List of Abstracts 2016: SESSION E - ULTRA-FINE GRAINED MATERIALS

Severe Plastic Deformation (SPD) methods are the key to producing bulk ultrafine-grained (UFG) materials. This contribution highlights innovative research results associated with Equal Channel Angular Pressing (ECAP) of aluminum alloys. We observe heterogeneous microstructures in a AA6060 alloy after processing by extrusion and subsequent ECAP. Electron microscopy reveals a UFG microstructure in the shear bands, while matrix bands are characterized by a high fraction of low-angle grain boundaries and dislocation cells. These characteristic microstructural features affect the local mechanical properties, which are found to be distinctly different for the two types of bands, as well as the macroscopic properties. Moreover, we study low-temperature (-60 °C) ECAP of AA7075 as a new method to SP-deform even hard to deform, high-strength aluminum alloys. This approach takes into account the fundamental deformation mechanisms (cross-slip of dislocations) and their dependence on temperature, which results in increased formability at lower temperatures. We demonstrate that low temperature SPD results in homogeneously deformed billets without cracking. Finally, we discuss how fundamental research into SPD processing can be transferred to practical applications, considering two examples: room temperature extrusion (as a first shape-changing SPD method) and use of UFG high-strength aluminum as tailor-made damping materials in competitive sport sleds.

To evaluate the influence of the initial structure on nanostructuring processes various types of preliminary thermal treatment of medium carbon steel C45were used: 1) quenching with water austenitization at 800 ° C for 1 hour, followed by the release of high tempering (spheroidized condition); 2) austenitization at 800 ° C, isothermal transformation from austenite to a supercooled salt melt at 450 ° C and cooling in water (patented state); 3) quenching in water with austenitization at 800 ° C for 1 and subsequent low tempering (tempered martensitic state); 4) quenching in water from austenitization at 800 ° C for 1 hour (martensitic state). Samples with different initial state were subjected to severe plastic deformation by HPT. The development of the microstructure was investigated by ТЕМ and SEM. To compare mechanical properties tensile and microhardness tests were made. The results show clearly that the initial state influences both, microstructure and mechanical properties in steels after HPT. The highest strength (over 2500 MPa) was attained by combined treatment (martensitic state +HPT). The nature of the super-strength state in carbon steels is discussed.

The present paper explores the effect of annealing on mechanical and structural properties of ultrafine-grained Ti GRADE 2 after it was mechanically worked in either CONFORM SPD or rotary swaging machine. The main objective of this study was to optimize the annealing schedule in order to achieve recovery in the severely deformed structure and at the same time retain the mechanical properties of the worked material. The temperatures of the recovery processes were determined by means of dilatometry. It was found that in an ultrafine-grained material the temperatures of the recovery processes are shifted towards lower values. Based on dilatometric analysis, annealing schedules were developed and mechanical and structural properties were evaluated.

Ultrafine-grained materials produced by severe plastic deformation (SPD) exhibit high strength and ductility. None of the established SPD processes incorporates a change of shape. The semi-products commonly need to be further processed to final shape by additional cutting processes. In the present study, we investigate microstructures and mechanical properties associated with a shaping extrusion process operating below recrystallization temperature. Cast billets of the age hardening aluminum alloy 6060 are solid-solution annealed and extruded at room temperature (RT) and at a slightly elevated temperature (170 °C, AT). Electron back-scatter diffraction measurements in the center of the circular rods show a coarse-grained microstructure with a double fiber texture. Near the outer surface, fine- and ultrafine-grained pancake structures are observed, with fiber textures that are significantly rotated due to the pronounced shear deformation during extrusion. After aging of the RT-deformed material, increased hardness/ strength values (105 HV1, 348 MPa) and a homogeneous distribution are observed. In contrast, the AT-extruded material exhibits higher values in the center (108 HV1, 368 MPa). The novel low-temperature extrusion approach leads to strongly accelerated precipitation kinetics, higher hardness and a significant reduction of aging time, which offers a high potential for the integration in industrial process chains.

The new concept of modelling the plasticity of nanostructured materials having the particle size in the range from 3 to 20 nm is proposed. The classical thermodynamic approach employing the minimum number of physical parameters and no dislocation theory is the basis of this approach. The volume density of the surface energy of individual nanoparticles was determined and showed to be a significant part of the internal energy during deformation. Moreover, the dependence of the melting temperature on the nanoparticle size was modelled and compared with experimental data. The yield point was interpreted as the result of changes of surface energy of nanoparticles/nanograins during deformation. The calculated dependence of the yield point on the particle/grain size was related to the Hall–Petch law and confirmed the assumption of the critical role of surface tension forces in initial stages of plastic deformation of nanostructured materials.

In recent time, severe plastic deformation (SPD) processing has become greatly developed. This type of processing drastically transforms a material's structure to the ultrafine-grained or even nanostructured state. A number of papers have demonstrated the positive effect of such structural states on the decomposition kinetics of the supersaturated solid solution and the formation of enhanced physico-mechanical properties. In this connection, in the present investigation SPD with different accumulated strains during the processing was used at the stage of cold deformation. This approach enables revealing the efficiency of SPD processing and, consequently, the effect of the level of accumulated strain and the regime of subsequent aging on the formation of physico-mechanical properties. The following SPD processing techniques were employed in the study: high-pressure torsion (HPT) and equal-channel angular pressing (ECAP).
For a comparative study, search for the rational regimes of aging was conducted for the Cu-1Cr-0.08Zr alloy samples with a coarse-grained (CG) and an ultrafine-grained (UFG) structure at different temperatures and under different holding times. The range of aging temperatures of 400˚C, 450˚C, 500˚C was selected on the basis of previous studies, which demonstrated that temperatures below this range provide a very insignificant increment in mechanical properties, as compared with the initial ones. The aging time was 10, 20, 30 min, and subsequently 1 hours – 10 hours, with an interval of 1 hour. This interval of time was selected in order to show the decomposition kinetics of the solid solution.
Analysis of the obtained data has revealed that SPD processing leads to a noticeable enhancement in strength, and this process runs especially actively when HPT processing is employed. For instance, after one revolution (е=30) and five revolutions (е=270) hardness reaches 1840 MPa, in contrast to the initial hardness of 860 MPa, and the increment in hardness is 980 MPa. A similar picture, showing an increase in hardness, is observed after ECAP processing, but the increment in hardness is much lower. Subsequent heat treatment (aging) leads to a further growth in hardness, and this process runs most actively after HPT processing under both е=30 (1 revolution) and е=270 (5 revolutions).
After ECAP+HT, the best values of hardness (маx. 2320 MPa) are exhibited by the samples produced by 4 processing cycles and at the aging temperature of 450°С. The distinctive feature of this processing regime is the high thermal stability of the hardness value, as the decrease in hardness during 10 hours of holding time at the aging temperature was only 60 MPa.
Thus, regimes have been experimentally established that enable producing the best combination of strength and electrical conductivity in the Cu-1Cr-0.08Zr alloy. For ECAP processing, such a regime is deformation processing through 4 passes and aging at 500°С for 1 hour (strength 2320 MPa and electrical conductivity 80% IACS). For HPT processing, such a regime is 5 revolutions with subsequent aging at 450°С for 2 hours (strength 2650 MPa and electrical conductivity 80% IACS). ACKNOWLEDGEMENT: The authors acknowledge the financial support by the Ministry of Education and Science of the Russian Federation under Grant agreement No. 14.586.21.0025 (unique identification number RFMEFI58616X0025).