Materials Science Forum Vols. 503-504

Abstract: Grain refinement is attempted using severe plastic deformation (SPD) through the severe torsion straining process (STSP) which we have developed recently. The STSP is a continuous process for grain refinement without requirement of any die. In this study, an AZ61 Mg alloy was subjected to STSP at a temperature of 573 K with a rotation speed of 10 rpm and a moving speed of 200 mm/min. With this process, an initial grain size of ~20 μm was reduced to ~2~3 μm. Room temperature compression tests revealed that there were no cracks after 15% of compression for the STSP sample whereas fracture occurred for a conventionally extruded sample. For compression tests at 473 K, no cracks occurred in the STSP samples after 80% compression but compression was feasible without cracking only up to 20% for an extruded sample. It is shown that the STSP can be useful for grain refinement and ductility improvement of the AZ61 Mg alloy.

Abstract: Severe plastic deformation (SPD) makes it possible to refine grain size in many metallic materials. Recently, we have developed a new SPD process designated the severe torsion straining process (STSP). This process requires no die but one side of a rod is rotated with respect to the other while producing a local heated zone in the rod and cooling both sides of the heated zone. Torsion strain is then introduced in the local heated zone. The STSP can be a continuous process because the rod is moved in the longitudinal direction while introducing torsion strain through the rotation. For grain refinement using the STSP, various factors may affect, which are the rotation speed, moving speed, straining temperature, cooling rate and diameter of the rod. In this study, the STSP is applied to grain refinement of an A5056 Al-Mg commercial alloy and the factors affecting the grain refinement are optimized. STSP was conducted at a temperature in the range from 573K to 723K. Microstructure was observed by optical microscopy, scanning electron microscopy with an orientation imaging system, and transmission electron microscopy. Microscopy observations revealed that the grain size was reduced to ~0.9 μm, when STSP was conducted at 573K with a rotation speed of 10 rpm and moving speed of 50 mm/min. There is a critical ratio of rotation speed to moving speed above which the rod breaks. The grain size tends to be finer as the straining temperature is lower, the cooling rate is faster and the ratio of rotation speed to moving speed is closer to the critical value.

Abstract: In this work severe plastic deformation (SPD) was applied to magnesium base alloys of the Mg-Sm system (2.8-5.5 mass %Sm). These alloys are characterized by high strength at elevated temperatures and high strengthening effect during aging. SPD was performed by torsion under pressure of 4 GPa at 20 and 200°C to ε ∼ 6. SPD results in significant strengthening of the Mg-Sm alloys due to the formation of submicrocrystalline structure. In all cases SPD accelerates the solid solution decomposition upon subsequent aging. The highest strengthening can be obtained if the solution treated alloy is aged at 200°C after SPD at room temperature. The state of high strength can be also reached if the following sequence of the operations is used: solution treatment + aging at 200 °C up to maximum hardness + SPD at 20°C + aging at 200°C accompanied by Sm –rich phase precipitation in the submicrocrystalline matrix.

Abstract: Strengthening is a complex process involving such basic mechanisms as dislocation accumulation (work hardening), Hall-Petch hardening due to grain refinement, solid solution hardening and precipitation hardening in various combinations. The contribution of different mechanisms into resultant strength can vary significantly depending on chemical composition and processing. The purpose of the present work is to explore the significance of SPD for hardening and to clarify the role of different strengthening mechanisms. The model Au-based system was employed using pure Au, single phase solid solution Au-25Ag and precipitation hardenable Au-12.5Ag-12.5Cu (in mass %) alloy subjected to ECAP. The additive character of different strengthening mechanisms is clearly demonstrated. The extremely high strength exceeding 1 GPa is achieved in the Au-Ag-Cu after ECAP followed by aging, which has never been attained in conventional processing schemes.

Abstract: The present work is aimed at linking the microstuctutral features obtained after severe plastic deformation via ECAP to the tensile behavior and thermal stability of pure (99.98%) copper processed by routes A and Bc to different number of passes. The main conclusion one can draw unambiguously from the currently available results is that the strain path exerts relatively little effect on the resultant tensile properties when the number of pressing is sufficiently large, although there have been some marked differences in crystallographic textures and distribution of grain-boundaries. The effect of the number of pressings on the tensile ductility is considerable.

Abstract: An Fe-2%Si alloy, which was designed for electromagnetic applications was submitted to a series of plane strain compression (PSC) tests with reductions of 25, 35 and 75% at temperatures varying from 800 to 1,100°C and at a constant engineering strain rate corresponding to a constant cross velocity of 20 mm/s. The initial structure of the material displayed nearly equi-axed grains with an average size of 80 μm. The as-received texture was characterised by a nearly random cube fibre (<100>//ND) with a relatively weak maximum on the rotated cube component ({001}<110>). After deformation the samples were water quenched in order to avoid post-process static recrystallization events. The microstructures were analysed by orientation imaging microscopy (OIM) revealing that the zone of PSC was restricted to the central layers of the sample but minimally covering 50% of the sample thickness. After deformation at 800°C the conventional lamellar deformation structures were observed on the sections perpendicular to the transverse direction of PSC. At higher deformation temperatures the structure was of a bimodal nature consisting of lamellar deformation bands and equi-axed small grains. The volume fraction of these small equi-axed grains increased from 19.9% after 75%reduction at 800°C to 67.8% after 75% reduction at 1.100°C. After 75% reduction the equi-axed grains exhibited an average size of 10 μm which represents a strong grain refinement with respect to the initial size of 80 μm prior to PSC. Ferrite Silicon steels undergo extensive dynamic recovery during hot working. Dynamic recrystallization (DRX), though, has not yet been reported for these alloys although the present data suggest that a DRX mechanism might be responsible for the remarkable grain refinement after relatively low amounts of strain applied at high temperatures.

Abstract: The effect of equal-channel angular pressing (ECAP) on the structure-phase state and superplasticity development was investigated using the 1421 Al-Mg-Li alloy. The physical reasons for the displacement of the temperature range of superplasticity to lower temperatures after ECAP by comparison with the initial state are considered. Possible reasons are discussed for the decrease in the activation energy of true grain boundary sliding in the alloy produced using ECAP by comparison with the initial condition.

Abstract: A self-organization model for repartition of dislocation cell structures and transition of subgrains on a three-stage hardening of single crystal are developed. Stress-effect coefficients models are proposed in order to introduce stress information into the reaction-diffusion equations. A FD simulation for dislocation patterning and a FE one for crystal deformation are simultaneously carried out for an FCC single crystal. It is numerically predicted that a cell structures are repartitioned and the generated dislocation pattern in stage III can be regarded as a subgrain.

Abstract: It has been revealed that in Iridium influenced be severe plastic deformation (SPD) a ultrafine grained (UFG) structure is formed (the grain size of 20-30 nm), but in the bodies of grains there are practically no defects of structure, however, after irradiation a subgrain structure, (subgrain size of 3-5 nm) is formed, and in the bodies of subgrains there are defects. The subgrain structure was also revealed in UFG Nickel and Copper after SPD (subgrain size of 3-15 nm), but in the latter case the observed boundary region is broader and subgrain are highly disoriented.