Materials Science Forum Vols. 667-669

Abstract: The effect of ECAE pass number on the electrochemical properties of CP-Ti was investigated by electrochemical techniques (potentiodynamic polarization test, potentiodynamic polarization test and surface analyses (OM, scanning electron microscopy (SEM) in Ringer’s solution at 37°C. The results show that the corrosion resistance of muli-pass ECAE CP-Ti samples is superior to the coarse grain CP-Ti. The effect of texture of ECAEed samples parallel to the surface on the corrosion properties has been discussed.

Abstract: For thermoelectrics it is important to produce thermodynamically stable bulk nanostructured materials. Ball milling/hot pressing was shown to reduce the crystallite size by a factor of 100 and to reach about 100 nm with dislocation densities of 1012 – 1013m-2. Thereby thermoelectric properties of single, double and multifilled Sb-based skutterudites were improved significantly leading to figures of merit ZT, which in some cases are twice as high as those of their microstructured counterparts. With HPT treatment the crystallite size can be decreased to even 50 nm with dislocation densities as high as 1015m-2. The small grains as well as the high dislocation density result in a further lowering of thermal conductivity holding a high potential for future enhancement of ZT.

Abstract: Various different severe plastic deformation processes (SPD) have been developed to produce ultra-fine grained (UFG) materials during the last two decades. One very important material property that the UFG materials should have for structural materials application is good wear resistance. This review paper presents some recent work related to the wear resistance of materials processed by SPD, in particular for alloys processed by using equal-channel angular pressing (ECAP) and accumulative roll-bonding (ARB).

Abstract: An Al-1050 alloy was processed by ECAP and HPT, respectively. Dry sliding wear tests were conducted on the as-received and SPD-processed samples under various sliding conditions. A comparison of wear rate indicated that SPD processing decreased the wear resistance. Two main wear mechanisms were observed. The initial wear stage was dominated by severe platelet wear mechanism and later wear was dominated by an oxidation wear mechanism. The results show the severe wear stage of SPD-processed Al-1050 is much longer than that of the as-received Al-1050, which is attributed to the loss of work hardening capacity after SPD processing.

Abstract: Sliding wear behavior of severely deformed 6061 aluminum alloy sheets by accumulative roll bonding (ARB) process subjected to dry sliding wear at different loading and sliding velocities was investigated using a pin on disc wear machine. The sheets were processed up to five ARB cycles in order to induce a high strain (~ 4.0) to the samples. EBSD results showed that after five cycles of ARB, sheets were found to contain ultra-fine grains with high fraction of high angle grain boundaries. Wear was continuously monitored by measuring the weight losses and morphologies of worn surfaces by scanning electron microscope (SEM), and a model for the wear of the ARB-processed samples was proposed.

Abstract: Bulk nanocrystalline 304 stainless steel (nanocrystalline 304ss) discs had been successfully prepared by the commercial microcrystalline 304 stainless steel (microcrystalline 304ss) plate using severe rolling technique. Micro-hardness was measured to reveal the different mechanical behavior after the severe plastic deformation. The electrochemical corrosion resistance and ion release behavior after immersion of the samples were investigated in Hank’s solution for its potential use as implant and orthodontic appliance in body. Furthermore, murine fibroblast cells were indirectly employed to detect cytotoxicity by co-incubation with the extraction from the given materials. Haemocompatibility, consisting of hemolysis test and adhesion of the platelets, was also measured with fresh human whole blood and platelet-rich plasma respectively. Polarization resistance trials indicate that nanocrystalline 304ss is more corrosion resistant in the Hank’s solution, with lower current density and superficial corrosion morphologies. The release values of the biotoxic ions after immersion do not exceed the set limit and turn to be well below the critical value necessary to induce allergy and below daily dietary intake level. Cellular interaction is observed via the proliferated feature of the cell line. Hemolysis and platelet adhesion results elucidates that nanocrystalline 304ss is biological and hematologic compatible.

Abstract: The initial and deformed states of austenitic stainless steel AISI 321 before and after neutron irradiation with a damage dose up to 5.3 dpa at reactor core temperature of 350 °С were comparatively investigated. Corrosion behavior, mechanical properties are analyzed in the temperature range of 20…650 °С. It is shown that radiation damage of the steel in the deformed state significantly decreases in comparison with the initial coarse-grained state at the same level of corrosion resistance.

Abstract: The corrosion behavior of ultra-fine grain (UFG) copper bulk prepared by equal channel angular pressing (ECAP) was studied in 3.5% NaCl solution. The effect of ECAP deformation on the copper corrosion is controversial in the literature, and worth to verify by means of various experimental techniques. Corrosion performances of UFG copper were investigated in comparison with that in recrystallized coarse grain (CG) copper by polarization curves, Tafel extrapolation method, electrochemical impedance spectroscopy(EIS). The shape of polarization curves and type of corrosive attack remains the same in the UFG and the coarse-grain state. UFG copper exhibited a lower corrosion current and high self-corrosion potential in comparison with CG copper. Electrochemical experimental results showed that UFG copper increased in resistance to corrosion compared with CG copper. This decrease in corrosion resistance was mainly attributed to the more compact corrosion film of UFG copper. The compact passive film led to decrease of the diffusion capability of ions within the corrosion film and corrosion rate.

Abstract: With the development of the ECAP technology, the corrosion behavior of the UFG materials has been investigated in some degree. Two categories of corrosion behaviors of UFG material were discussed in this paper. The large proportion of the non-equilibrium grain boundaries and high residual stress inside of the grain were regarded as the key to affect the corrosion behavior of the UFG materials. The corrosion behavior was also affected by the ECAP factors especially the press pass number and the pressing temperature. Finally some prospects of making UFG materials combined with high mechanical property and good corrosion resistance by ECAP technology were expounded.

Abstract: A Ni-rich Ti-50.7.at%Ni alloy was processed by Equal Channel Angular Extrusion (ECAE) at 500°C. After 8 passes ECAE, microstructure was refined to sub-micron scale, approximately 0.2 μm～0.3 μm. TEM observation reveals that Ti3Ni4 phase precipitated in Ni-rich Ti-50.7.at%Ni alloy during the preheating treatment before each ECAE pass, but re-dissolved during sequent ECAE processes. After ECAE treatment, the B2R transformation occurred within a larger temperature range. Comparing with the solution-treated TiNi specimen, the martensitic transformations start (Ms) and peak temperatures (Mp) of TiNi specimens ECAEed were dramatically lowered. Super-elasticity characteristics of TiNi alloy were tested by tensile loading and unloading cycles. The results reveal that at a tensile strain of 4% or smaller, ultrafine-grained (UFG) TiNi alloy processed by 4 passes ECAE shows better super-elasticity than solution-treated sample. Microstructure evolution and its effect on phase transformations and super-elasticity characteristics have been discussed.