Papers by Keyword: High Pressure Torsion (HPT)

Abstract: This study presents the result of the researches regarding the obtaining of NiTi alloy through powder metallurgy (PM) as a possible alternative to present technologies (melting through induction in vacuum—MIV and re-melting with electric arch in vacuum—VAR). The researches made by the authors have aim at the obtaining of Ni-Ti materials with fine grain or ultrafine grain through powder metallurgy techniques, starting from ordinary metallic powders of Ti, Ni, Cu, with grain size less than 100 micrometers, and also using processing through severe plastic deformation (HPT — high pressure torsion). The fabrication through PM has an important advantage because a product requires low processing subsequent considering that it can get with sizes and shape very similar to the final ones, which is not negligible if one takes into account that the alloys Ni-Ti do not excel on cutting processability. Cylindrical samples were produced by cold uniaxial compression, at the specific pressure of 600 MPa, dosed in a proportion of 52.5 % Ni + 43.5 % Ti + 4.0 % Cu, mass composition. The compressed samples, after the sintering in vacuum and severe plastic deformation have been characterized by X-ray diffraction (XRD) , differential scanning calorimetry (DSC) and optical microscopy.

Abstract: High speed high pressure torsion (HSHPT) processing technology, engineered to achieving (ultra) fine bulk metallic structure under high pressure (~ GPa) and torsion by applying supplementary elevated rotation speed of superior anvil. Coned-disk spring shape modules were processed from an as cast Fe-28Mn-6Si-5Cr (mass %) shape memory alloy (SMA). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies revealed that the structure of modules became submicron as an effect of HSHPT processing. After severe plastic deformation, a grain size gradient was obtained along the truncated cone generator, increasing from inner to outer areas, due to different deformation degrees in these zones. The mechanical and shape memory properties was performed in order to relate the structural changes caused by severe plastic deformation.

Abstract: In this study, strength of Cu-9Fe-1.2X (X 5 Ag or Cr) microcomposite disks obtained HPT(high pressure torsioning) have been investigated. The dendrite arms were aligned along the drawing direction and elongated into filaments. The filament spacings were found to small in Cu-9Fe-1.2Ag compared to those of other microcomposites. The ultimate tensile strength of the Cu-Fe-Ag microcomposites were higher than those of Cu-Fe-Co and Cu-Fe-Ni microcomposites, suggesting the strengthening of the matrix is more effective than the strengthening of the filaments in strengthening the microcomposites. The strength of Cu-Fe-Xi microcomposites increased more rapidly when the finer microstructure was developed by the plastic flow and refinement of filaments by the effect of strong Cu matrix strengthened by Ag atoms. With increase of strain up to 7 rotations, the second phase lamella was observed to be fragmented by HPT rotations and small second phase particles were observed.

Abstract: Investigations of the microstructure of materials processed via severe plastic deformation methods such as high pressure torsion (HPT) and their recrystallization behaviour is of great interest as they are capable of producing ultra fine grained material (UFD) with good mechanical properties.

Abstract: ntense plastic deformation is generally effective in producing grain refinement. IPD methods include equal channel angular pressing/extrusion (ECAP/ECAE), high-pressure torsion (HPT), accumulative roll bonding (ARB), and friction stir processing (FSP), among others. In this work, we summarize the main results on grain refinement by these processing methods and present our own data on microstructure and texture evolution in metals and alloys during ECAP, HPT and FSP. Whereas ECAP and HPT are usually performed with the work piece material initially at room temperature or even at liquid nitrogen temperature to enhance refinement, FSP involves a brief but complex thermomechanical cycle with peak temperatures up to 0.7 0.9 T_Melt. Apparently, materials undergo dynamic recrystallization (DRX) during FSP. DRX also occurs also in metals and alloys of low T_Melt due to adiabatic heating during HPT performed at room temperature. The paper is devoted to revisiting of previous as well as new results and a comparative analysis of microstructure and texture evolution in commercially pure aluminum and selected pure metals and alloys during ECAP, HPT and FSP in order to illustrate the limits of grain refinement.

Abstract: The present work deals with the evolution of mechanical properties and structure of low-carbon Fe-1,12Mn-0,08V-0,07Ti-0,1C (wt.%) steel after severe plastic deformation (SPD) and high-temperature annealing. Steel in initial ferritic-pearlitic state was deformed by equal channel angular pressing (ECAP) at T=200°C and high pressure torsion (HPT) at room temperature. The evolution of ultrafine grained structure and its thermal stability were investigated after annealing at 400-700°C for 1 hour. The results shown that SPD leads to formation of structure with an average size of (sub-) grain of 260 nm after ECAP and 90 nm after HPT. Ultrafine grained structures produced by SPD reveal a high thermal stability up to 500°C after ECAP and 400°C after HPT. At higher annealing temperatures a growth of structural elements and a decrease in microhardness were observed.

