Papers by Keyword: Ultrafine Grained

Abstract: The temperature-dependent deformation and damage behaviors of ultrafine-grained (UFG) Cu and Ti produced by equal channel angular pressing (ECAP) were investigated and compared. It was found that ECAPed materials with different crystalline structures, e.g. the present fcc Cu and hcp Ti, exhibited significantly distinctive high-temperature deformation and damage characteristics. As the testing temperature is below recrystallization, small- and large-scale cracks or voids formed along the shear bands (SBs) on the surface of UFG Cu, whereas only a few fine shear lines and some non-propagation voids appeared on the surface of UFG Ti. As the temperature is above recrystallization, some small cracks (or voids) formed along grain boundaries and slip deformation took place in many coarsened grains, while only extrusions and intrusions instead of obvious cracks or voids are observable for UFG Ti. The corresponding microstructual changes after compressive deformation, e.g. grain coarsening, were also examined and confirmed by TEM observations.

Abstract: Future applications of ultrafine-grained, high performance materials produced by equal-channel angular pressing (ECAP) will most likely require processing on an industrial scale. There is a need for detailed microstructural and mechanical characterisation of large-scale, ECAP-processed billets. In the present study, we examine the microstructure and mechanical properties as a function of location and orientation within large (50 x 50 x 300 mm³) billets of an Al 6060 alloy produced by ECAP (90° channel angle) with different magnitudes of backpressure. The internal deformation is analysed using a grid-line method on split billets. Hardness is recorded in longitudinal and cross-sectional planes. In order to further characterise the local, post-ECAP mechanical properties, tensile tests in different layers are performed. Moreover, low voltage scanning transmission electron microscopy observations highlight relevant microstructural features. We find that the homogeneity and anisotropy of mechanical properties within the billets depend significantly on the geometry of the shear zone. We demonstrate that deformation gradients can be reduced considerably by increasing the backpressure: The opening-angle of the fan-shaped shear zone is reduced from ψ ≈ 20 ° to ψ ≈ 7 ° when the backpressure is increased from 0 to 150 MPa. Backpressures of 150 MPa result in excellent homogeneity, with a relative variation of tensile mechanical properties of less than 7 %. Our investigation demonstrates that ECAP is suitable for processing homogenous, high performance materials on a large scale, paving the way for advanced industrial applications.

Abstract: The corrosion behavior of ultra-fine grain (UFG) copper bulk prepared by equal channel angular pressing (ECAP) was studied in 3.5% NaCl solution. The effect of ECAP deformation on the copper corrosion is controversial in the literature, and worth to verify by means of various experimental techniques. Corrosion performances of UFG copper were investigated in comparison with that in recrystallized coarse grain (CG) copper by polarization curves, Tafel extrapolation method, electrochemical impedance spectroscopy(EIS). The shape of polarization curves and type of corrosive attack remains the same in the UFG and the coarse-grain state. UFG copper exhibited a lower corrosion current and high self-corrosion potential in comparison with CG copper. Electrochemical experimental results showed that UFG copper increased in resistance to corrosion compared with CG copper. This decrease in corrosion resistance was mainly attributed to the more compact corrosion film of UFG copper. The compact passive film led to decrease of the diffusion capability of ions within the corrosion film and corrosion rate.

Abstract: Steel plates with lath martensite microstructure were rolled up to 68% reduction at 673 K and then annealed at 473-973 K. The microstructure evolution was studied by using an optical microscope and a transmission electron microscopy. And the properties were investigated by using tensile tests and hardness tests. Results show that ultrafine grains + nano-carbides are obtained in the steel plates. The specimen annealed at 823 K has a good combination of strength and ductility. The tensile strength and total elongation are 1028 MPa and 7.2%, respectively. And the hardness is 338 Hv.

Abstract: In the present investigation, annealed billets of commercially pure Al (1050) with coarse-grained microstructure of 0.6 mm were ECAPed through a die with an internal angle of 90o using two routes A and BC. The samples were processed up to four passes using both routes. The change in the processing route results in the change of the shear plane, and consequently the change in the produced microstructure. The microstructure study was conducted on the extrusion direction and the shear plane. The cell size, misoriention and the fraction of high angle boundaries were determined by using electron back scattered diffraction (EBSD). A study of mechanical behavior was conducted by cutting tensile and compression specimens from the ECAPed specimen in the extrusion direction to study the effect of processing route and the number of passes on the deformation characteristics. Enhanced strength was observed but with anisotropic behavior between tension and compression. Cyclic deformation under load control (HSF) was also performed and the S-N curves were established as a function of number of passes and processing route. The fractography of fractured tensile specimens was also investigated.

