Materials Science Forum Vols. 503-504

Abstract: Microstructural evolution during equal channel angular extrusion (ECAE) was investigated in a coarse-grained dilute aluminum alloy, Al-3%Cu, at a temperature of 250oC. Scanning electron microscopy (SEM) with electron back scattering diffraction (EBSD) and optical metallography (OM) was used to reveal the structural changes in the alloy deformed up to a strain of ε=12. The microstructural evolution at initial and moderate stages of deformation is characterized by the formation of low angle boundaries and deformation bands with moderate misorientations in grain interiors. With further deformation the number and the misorientation of the deformation bands increase, finally leading to the subdivision of original grains by these bands and then the development of fine grains with an average size of about 6 μm at ε=12. The evolution of deformation bands in initial grain interiors and their role on new grain formation are discussed in detail.

Abstract: Mechanical spectroscopy was employed to investigate the microstructure evolution of a Zr-modified 6082 Al-Mg-Si alloy and the same alloy with Sc addition after ageing and following severe plastic deformation through equal channel angular pressing. Measurements of the internal friction and dynamic young modulus have been performed in isothermal and isochronal runs in the frequency range 0.1 - 104 Hz. The anelasticity spectra reveal in the temperature range 470-870 K both structural and anelastic relaxation processes. Two structural damping maxima connected with inverse temperature trend of the modulus occur in the alloys submitted to equal channel angular pressing, the first one is strongly suppresed by Sc and Zr addition. An anelastic relaxation peak whose strength depends on the nature and morphology of precipitates and dispersoids and on the deformation and ageing condition was observed in all samples investigated. The high background damping occurring before the first structural damping maximum is analyzed with reference to a superplastic behavior of the equal channel angular pressing processed alloys.

Abstract: The microstructural evolution with strain was investigated either in a Zr-modified 6082 Al-Mg-Si alloy and in the same alloy added with 0.117wt.% Sc, subjected to severe plastic deformations. Materials were deformed by equal-channel angular pressing using route BC, up to a true strain of ∼12. A strain of ~4 produced a sub-micrometer scale microstructure with very fine cells (nanometer scale) in the grain interior. The role of fine dispersoids (Al3(Sc1-x,Zrx)) was investigated by transmission electron microscopy techniques and discussed. Dispersoids were responsible for a more complex dislocation substructure with strain. Compared to the commercial parent alloy, block wall formation and propagation were favored by the presence of Sc-Zr containing dispersoids, while cell boundary evolution was less affected, compared to the commercial parent alloy. Mean misorientation across block walls increased with strain much more in the Sc-Zr containing alloy, reaching a plateau, starting from a true strain of ∼8. Misorientation across cell boundaries continuously increased to ∼8° and ∼5° for the Sc-Zr and Zr containing alloy, respectively.

Abstract: The mechanical properties of fine-grained Aluminium AA1050 produced by ECAP at room temperature, have been investigated under various deformation modes. Because ECAP leads to an unstable microstructure, some samples were annealed at 300°C for 10 minutes in order to stabilise the microstructure. For mechanical testing, cylindrical samples were used in three types of monotonic deformation tests: axisymmetric compression, uniaxial tension and simple shear by torsion. The influence of the deformation mode on the yield locus and strain hardening behaviour has been studied and will be discussed for both hot rolled AA1050, ECAP samples without annealing and annealed ECAP samples. To achieve a better understanding of the strain hardening, some preliminary tests with a change in strain path were also performed. The results of tension tests followed by compression will be reported.

Abstract: New data on structure and texture features of Cu and Ti rods, subjected to ECAP at 20oC and 400oC respectively, were obtained by means of advanced X-ray diffractometric methods. A deformation inhomogeneity through rod’s cross-section was studied by reduced cubic samples 3x3x3 mm, cut out from different regions of rods. The inhomogeneity of ECAP rods is characterized by distributions of texture and substructure parameters. Main tendencies in structure formation by ECAP are revealed.

