Peculiarities in the Texture Formation of Intermetallic Compounds Deformed by High Pressure Torsion (original) (raw)
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ADVANCED ENGINEERING MATERIALS 2011, 13, No. 4
Ultrafine-grained (UFG) materials processed by severe plastic deformation are known to exhibit good mechanical properties. Much about the annealing behavior of such materials is still unknown, and this work aims to provide a better understanding of the thermal properties of UFG materials. For this purpose a Cu–0.17 wt%Zr alloy was subjected to high pressure torsion (HPT) with a maximal pressure of 4.8 GPa at room temperature. The microstructures of the specimens were characterized using electron back scatter (EBSD) measurements, transmission electron microscopy (TEM), and hardness measurements. During annealing of the samples, dispersoids were formed which improved the thermal stability of the alloy. At higher strain levels the fraction of high angle grain boundaries (HAGBs) increased above 70% of the total grain boundaries.
Journal of Alloys and Compounds, 2004
Different samples prepared by high-pressure torsion deformation (Cu, Cu with addition of different amounts of Al 2 O 3 , Fe, Mg and Mg with 10 wt.% Gd) were investigated by X-ray powder diffraction (PXRD), positron lifetime spectroscopy (PL) and transmission electron microscopy (TEM). Conventional PXRD studies were carried out in order to determine lattice parameters, texture coefficients and especially crystallite size and microstrain (and dislocation densities). In simplified analysis, the profiles were analyzed in terms of the modified Williamson-Hall (WH) plots and more sophisticated analysis was performed by total powder diffraction pattern fitting. Small grains of the order of 50-300 nm and dislocation densities within the range of 1 × 10 13 m −2 up to 1 × 10 15 m −2 were determined. Observed line broadening anisotropy could be explained well for all the materials by the dislocation-induced line broadening and elastic anisotropy. Positron lifetime spectra have shown two major components-from positrons trapped at dislocations inside the distorted regions and the one attributed to positrons trapped in microvoids with the size of 4-5 vacancies. The values of crystallite size and dislocation density agree quite well with the estimations made from TEM. It was found that the addition of more than 0.5 wt.% Al 2 O 3 prevents grain growth and keeps the dislocation density high up to about 400 • C. The amount of 0.3 wt.% is insufficient for that and significant grain growth is observed at about 200 • C.
Structure of Cu deformed by high pressure torsion
Acta Materialia, 2005
Pure copper is deformed by high pressure torsion and the resulting microstructure is studied. Small structural elements are formed. Their size decreases with increasing strain and reach a steady-state. The misorientation between neighbouring structural elements increases with strain and finally reaches a nearly random distribution. The steady-state size decreases with increasing pressure and decreasing temperature. The shape of the elements suggests the continuous formation of new elements during steady-state deformation. This would be a process similar to dynamic recrystallisation.
Materials Science and Engineering: A, 2013
Several pure metals exhibit softening when imparting large strains at room temperature. This study investigates the nature of this strain softening. Torque measurements during high-pressure torsion using ring specimens, which appear to be more suitable than disk specimens for the evaluation of the strain response on the in situ flow stress, suggest that the softening in aluminum occurs mainly by dynamic recrystallization and recovery, whereas no appreciable dynamic softening occurs in copper. The softening in aluminum is associated with decreasing the dislocation density and increasing the grain size and missorientation angles, whereas copper exhibits ultrafine grains of higher dislocation density and few nanotwins. A significant static recrystallization is detected in HPT-processed pure copper by argon irradiation during ion milling.
A model of ultrafine microstructure evolution in materials deformed by high-pressure torsion
The proposed crystal plasticity model outlines a possible mechanism of a material response under severe plastic deformation as observed in high-pressure torsion experiments. A simplified version of the model based on an assumption of uniform deformation of plane-strain double slip reveals rotations of slip systems caused by the imposed shear strain. An axial compression and a shear stress of twist govern this process. The accompanied continuous reconstruction of the deformation substructure is probably one of the main reasons for the observed strengthening. Local variations in the crystal lattice orientation are responsible for the microstructure fragmentation.