Abstract: Precipitation effects in age-hardenable Mg-13wt.%Tb alloy were investigated in this work. The solution treated alloy was subjected to isochronal annealing and decomposition of the supersaturated solid solution was investigated by positron annihilation spectroscopy combined with transmission electron microscopy, electrical resistometry, differential scanning calorimetry and microhardness measurements. Peak hardening was observed at 200°C due to precipitation of finely dispersed particles of β phase with the D0₁₉ structure. Vacancy-like defects associated with β phase particles were detected by positron annihilation. At higher temperatures precipitation of β and subsequently β phase takes place. Formation of these phases lead to some additional hardening and introduces open volume defects at precipitate/matrix interfaces. To elucidate the effect of plastic deformation on the precipitation sequence we studied also a Mg-13wt.%Tb alloy with ultra fine grained structure prepared by high pressure torsion. In the ultra fine grained alloy precipitation of the β phase occurs at lower temperature compared to the coarse grained material and the peak hardening is shifted to a lower temperature as well. This effect can be explained by enhanced diffusivity of Mg and Tb atoms due to a dense network of grain boundaries and high density of dislocations introduced by severe plastic deformation. Moreover, dislocations and grain boundaries serve also as nucleation sites for precipitates. Hence, precipitation effects are accelerated in the alloy subjected to severe plastic deformation.

Abstract: In this paper the in vitro degradation of ultrafine grained (UFG) Mg-Zn-Ca alloy produced by HPT was investigated by electrochemical measurements and immersion tests in SBF. It was found that UFG Mg alloy had better degradation properties and also higher microhardness value than as-cast Mg alloy. The corrosion current density of UFG Mg alloy decreased by about two orders of magnitude, compared with that of as-cast alloy. Through electrochemical impedance spectroscopy (EIS) test，UFG Mg alloy showed a higher charge transfer resistance value. In immersion test, UFG Mg alloy in SBF exhibited more uniform corrosion and lower degradation rate (0.0763 mm/yr) than as-cast alloy. The degradation properties were related with the microstructure evolution, namely the grain refinement and redistribution of second phase. Keywords: Mg-Zn-Ca alloy; High-pressure torsion (HPT); Degradation behavior; Simulated body fluid (SBF); Microhardness

Abstract: In this study ultrafine grain structure evolution during high pressure torsion (HPT) of commercial aluminium alloy AA6082 at increased temperature is presented. Two different initial structural states of the alloy were prepared by thermal treatment. The progress in structure refinement in dependence on the shear strain level strain was investigated by TEM of thin foils. The impact of different amount of strain (ε_ef) introduced was analyzed with respect to the effect of increased temperature. The microhardness results measured across the deformed discs pointed out that some data scattering. The results of microstructure analyses showed that ultrafine grain (ufg) structure was already formed in deformed disc upon the first turn, regardless the initial structure of alloy, resulting from prior thermal treatment. The observed heterogeneity in ufg structure formation across the deformed disc was observed, supporting microhardness results scattering. By increasing the strain level (number of turns N-2,4,6), more effectively homogenized ufg structure was observed across the deformed discs. The effect of increased deformation temperature became evident and dynamic recrystalization modified locally ufg structure.. The retardation of new grains growth and higher thermal stability of ufg structure was observed, when two steps thermal treatment of alloy (quenching and ageing) was executed prior deformation. Strength measurements results yielded form tensile tests showed that the effect of structure strengthening was degraded by local recrystallization. The results of torque measurement versus the time showed that the torque required to deform the sample was increasing until the first turn and then kept stable or even decreased.

Abstract: High-pressure torsion (HPT) is a processing technique in which samples, in the form of thin disks, are subjected to a high applied pressure and concurrent torsional straining. In principle, the strain introduced into the disk during the straining varies across the disk and there is a direct proportionality between the estimated strain and the radial position on the disk. This means that the strain is zero at the center of each HPT disk and it reaches a maximum value at the outer periphery. Contrary to these expectations, recent experiments show there is a gradual evolution with increasing numbers of revolutions such that the hardness of the disk gradually becomes reasonably homogeneous. This report examines the development of hardness and microstructural homogeneity with special emphasis on the evolution in hardness homogeneity along vertical sections of disks of high-purity aluminum processed by HPT. The results demonstrate that, at least for pure aluminum, the distributions in the hardness values are independent of the plane of sectioning.