Abstract: Copper based materials are still the most attractive low resistivity materials for microelectronics and electrotechnics applications, though, all variants developed to combine strength and conductivity, such as solid solutions and composites, suffer from decay in electric conductivity while strength is increased . In a addition, linear decay was also conjectured for pure copper when grain size is refined below the UFG and nanostructured domains (except when grain boundaries are pure twins). Copper alloys with low content of silver and chromium were prepared by high pressure torsion (HPT) with various annealing conditions. Vickers hardness and electric resistivity in the temperature range of 4K-340K, were measured as well as microstructural characterizations were performed using quantitative X-ray diffraction. Depending on the annealing conditions the alloys exhibit from 25% to 75% of IACS electric conductivity at room temperature and hardness in the range of 200 Hv. Origins of both high strength and high electric conductivity were investigated from microstructures analysis, using transmission electron microscopy and mechanical testing.

Abstract: Motivated by the large variety of enhanced properties of ultrafine and nanocrystalline materials such materials are under extensive investigation. Besides focusing on classical material parameters, like strength and ductility, the fracture toughness of these materials is also of great importance, especially when the damage tolerance is required. In this contribution an overview of the fracture behavior of different metals covering ultrafine-grained iron and nickel as well as a nanocrystalline steel processed via high pressure torsion (HPT) will be given. It will be shown that the specimen orientation can have a tremendous influence on the fracture behavior and toughness. Due to this toughness anisotropy an unexpectedly good combination of high strength and high fracture toughness can be achieved very often in these materials.

Abstract: The deformation behavior and the microstructure evolution in a 304-type austenitic stainless steel were studied in multiple forging tests at temperature of 700°C. The flow stresses increased to its maximum value with straining to about 1 and, then, slightly decreased resulting in a steady state deformation behavior at strains above 3. The structural changes were characterized by the development of a spatial net of deformation subboundaries, the misorientations of which increased to the values typical of conventional grain boundaries. The number of ultrafine grains increased with straining, leading to development of submicrocrystalline structure. The fraction of submicrocrystalline structure composed of ultrafine grains with an average size of about 300 nm exceeded 0.7 after straining to 2.

Abstract: The ultra-fine grained (UFG) materials have been widely investigated due to their mechanical properties such as super high strength and super high plasticity. The finite element simulation schemes are planned for the deformation mechanism of ECAP pressing. It will greatly affect the extrusion process when processing parameter changed such as die geometrical parameters and friction condition. It is considerable to determinate the range of die channel angle and die corner angle during ECAP process, and a moderate die corner angle is usually chosen. The friction condition of the ECAP should be lubricated as good as possible in the pressing on the basis of optimal die geometrical conditions. The ECAP process is a non-uniform shear deformation process in the cross-section of the workpiece for first-pass pressing. There have low angle grain boundaries along the cross-section of the grain microstructures. For the multi-pass pressing, although the pressing pass numbers are same, the processing routes were of important significance on the grain sizes and grain distribution and grain boundaries (GBs) orientations of workpieces. The formation mechanism of nanostructure was given for the ECAP through studying the deformation behavior and dislocation’s evolution. The dislocation is one of the main defects in the processed materials, but the form of the dislocations and its density are difference for different processing route. The processed workpiece with high surface quality, refined grain microstructure and optimization grain boundaries were obtained with optimal processing parameters and routes.

Abstract: An age-hardenable Cu-2.9%Ni-0.6%Si alloy was subjected to high-pressure torsion. Aging behavior was investigated in terms of hardness, electrical conductivity and microstructural features. Transmission electron microscopy showed that the grain size is refined to ~150 nm and the Vickers microhardness was significantly increased through the HPT process. Aging treatment of the HPT-processed alloy led to a further increase in the hardness. Electrical conductivity is also improved with the aging treatment. It was confirmed that the simultaneous strengthening by grain refinement and fine precipitation is achieved while maintaining high electrical conductivity. Three dimensional atom probe analysis revealed that fine precipitates with sizes of ~20 nm or smaller were formed in the Cu matrix and some particles consist of Ni and Si with no appreciable amount of Cu.