Materials Science Forum Vols. 584-586

Abstract: The mechanical properties and microstructural characteristics of a precipitation hardenable Al 7075 alloy subjected to rolling at liquid nitrogen temperature and room temperature are reported in this present work. The Al 7075 alloy was severely rolled at cryogenic temperature and room temperature and its mechanical properties were studied by using tensile tests and hardness. The microstructural characterization of Al 7075 alloy were made using SEM/EBSD, TEM and DSC. The cryorolled Al 7075 alloys have shown improved mechanical properties as compared to the room temperature rolled Al alloy. The cryorolled Al alloy after 90% thickness reduction exhibits ultrafine grain structure as observed from its TEM micrographs. It is observed that the strength and hardness of the cryorolled materials (CR) at different percentage of thickness reductions are higher as compared to the room temperature rolled (RTR) materials at the same strain due to suppression of dynamic recovery and accumulation of higher dislocations density in the CR materials.

Abstract: The strain rate sensitivity of the aluminium alloy AA6061 has been investigated in a conventional grain sized (CG) state and in two different ultrafine grained (UFG) conditions processed by Equal Channel Angular Pressing (ECAP) for 2 and 6 passes at 100o C. Strain rate jump tests in compression were performed at different temperatures and the strain-rate sensitivity exponent m was determined. The tests were accomplished by microstructural investigations before and after compression testing in CG and UFG conditions. It is shown that all UFG microstructures exhibit strongly increased strain-rate sensitivity (SRS) compared to the CG state. The SRS increases with increasing temperature and is more pronounced for the UFG material processed using 6 ECAP passes. The microstructural investigations show a rather high stability of the grain structure for the UFG conditions up to 250o C. The results are discussed with respect to the relevant deformation mechanisms.

Abstract: Crystallographic texture includes much information on a material’s behaviour that depends on the processed material itself, the particular thermomechanical processing it has undergone, and the quality of the process. ECAP processed pure Mg and Mg alloys were characterised by their texture and are discussed in terms of texture symmetry. For all types of deformation, including ECAP, the basal plain orientation has to follow certain general rules. Particularly for larger number of passes the ECAP texture can be explained in terms of standard rolling or extrusion textures.

Abstract: A fine-grained magnesium alloy AZ31was obtained through equal channel angular pressing (ECAP).Mechanical properties and the microstructures after deformation under tension and compression were investigated. The tensile strength, compressive strength and the elongation to failure of the fine-grained AZ31 are enhanced due to the reduction of grain size. The compressive ultimate strain (CUS) of the fine-grained AZ31 magnesium alloy is lower than that of the initial state due to the formation of shear bands during compression. The ECAP processed AZ31 magnesium alloy exhibited no tension/compression asymmetry in yielding.

Abstract: The paper considers changes in microstructure, texture and mechanical properties of commercial titanium with initial coarse-grained structure during cold sheet rolling. It has been shown that rolling above 75% leads to formation of a uniform nanocrystalline (NC) structure with a grain size of approximately 0.2 'm in the sheet material. The sheets have a high level of mechanical properties which is comparable with the properties of bulk specimens of titanium with NC structure produced by some other method of severe plastic deformation.

Abstract: In this article we present the results of the experimental research and those of the processes developing the crystallographic texture of computer modeling in CP Ti in the process of 1-4 equal channel angular pressing (ECAP) passes along the route ВС. The goal of the research was to determine the active deformation mechanisms, depending on the strain degree, accumulated in the ECAP process. The research was carried out by the method of X-ray analysis and by computer modeling. Computer modeling was carried out on the example of visco-plastic self-consistent model. Thereby, the basal, the prismatic and the pyramidal (of the 1st and of the 2nd order) slip systems were considered as possible active slip systems. Besides, the possibility of activating the tensile and the compressive twinning systems were taken into consideration. As the result of the carried out experimental research, the objective laws of forming preferred orientations were determined. For the first time, with the help of computer modeling, made up to the 4th ECAP pass, it was shown that the crystallographic texture development processes in CP Ti in ECAP, realized at temperature of 723 K, can be explained by activation of the basal, prismatic and pyramidal (of the 1st order) slip systems and compressive twinning systems. Therefore, the increase of the ECAP passes can lead to amplification of contribution of the basal and prismatic slip systems, as well as the insignificant weakening of the contribution of pyramidal slip systems (of the 1st order). Moreover, the compressive twinning can become obvious only in the 1st ECAP pass.

