Materials Science Forum Vols. 667-669

Abstract: The Al-Cu-Mg-Ag heat-resistant alloy were prepared by ingot metallurgy technology. Effects of serve plastic deformation on microstructure and mechanical properties of the alloy were investigated by microscopy, differential scanning calorimetry and hardness tester. It has been shown that serve plastic deformation in the solid solution and quenching state with aging treatment was found to be quite effective in refining the grains to 2 μm, and improving the mechanical properties and heat-resistant properties of the extruded Al-Cu-Mg-Ag alloy. Serve plastic deformation treatment accelerates the aging hardening process of the extruded alloys, increases the density of precipitate phase.

Abstract: The effect of ultrasonic treatment on the microstructure, microhardness and thermal stability of pure nickel after high pressure torsion (HPT) was studied. It was shown that the ultrasonic treatment reduces internal stresses induced by severe plastic deformation. The higher the intensity of ultrasound in the range studied, the stronger is this effect. Also it was revealed that grain growth in nickel processed by HPT followed by ultrasonic treatment occurs at higher temperatures than that in nickel as-processed by HPT, i.e. the thermal stability of nanostructured nickel is increased.

Abstract: The ultrafine copper with twins was prepared by asymmetrical accumulative roll-bonding (AARB) and heat treatment. The characteristic of grain refinement and the texture transformation during recrystallization were investigated. The results show that the crossover areas, which were formed by the shear stress during AARB, were zones of compression deformation and shearing deformation when the deformation of copper occurred in these areas. The average grains size was reduced to 1μm from original average size of 30～50μm due to processing by AARB. The strong elastic stress accumulated by shearing texture is responsible for oriental growing of a great number of grains during recrystallization. The shearing texture {001}<110> rotates 54.7°and transforms into annealing texture {111}<211> i.e. twins microstructure.

Abstract: Within the framework of crystal plasticity the grain refinement observed in metals exposed to severe plastic deformation(SPD) is interpreted as an energy minimization process. For a crystalline solid it is less energetically costly to be fragmented in small regions(subgrains) deformed by a reduced number of slip systems and fitted together by local reorientations than to be deformed by homogeneous multislip.The energy minimization enhanced by hardening tends to reduce the size of the subgrains. On the other hand, the refinement increases the area of the boundaries between the subgrains, hence, it increases interface energy. Thus, the size of the subgrains is the result of a competition between these two tendencies:the internal and dissipative energy tend to decrease the refined structural size, whereas the interface energy opposes this tendency. Moreover, the formed substructure pattern follows the orientation of the active slip systems. The effective rotation of the active slip systems, which is a basic charac-teristic of SPD processing techniques, enhances the refinement process. It is proposed that the effective rotation causes a continuous reconstruction of the substructure pattern. The hindered destruction of the previous pattern leads to an increased hardening thus reducing the size of the subgrains. The proposed approach is demonstrated in the analysis of the refinement process induced by high pressure torsion and the results are correlated with the available observations.

Abstract: Strain-induced grain refinement in magnesium alloy AZ61 was studied by means of cyclic bending carried out at a temperature 623K. The deformed microstructures were investigated by optical and SEM/EBSD metallographic observation. The results show that the grain size of the surface layer can be significant refined to about 3 μm by dynamic recrystallization. The volume fraction of new grains increases with strain and reaches a value of 0.8 after 8 passes. Most of the new grains are separated by high angle boundaries. Hardness distribution through thickness for the cyclic bended sheet exhibits “V” shape with the marked difference between the surface and the inside, the hardness is critically increased in surface layer, i.e. ~82 Hv, which is about 1.39 times than the as-received AZ61.

