the deformation mechanisms was revealed by comparing the results obtained on ultrafine-grained and coarse-grained Zn. It was found that for coarse-grained Zn at room temperature and strain rates lower than 10^-2 s^-1 twinning contributed to plasticity besides dislocation activity. For strain rates higher than 10³ s^-1 the plasticity in coarse-grained Zn was controlled by dislocation drag. In ultrafine-grained Zn the relatively large dislocation density (~10¹⁴ m^-2) and the small grain size (~250 nm) limit the dislocation velocity yielding the lack of dislocation drag effects up to 10⁴ s^-1. For ultrafine-grained Zn, twinning was not observed in the entire strain rate range due to its very small grain size. During room temperature compression at strain rates higher than 0.35 s^-1 and in high temperature creep deformation of ultrafine-grained Zn besides prismatic and pyramidal <c+a> dislocations observed in the initial state, <a>-type basal and pyramidal dislocations as well as other <c+a>-type pyramidal dislocations were formed.

Abstract: Blends of Cu powders and 3 vol. % carbon nanotubes (CNTs), and an additional sample from pure Cu powder were consolidated by High Pressure Torsion (HPT) at room temperature (RT) and 373 K. The grain size, the lattice defect densities as well as the hardness of the pure and composite materials were determined. Due to the pinning effect of CNTs, the dislocation density is about three times larger, while the grain size is about half of that obtained in the sample consolidated from the pure Cu powder. The increase of the HPT-processing temperature from RT to 373 K resulted in only a slight increase of the grain size in the Cu-CNT composite while the dislocation density and the twin boundary frequency were reduced significantly. The flow stress obtained experimentally agrees well with the value calculated by the Taylor-formula indicating that the strength in both pure Cu and Cu-CNT composites is determined mainly by the interaction between dislocations. The addition of CNTs to Cu yields a significantly better thermal stability of the UFG matrix processed by HPT.

Abstract: The effect of the impurity content on the evolution of the ultrafine-grained (UFG) microstructure in low stacking fault energy Ag and its stability at room and elevated temperatures were investigated. Samples of silver having high (99.995%) and somewhat lower (99.99%) purity levels were processed by equal-channel angular pressing (ECAP) at room temperature (RT) up to 16 passes. Although, the minimum grain size achieved by ECAP was ~200 nm for both series, the lattice defect structure was strongly influenced by the impurity content. In the samples processed by 4-16 passes of ECAP a self-annealing occurred during storage RT that was promoted by the higher twin boundary frequency. Both room-and high-temperature thermal stability of 99.99% purity Ag were much better due to the pinning effect of impurities. It was found that a large number of dislocation loops remained in the microstructure even after recrystallization at high temperatures.

Abstract: The influence of the consolidation conditions on the microstructure and plastic behavior of ultrafine-grained Ni and Al sintered from nanopowders was studied. It was found that the smaller initial Ni powder particle size yielded a smaller grain size and a larger oxide content in the as-consolidated sample resulting in a higher strength and lower ductility. When the Ni nanopowder was in contact with air (instead of an inert atmosphere) during the short handling time before sintering, the oxide content increased without a considerable change of the grain size that also decreased the ductility. The reduced time and temperature in Spark Plasma Sintering compared to Hot Isostatic Pressing led to a smaller grain size that resulted in a higher strength of Ni. In the case of an Al nanopowder processed by Hot Isostatic Pressing at 450 °C, the consolidation was hindered by the strongly limited diffusion due to the presence of a rigid amorphous layer on the surface of particles. However, at the sintering temperature of 550 °C, the crystallization and the fragmentation of the layer occurred that yielded a better densification.

Abstract: Ultrafine-grained (UFG) CP titanium (Grade-4) sample was processed by electroplastic rolling (EPR) at room temperature which was compared to a specimen processed by conventional cold rolling (CR). EPR was performed using pulsed unidirectional current with a current density of 95 A/mm2, pulse duration of 10-4 s and frequency of 1000 Hz. It was found that the sample processed by EPR has slightly higher dislocation density and smaller crystallite size than for the CR specimen resulting in a higher tensile strength for the former specimen. In the case of EPR sample, the relative fraction of dislocations is lower than for CR specimen. During annealing the relative fraction of dislocations decreased for both samples which can be explained by the fact that the dislocations have larger Burgers-vector and consequently higher formation energy than the other two types.

Abstract: The thermal stability of ultrafine-grained (UFG) microstructure in face centered cubic metals processed by severe plastic deformation (SPD) was studied. The influence of the SPD procedure on the stability was investigated for Cu samples processed by Equal-Channel Angular Pressing (ECAP), High-Pressure Torsion (HPT), Multi-Directional Forging and Twist Extrusion at room temperature (RT). It is found that HPT results in the lowest thermal stability due to the very high dislocation density. Furthermore, the effect of the low stacking fault energy of Ag on the stability is also investigated. It is revealed that the UFG microstructure produced in Ag by ECAP is recovered and recrystallized during storage at room temperature. The driving force for this unusual recovery and recrystallization is the high dislocation density developed during ECAP due to the high degree of dislocation dissociation caused by the very low stacking fault energy of Ag.

Abstract: Most ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) exibit only limited ductility which is correlated with the low strain rate sensitivity (SRS) of these materials. Recently, it was demonstrated that SPD is capable of increasing the room temperature ductility of aluminum-based alloys attaining elongations up to 150%, together with relatively high strain rate sensitivity. In the present work, additional results and discussions are presented on the effect of grain boundary sliding (GBS) and SRS on the ductility of some UFG metals and alloys. The characteristics of constitutive equations describing the steady-state deformation process are quantitatively analyzed for a better understanding of the effects of grain boundaries and strain rate sensitivity.

Abstract: The evolution of the microstructure during processing by equal-channel angular pressing (ECAP) in silver having extremely low stacking fault energy was studied up to 16 passes. It was shown that at high strains the contribution of twinning to deformation increased at the expense of dislocation-controlled processes. It was also found that during storage at room temperature (i.e. at the temperature of ECAP) there was a self-annealing of the severely deformed microstructure after 1 month and its degree was revealed to have a strong dependence on the number of passes.