Abstract: Microstructure evolution and mechanical behavior of alpha/beta Ti-6Al-4V (VT6) and near-beta Ti-5Al-5Mo-5V-1Cr-1Fe (VT22) titanium alloys during uniaxial compression at 600°C to a high strain of 70% was studied. The plastic-flow response for both alloys is characterized by successive stages of strain hardening, flow softening, and steady-state flow. During compression the lamellae spheroidized to produce a partially globular microstructure. Globularization in VT6 is associated with the loss of the initial Burgers-type coherency between the alpha and beta phases and the subsequent individual deformation of each phase. The misorientations of boundaries increase to the high-angle range by means of the accumulation of lattice dislocations. In VT22 alloy the alpha phase evolves similar to that in VT6 alloy, while in the beta phase mainly low-angle boundaries are observed even after 70 pct. reduction.

Abstract: The potential application range of coarse-grained commercial purity titanium is limited by its low mechanical properties. A reduction of the grain size of titanium leads to a significant increase in its strength and hardness. This paper is concerned with application of hydrostatic extrusion (HE) for fabrication nano-grained titanium. In the present study titanium rods were subjected to hydrostatic extrusion with the aim to reduce the grain size to the nano-metric scale and thereby improve the mechanical properties. The obtained material can be an equivalent and compete with the commonly used Ti6Al4V alloy. The results were compared with those other SPD techniques reported in the literature and refered to Hall-Petch relationship.

Abstract: A comparative investigation of mechanical properties of Ti–6Al–4V titanium alloy with coarse-grained (400 m), microcrystalline (10 µm) and submicrocrystalline (0.4 µm) structures in the temperature range 20–500°C has been carried out. The submicrocrystalline structure was obtained by multiaxial isothermal forging. The alloys with the coarse-grained and microcrystalline structures were used in a heat-strengthened condition. The microstructure refinement increases both the strength and fatigue limit of the alloy at room temperature by about 20%. The strength of the submicrocrystalline alloy is higher than that of the microcrystalline alloy in the range 20 - 400°C. Long-term strength of the submicrocrystalline specimens below 300°C is also considerably higher than that of the other conditions. However, the creep strength of the submicrocrystalline alloy is slightly lower than that of the heat-strengthened microcrystalline alloy already at 250°C. The impact toughness in submicrocrystalline state is lower especially in the samples with introduced cracks. Additional surface modification of submicrocrystalline alloy by ion implantation gives a considerable increase in the fatigue limit. Advantages of practical application of submicrocrystalline titanium alloys produced by multiaxial isothermal forging have been evaluated.

Abstract: Aging behavior and mechanical properties of ultra fine grained Cu-Cr-Zr alloy sheet produced by accumulative roll bonding (ARB) process were investigated. A Cu-0.85Cr-0.07Zr (in mass%) alloy was solution treated and then cold-rolled at ambient temperature in the sheet of 1 mm thick. The sheets were heavily deformed by ARB process at ambient temperature up to 5 cycles. The grain size was reduced down to 210 nm and the fraction of high angle grain boundaries (HAGB’s) in the specimen after ARB process was 63%. The proof stress ( σ 0.2) and elongation were 540 MPa and 10%, respectively. Due to the aging treatment, a little grain growth took place (240 nm) and the fraction of HAGB’s was increased to 67%. The proof stress and elongation of the aged one increased to 605 MPa and 15%, respectively. It was noteworthy that the electrical conductivity remarkably increased from 35% to 79%IACS by the aging treatment. It was concluded that the aging treatment after ARB process enhanced not only the mechanical properties but also the electrical conductivity in the Cu-Cr-Zr alloys.