Abstract: The deformation behavior of commercial Zr alloys with 1% and 2,5%Nb under compression at temperatures of the (α+β)-region of Zr-Nb phase diagram is considered on the basis of experimental data obtained by X-ray texture study of deformed samples. Mechanisms, responsible for plastic deformation of alloys by different temperature-rate regimes were determined on the basis of resulting textures. Among these mechanisms there are crystallographic slip and mutual displacements of crystallites along interphase boundaries. The latter mechanism sharply intensifies by grain fragmentation down to nanostructuring under conditions of α«β phase transformations. Texture features of deformed samples testify about interaction of plastic deformation with phase transformations and indicate that due to this interaction compression by optimal regimes promotes the utmost refinement of structure elements.

Abstract: The Mg-5Zn-2.5Er matrix composite reinforced with the in-situ synthesized Mg2Si second phase particles was fabricated via repeated plastic working (RPW) process. The microstructures and the nanocrystals in the composite have been investigated using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HREM) and energy dispersive X-ray (EDX). Great deals of nanocrystals were found in the matrix, and they were around the in-situ synthesized Mg2Si. The HREM analysis showed that the size of nanocrystals was in the range of 5-10 nm, and the difference in their crystallographic orientation was bigger than 15°. It is suggested that the formation of nanocrystals in the matrix is attributed to the RPW deformation process and to the intensive stresse fields around the in-situ synthesized Mg2Si particles, which suppress the growth of nanocrystals by forming nonequilibrium grain boundaries containing disordered dislocation networks and junction disclinations.

Abstract: Copper fragments are found to adhere on penetration channel wall after copper jets penetrate steel target, and the research on it is helpful to know microstructure evolution of jets in the process of penetration and cooling time. This paper is based on the observation of bore in steel target penetrated by copper jets, and uses numerical simulation to study the process of copper jets penetration, then the change of temperature and grain size of jets adhered on penetration channel wall can be gotten, and the results agree with the observation of penetration channel wall taken by SEM. From the observations of copper and steel, we can get the conclusion that copper jets are not melted but have dynamic recrystallization in the process of penetration, then copper grain size increases obviously in cooling time, and twins are formed at the same time.

Abstract: Instrumented high pressure torsion, i.e. mechanical test in a torsion mode under high pressure, allows interesting possibility of materials testing, because materials mechanical response can be studied in a practically unlimited shear strain range. We have studied microstructures formed in initially coarse crystalline and nanocrystalline (nc) Pd and its alloys after instrumented HPT up to shear strain 300, and revealed signatures of similar processes occurring in all these materials. In particular, we found traces of cooperative grain boundary sliding in the form of aligned in parallel segments of boundaries of several grains with straightened triple points. Fracture surfaces contained shear bands. Texture measurements revealed lower dislocation activity in nanocrystalline state as compared with coarse crystalline one. Therefore we argue that cooperative grain boundary sliding is an important deformation mechanism at large strain which develops in both ultrafine grained (ufg) and nanocrystalline materials. In nc and ufg materials planes of cooperative grain boundary sliding act as precursors of shear bands and shear occurs along planes formed by numerous grain boundaries.

Abstract: The presence of a hydrostatic pressure as a general feature of SPD methods is essential for achieving the high strains and the introduction of the high amount of lattice defects, which are necessary to establish new grain boundaries. Systematic investigations of High Pressure Torsion (HPT)-deformed Cu under variation of strain and hydrostatic pressure revealed marked differences between the in-situ torsional stress (torque measurement) and the post-HPT strength of the ultrafine-grained materials. These facts let assume the occurrence of relaxation processes (recovery/recrystallisation) of static character with respect to the release of the hydrostatic pressure after straining. In order to gain insight into the processes behind, a special experimental procedure was designed to simulate the hydrostatic pressure release. Investigations by X-ray line profile analysis and hardness measurement show marked influences of the pressure release on microstructure and strength. While the size of the coherently scattering domains is not strongly affected, the dislocation density decreases drastically and the arrangement of the dislocations within the subgrain structure changes to a less stress intensive one, upon the pressure release. In parallel the hardness decreases significantly and confirms the discrepancy between in-situ torque-stress and post-HPT strength.