Materials Science Forum Vols. 584-586

Abstract: Severe plastic deformation (SPD) has received considerable attention for its capability to produce ultrafine and nano structured materials. On the one hand, SPD, especially in the forms of equal channel angular pressing (ECAP) and high pressure torsion (HPT) is able to refine bulk materials with coarse grain structures. On the other hand, SPD has been used to synthesise bulk materials from particles. It enables particles from nano to micro scales to be consolidated into fully dense materials at much lower temperatures and shorter times, compared to the conventional sintering processing. It is particularly relevant to consolidating particles with non-equilibrium microstructures and to producing complex multiphase alloys. In this summary, ECAP as an effective process to synthesise a range of light metal based materials from particles with various sizes and structures, including aluminium and aluminium composites, titanium and magnesium, will be demonstrated. Full density and good bonding are achieved easily with the application of a back pressure. Microstructures from nano to ultrafine scales have been produced, resulting in significantly enhanced strength. Simultaneous increase in ductility has also been achieved in some alloys by virtue of multi-scale structures.

Abstract: The aim of this paper is the investigation of electroplastic deformation (EPD) and subsequent annealing influence on martensitic transformation in the shape memory Ni50.7Ti49.3 alloy. Using differential scanning calorimetry method it was shown that EPD at the low strain stimulates structure relaxation and recovers martensitic transformation in cooling, which is usually suppressed by cold rolling.

Abstract: All the SPD techniques introduce reversal straining principally, but effects of the reversal deformation on structure evolution were not studied directly yet. In the present work, an attempt was made to manage structure in pure (99.99%) Al by strain reversal through high pressure torsion (HPT). Total accumulated deformation up to equivalent strain ~8 was used. General trend of the grain refinement is similar for both deformation modes; and it is typical with all other SPD processed FCC metals. At the same time, the difference in microstructure evolution at the vicinity of the specimen axis and with increasing distance in the radial direction introduces microstructural heterogeneities which are specific features of the reversal straining. In the monotonic deformation process the A ({111}<011>) fiber is gradually substituted by the C component ({ 0 0 1}< 1 1 0>) with increasing strain before it is found to weaken. In the reverse straining process the A fiber is found to dominate the deformation texture in the low strain region. In the reverse straining process at high strain level, a {001}<100> component appear.

Abstract: Batch SPD processes have a limited scope for being used on an industrial scale. More feasible are continuous processes among which the new SPD process of Incremental ECAP (IECAP) is an attractive option. In this paper, a double-billet version of I-ECAP, which doubles process productivity, is presented. The concept of the process is first checked using the finite element (FE) method. FE simulation results are the basis for the design of an experimental rig. Trials of nanostructuring of 10x10x200 Al 1070 billets are carried out with the forces on the reciprocating die and the feeder measured. Metallurgical samples after 4 and 8 passes of I-ECAP (route BC) are investigated using TEM. Tensile properties after 8 passes are established. All these results show that the new SPD process of I-ECAP gives the results comparable to those obtained by a classical batch ECAP with the added capability of dealing with much longer (possibly infinite) billets.

Abstract: The present work was performed in order to analyze the influence of the outer corner radius (R) of ECAP die channels on the strain field of billets subjected to ECAP deformation in a Φ = 120o die, employing three different methods: (i) physical simulation, consisting of the direct measurement of deformations of a grid inscribed in longitudinally cut mid-planes of ECAPed billets; (ii) numerical simulation employing an explicit finite element code for large displacements and large plastic deformations, and (iii) calculation by the Iwahashi formula. Materials employed were Al-4%Cu and an eutectic Pb-62Sn alloy, and the dependence of shear strain with R was satisfactorily described using the three methods. The experimental method showed a small deviation from the other two, which was explained making use of the corner die formation concept. Similarly, this concept helped to understand the increase of strain heterogeneity with R. Also, it was shown that large corner radii decrease ECAP pressing loads, facilitating deformation of high strength materials. Finally, the data show that the deformation characteristics of the materials here studied do not exert a measurable influence on the shear strain magnitude and distribution.

Abstract: Homogeneous nanocrystalline structure with the average grain size of about 300 nm was produced in Ti2AlNb-based intermetallic alloy by a thermomechanical processing which included multistep isothermal forging at temperatures below the β-transus and intermediate annealings. Nanostructured material possessed excellent mechanical properties. At room temperature, elongations up to 25% were obtained and the ultimate strength reached 1400 MPa. The alloy exhibited superplastic behavior in the temperature range of 850-1000°C. The maximum elongation of 930% and steady state flow stress σ50 of about 125 MPa were obtained at 900°C and strain rate of 4.2×10-3 s-1. The rolling temperatures of nanostructured alloy were defined from analysis of its mechanical behavior at a typical rolling strain rate of about 10-1 s-1 and intermetallic sheets with improved mechanical properties were produced.

Abstract: The stress-strain curves at high strain rate superplasticity were analyzed for Ti-6Al-4V and aluminum 1420 industrial alloys in ultra-fine grained state produced by severe plastic deformation. For both alloys the observed strengthening effect can be caused by grain growth under the above mentioned conditions resulting to increase of grain boundary sliding resistance. In the case of aluminum alloy the grain recovery can be accompanied by significant change in phase composition which is also stimulates the recrystallization process.

Abstract: Equal-channel angular pressing (ECAP) at 443 K was used to introduce an ultra-fine grained (UFG) microstructure to a Zr and Sc modified 7075 aluminum alloy. Using the methods of TEM and EBSD, an average grain size of 0.6 1m was recorded after the pressing. The UFG microstructure remained very stable up to the temperature of 723 K, where the material exhibited high strain rate superplasticity (HSRSP) with elongations to failure of 610 % and 410 % at initial strain rates of 6.4 x 10-2 s-1 and 1 x 10-1 s-1, respectively. A strain rate sensitivity parameter m in the vicinity of 0.45 was observed at temperatures as high as 773 K. At this temperature, the material still reached an elongation to failure of 430 % at 2 x 10-2 s-1. These results confirm the stabilizing effect of the Zr and Sc additions on the UFG microstructure in a 7XXX series aluminum alloy produced by severe plastic deformation.

Abstract: Tensile tests were carried out at 473 K with initial strain rate of 10-4 s-1 in samples of a ZK60 alloy (Mg-5.5% Zn-0.5% Zr) processed by different number of passes using Equal-Channel Angular Pressing (ECAP). The measured superplastic elongations ranged from ~930% to a record of ~3050%. The flow behavior was found to vary with the number of passes of ECAP. It is shown that strain-hardening due to grain growth and the evolution of the strain rate sensitivity with the strain determines the flow behavior and final elongations. The results are consistent with theories of plastic flow in tensile testing.

Abstract: Microstructure and mechanical properties of the ultrafine-grained (UFG) 1421 aluminum alloy processed by equal channel angular pressing (ECAP) have been studied. This UFG material was successfully rolled under the conditions of superplasticity. It was established that the rolled material exhibited not only the enhanced superplasticity, but also high strength retaining initial ductility at room temperature after additional short-term annealing and low-temperature aging.