Materials Science Forum Vols. 667-669

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 (εef) 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 from 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: A new approach is proposed to control the processes of billets plastic deformation during metal treatment by the methods of severe plastic deformation (SPD). High strength and plasticity have been attained for the processed copper billets after multiple repetitions of equal channel angular hydroextrusion (ECAH) and direct hydroextrusion (HE) techniques and with ECAH and HE implementation in the fractional mode. The combined SPD treatment including ECAH, HE and drawing (D) provided for fire refined tough pitch copper (Cu-FRTP) the ultimate tensile strength =686 MPa, the elongation to failure =2% and the electrical conductivity (EC) at a level of 86.4% IACS and for oxidant free copper (Cu-OF) =576 MPa, =1.9%, EC=96.7% IACS in the 0.5 mm diameter wire. Such treatment is efficient due to the alternative schemes of deformation, the fractional mode and the optimum degrees of plastic deformation and periodic creation of favorable conditions for relaxation and dynamic recrystallization processes in the material.

Abstract: Deformation mechanisms occurring by tension of ultrafine-grained aluminum processed by equal-channel angular pressing at room temperature are investigated using comparative study of the microstructure before and after tensile testing as well as deformation relief on the pre-polished surface of the sample tested. Deformation behavior and structure evolution during tension suggest development of grain boundary sliding in addition to intragrain dislocation slip. Contribution grain boundary sliding to the overall deformation calculated using the magnitude of shift of grains relative to each other is found to be ~40%.

Abstract: The possibility of equal-channel angular pressing (ECAP) of steel billets with welded seams, the study of the microstructure and determine the characteristics of the weld strength and ductility is investigated. Two cylindrical work pieces (20 mm) of steel St3sp obtained by welding were subjected to ECAP. Investigation of the weld microstructure was carried out for three zones: base metal, weld metal and heat affected zone. Determining results the strength and ductility characteristics of the weld in the initial state and after ECAP are presented.

Abstract: The effects of severe plastic deformation (SPD) by isothermal rolling at the temperature of liquid nitrogen combined with prior- and post-SPD heat treatment, on microstructure and hardness of Al-4.4%Cu-1.4%Mg-0.7%Mn (D16) alloy were investigated. It was found no nanostructuring even after straining to 75%. Сryodeformation leads to microshear banding and processing the high-density dislocation substructures with a cell size of ~ 100-200 nm. Such a structure remains almost stable under 1 hr annealing up to 200oC and with further temperature increase initially transforms to bimodal with a small fraction of nanograins and then to uniform coarse grained one. It is found the change in the alloy post–SPD aging response leading to more active decomposition of the preliminary supersaturated aluminum solid solution, and to the alloy extra hardening under aging with shorter times and at lower temperatures compared to T6 temper.

Abstract: An AA2014 alloy was subjected to equal channel angular pressing in quenched condition up to fixed true strains of 1, 2 at a temperature of 170oC followed by artificial aging at the similar temperature. ECAP provides accumulation of a high dislocation density after a true strain of 1 and the formation of subgrain structure and separate chains of nanoscale grains after a true strain of 2. Yield stress (460 MPa) of the AA2014 alloy after 1 and 2 passes was the same. In contrast, increasing strain from 1 to 2 leads to three-fold increase in ductility.

Abstract: The microstructure and mechanical properties of 304 stainless steel were investigated which was subjected to equal channel angular pressing (ECAP). Tensile strength, elongation, Vickers hardness of as-ECAPed and annealed ECAPed 304 stainless steel were systematically measured and compared and microstructure evolution during ECAP and ECAP+annealing was observed by OM and TEM. It was found that with the increasing of ECAP passes, the grain size of stainless steel was effectively refined to nanoscale, such as about 50 nm after 8 ECAP-passes. In addition, the dislocation density in ECAPed samplel increased greatly, consequently, the tensile strength and hardness of ECAPed 304 stainless steel increased and elongation decreased remarkably. After annealing at 600°C for 10 min，the ductility of ECAPed stainless steel was improved greatly while grains did not have obvious growth, and strength did not change much. The above results showed that the optimization of strength and ductility in ultra-fined 304 stainless steel can be achieved by appropriate ECAP plus annealing processes.

Abstract: This work is devoted to enhancement of strength and ductility of the Ti-6Al-7Nb ELI alloy, which is less harmful from medical point of view for human body in comparison to Ti-6Al-4V. It has been demonstrated that formation of an ultrafine-grained structure in the alloy with the help of equal-channel angular pressing in combination with heat and deformation treatments allows reaching high strength (UTS = 1400 MPa) and sufficient ductility (elongation 10 %).

Abstract: An Al-4.57%Mg–0.2%Sc was subjected to equal channel angular pressing up to fixed true strains of 1, 2, 4, 8 and 12 at a temperature of 300oC. It was shown that extensive grain refinement occurs in this alloy through continuous dynamic recrystallization. As a result, ECAP can provide the formation of subgrain structure, partially recrystallized structure and fully recrystallized structure. The type of structure evolved is dependent on strain imposed. At ε2, the formation of three-dimensional arrays of low-angle boundaries takes place. Next, in the strain interval from 4 to 8 these low-angle boundaries gradually convert into high-angle boundaries. At ε12, fully recrystallized structure is evolved. Yield stress and ultimate strength gradually increases with increasing strain. Mechanisms of strengthening are discussed.

Abstract: In most papers dealing with tension and/or compression tests, the conventional yield stress is determined either by an offset method (usually 0.2% strain) or by back extrapolation from the stress-strain curve. In our experiments on ECAP’ed Aluminium a transient hardening saturation (THS) is always observed during the compression tests, but not during the tensile tests. This THS occurs at a significantly lower stress than the conventional yield stress. The aim of the present paper is to determine which the “real” start of yielding is. Two different experimental approaches have been adopted, confirming that the THS stage is exactly the yielding stage. This is not unimportant because it increases the tension-compression asymmetry and hence the back-stress and kinematic hardening. The reason for this different behaviour between tension and compression can be ascribed to a different change in strain path with respect to the ECAP deformation.