Abstract: A detailed X-ray study of Zr rods, subjected to ECAP at 350oC by routes C and BC, was conducted by the new X-ray method of Generalized Pole Figures, combining texture measurement with registration of X-ray line profiles. The data analysis is based on conceptions of the texture formation theory, connecting features of grain reorientation with activated deformation mechanisms. A degree of reproduction of the same distinctive texture by successive ECAP passes with antecedent rotation of the rod reflects attendant structure changes in material.

Abstract: Experiments were conducted both to evaluate the potential for grain refinement, the subsequent mechanical properties at room temperature in samples of AZ31 Mg alloy and also to investigate the relationship between one-step and two-step high ratio extrusion (HRE). The one-step HRE was undertaken using a high extrusion ratio of 70:1 at 250, 300 and 350°C. And the two-step HRE was conducted with an extrusion ratio of 7 for the first step at 250, 300 and 350°C, followed by a second-step extrusion with an extrusion ratio of 10 at 250, 300 and 350°C. The initial grain size in the AZ31 ingot was 100μm and that after one-step HRE became similar to 5μm, after two-step HRE at 250, 300 and 350°C were 2, 4, 7μm, respectively, resulting in superior mechanical properties at ambient temperature. The microstructure of two-step HRE was finer and uniformer than that of one-step HRE and the strength of one-step and two-step HRE were similar, moreover, the elongation of one-step HRE was improved markedly than that of two-step HRE. Dynamic recrystallization and adjacent grain broking during HRE is introduced to explain the effects of one-step and two-step HRE on the microstructure and mechanical properties of AZ31 Mg alloy. The current results imply that the simple HRE method might be a feasible processing method for industry applications, and the multiply steps extrusion are effective to fabricate high strength of fine grained hcp metals.

Abstract: The enhanced mechanical properties of crystalline materials are linked to very small grain sizes. The AA8090 is a commercial aluminium lithium alloy is referred in the metallurgical literature by its superplastic behavior linked as well to nano-grains as obtained by ECAP. On this research-work in progress, the 8090 aluminium alloy is studied after being processed in a 400 kN ECAE press up to nine passes through a 90º angle-die at 150 °C following Route A (constant path) at a processing speed of 10 mm/min. The room temperature mechanical properties of the extruded alloy are investigated through hardness measurements and tensile tests. Elastic measurements have been carried out by means of nanoindentation and ultrasonic testing, showing that both texture and nanostructuration of the processed alloy influence on the elastic constant of the processed material. In order to understand the mechanisms involved in the SPD-ECAP process, microstructural characterisation of the alloy has been carried out using different high resolution techniques such as Transmission Electron and Atomic Force Microscopy. TEM results have also been used in order to evaluate grain size evolution. From this analysis it is possible to see that after 4 passages the initial grains have already a high degree of dislocations. Among the characterisation techniques, AFM in the surface potential mode has been used to evaluate discontinuities in the matrix and /or decohesion between matrix and precipitates. After nine ECAP passages, no signs of decohesion at the interface have been observed neither using FEG-SEM nor using AFM.

Abstract: In this paper indirect ECA drawing method was supposed to be feasible for application. This method will ensure same deformation degree as that in ECAP while keep the size of the billet unchanged so that multiple processing can be done. With this method, materials with both rectangular and round section can be processed and there is no limit to the length of the materials. Die configuration was set up and the equations for relationship among die parameters, billet parameters and its properties, drawing forces and applying pressures are given.

Abstract: Microstructure evolution and corrosion behaviour of ultrafine-grained copper processed by equal channel angular pressing (route Bc) were studied. The results of TEM investigation of the microstructure evolution are presented along with the measurements of the corrosion potential, the corrosion current density and the anodic current density for two aggressive media, viz. 3% NaCl and 1M H2SO4. An important finding and a good news is that the corrosion behaviour of ECAP copper is not inferior to and does not qualitatively differ from that of the coarse grained material. Moreover, it was shown by SEM investigation that the corrosion damage is more homogeneous in ultrafine grained ECAP processed copper than in its coarse grained counterpart.