2009
In the present work, deformation behavior, texture and microstructure evolution of commercially pure titanium (CP Ti) are investigated by electron backscattered diffraction (EBSD) after compression tests at elevated temperatures. By analysing work hardening rate vs. flow stress, the deformation behaviour can be divided into three groups, viz. three-stage work hardening, two-stage work hardening and flow softening. A new deformation condition map is presented, dividing the deformation behavior of CP Ti into three distinct zones which can be separated by two distinct values of the Zener-Hollomon parameter. The deformed microstructures reveal that dynamic recovery is the dominant deformation mechanism for CP Ti during hot working. It is the first time that the Schmid factor and pole figures are used to analyse how the individual slip systems activate and how their activities evolve under various deformation conditions. Two constitutive equations are proposed in this work, one is for single peak dynamic recrystallization (DRX), the other is specially for CP Ti deformed during hot working. After the hot compression tests, some stress-strain curves show a single peak, leading to the motivation of setting up a DRX model. However, the examinations of EBSD maps and metallography evidently show that the deformation mechanism is dynamic recovery rather than DRX. Then, the second model is set up. The influence of the deformation conditions on grain size, texture and deformation twinning is systematically investigated. The results show that 101 2 twinning only occurs at the early stage of deformation. As the strain increases, the 101 2 twinning is suppressed while 101 1 twinning appears. Three peaks are found in the misorientation frequency-distribution corresponding to basal fiber texture, 101 1 and 101 2 twinning, respectively. A logZ-value of 13 is found to be critical for both the onset of 101 1 compressive twinning and the break point for the subgrain size. The presence of 101 1 twinning is the key factor for effectively reducing the deformed grain size. The percentage of low angle grain boundaries decreases with increasing Z-parameter, falling into a region separated by two parallel lines with a common slope and 10% displacement. After deformation, three texture components can be found, one close to the compression direction, CD, one 10~30 to CD and another 45 to CD.
Microstructural evolution of cryomilled Ti/Al mixture during high-pressure torsion
Journal of Materials Research, 2014
To provide insight into the influence of the length scale on the kinetics of phase evolution during severe plastic deformation, we studied the microstructure evolution of cryomilled Al and Ti mixture, which is further subjected to high-pressure torsion (HPT). The cryomilled microstructure consisted of elemental Al and Ti, and the subsequent HPT deformation at ambient temperature led to the solid state formation of Al-rich intermetallics. X-ray diffraction peaks originating from TiAl 2 and TiAl 3 were observed after one revolution of HPT, suggesting a shear strain-assisted formation of the intermetallics. A high resolution transmission electron microscope confirmed the formation of TiAl 2 following HPT for one revolution. Further HPT straining led to microstructure refinement and a mixing of the Ti and Al, as well as of any phases formed initially. The solid state formation of the intermetallics and the overall evolution of the microstructure are discussed based on the generation of a high density of lattice defects that evolve under the strain conditions present during HPT.
Practical Metallography, 2015
In this contribution, the microstructure of two intermetallic γ-TiAl-based alloys with different Al contents were examined after high-temperature deformation. To investigate the dynamic recrystallization of these alloys, isothermal compression tests were performed using a Gleeble® 3500 simulator. For the experiments, a temperature range of 1 150 °C to 1 300 °C and strain rates of 0.005 s−1, 0.05 s−1 and 0.5 s−1 were applied, up to a true strain of 0.9. The deformed microstructural states, particularly the multiphase alloys' dynamically recrystallized grain sizes were characterized via Scanning Electronic Microscopy (SEM) and Electron Back Scatter Diffraction (EBSD). The recrystallized grain sizes obtained from the experiments could be linked with the calculated Zener-Hollomon parameter through a power law.
Microstructure and texture evolution of commercial pure titanium deformed at elevated temperatures
Materials Science and Engineering: A, 2009
In the present work, deformation behavior, texture and microstructure evolution of commercially pure titanium (CP Ti) are investigated by electron backscattered diffraction (EBSD) after compression tests at elevated temperatures. By analysing work hardening rate vs. flow stress, the deformation behaviour can be divided into three groups, viz. three-stage work hardening, two-stage work hardening and flow softening. A new deformation condition map is presented, dividing the deformation behavior of CP Ti into three distinct zones which can be separated by two distinct values of the Zener-Hollomon parameter. The deformed microstructures reveal that dynamic recovery is the dominant deformation mechanism for CP Ti during hot working. It is the first time that the Schmid factor and pole figures are used to analyse how the individual slip systems activate and how their activities evolve under various deformation conditions. Two constitutive equations are proposed in this work, one is for single peak dynamic recrystallization (DRX), the other is specially for CP Ti deformed during hot working. After the hot compression tests, some stress-strain curves show a single peak, leading to the motivation of setting up a DRX model. However, the examinations of EBSD maps and metallography evidently show that the deformation mechanism is dynamic recovery rather than DRX. Then, the second model is set up. The influence of the deformation conditions on grain size, texture and deformation twinning is systematically investigated. The results show that 101 2 twinning only occurs at the early stage of deformation. As the strain increases, the 101 2 twinning is suppressed while 101 1 twinning appears. Three peaks are found in the misorientation frequency-distribution corresponding to basal fiber texture, 101 1 and 101 2 twinning, respectively. A logZ-value of 13 is found to be critical for both the onset of 101 1 compressive twinning and the break point for the subgrain size. The presence of 101 1 twinning is the key factor for effectively reducing the deformed grain size. The percentage of low angle grain boundaries decreases with increasing Z-parameter, falling into a region separated by two parallel lines with a common slope and 10% displacement. After deformation, three texture components can be found, one close to the compression direction, CD, one 10~30 to CD and another 45 to CD.