Materials Science Forum Vols. 539-543

Abstract: We present some attempts to simulate nanoscale phenomena, which involve different length-scales and time-scales, using multiscale molecular-dynamics approaches. To simulate realistically an impurity-segregated nanostructure, we have developed the hybrid quantum/classical approach. The method can describe seamlessly both dynamical changes of local chemical bonding and nanoscale atomic relaxations. We apply the method to hydrogen diffusion in Si grain boundary. We find that the hydrogen is strongly trapped in (001)Σ5 twist boundary below 1000K, whereas it starts diffusing along the grain boundary above 1000K. For long-time processes in nanostructure formation, we apply the stochastic-difference-equation method to accelerate the simulations for microstructure evolution. The method bridges the states separated by high-energy barriers in a configuration space by optimizing an action, defined as an error accumulation along a reaction pathway. As an example, a SDE simulation is performed for Cu thin-film formation via nanocluster deposition. We show that the method can be applied effectively to search for the long-time process which involves a rare event due to a large potential barrier between two atomic configurations.

Abstract: In this study, a deformation mechanism map of metallic nanocrystalline materials (NCMs) using the phase mixture model is proposed. It is based on recent modelling that appears to provide a conclusive description of the phenomenology and the mechanisms underlying the mechanical properties of NCMs. The proposed models adopted the concept of a ‘phase mixture’ in which the grain interior and the grain boundaries are treated as separate phases. The volume fraction of this grain boundary ‘phase’ may be quite appreciable in a NCM. Based on the theoretical model that provides an adequate description of the grain size dependence of plasticity covering all grain size range from coarse down to the nanoscale, the tensile deformation response of NCMs, especially focusing on the deformation mechanisms was investigated. The deformation mechanism map is newly proposed with axes of strain rate, grain size and temperature.

Abstract: Carbon deposition on Ni/YSZ cermets and the role of Zr-doped ceria catalysts on carbon deposition have been investigated in the present study. Upon exposure to methane, large amounts of filamentous carbon formed on the surface of the cermets. In addition, carbon dissolved into the Ni particles, significantly expanding the dimensions of the entire pellet. The addition of Zr-doped ceria catalyst pellets on each side of the Ni/YSZ cermet significantly reduced the amount of deposited carbon and affected the structure of the deposits. In particular, the carbon was more weakly bound to the surface and appeared only to be deposited on the Ni/YSZ surface and not dissolved into the structure.

Abstract: The compressive behavior was investigated on an electrodeposited nanocrystalline Ni-20%Fe alloy with a grain size of about 22 nm at room temperature (RT), 298 K, and the liquid nitrogen temperature (LN2T), 77 K. The sensitivity of the yield strength and plastic strain to the test temperature at different grain sizes was discussed. Moreover, through the Transmission Electron Microscopy (TEM) examination and microhardness measurement, the microstructures before and after the compression test at RT and LN2T were studied.

Abstract: Ultra-low carbon steel (ferritic steel), commercial purity aluminum and high purity copper were heavily deformed by the accumulative roll bonding (ARB) process, and the microstructural evolution during the ARB was analyzed. Significant grain refinement by grain subdivision mechanism was confirmed in all three kinds of materials. On the other hand, microstructure refinement slowed down with increasing strain and the grain size stayed in nearly a constant value in the ultrahigh strain region. The mechanism of the grain size saturation was discussed.

Abstract: A coarse grained Zr-Hf alloy has been subjected to one rolling pass with different thickness reductions ranging from 10% to 80%. Rolling was performed at three temperatures: 300°C, room temperature (RT) and liquid nitrogen temperature (-196°C). It has been found that, with increasing strain per pass, i.e., with increasing strain rate, the deformation mechanism changes from twinning to dislocation slip. The minimum strain per pass necessary to trigger the transition in deformation mechanism decreases with decreasing temperature. High strain, high strain-rate deformation leads to the development of an ultrafine grained structure. Simultaneously, a basal type rolling texture forms. At the higher temperatures (RT and above) a recrystallization texture component is also present. Thus, nanostructuring of this Zr-Hf alloy during severe rolling is attributed to a combination of grain subdivision by the formation of geometrically necessary boundaries and to nucleation and growth phenomena taking place as a consequence of rapid adiabatic heating.

Abstract: The accumulative roll bonding (ARB) process is one of the methods to refine the grain size of metallic materials. The ARB process up to 8 cycles was performed for the pure Cu and Cu- Fe-P (PMC-90) alloy at ambient temperature under no lubricant conditions. In the pure Cu, the nano-sized grains were formed after third cycle with an average grain size of 200nm. Once the 200 nm grains formed, further reduction in the grain size was not observed up to the 8 ARB process cycles. On the other hand, the formation of the stable nano-sized grains in PMC-90 alloy was retarded compared to the pure Cu due to the alloying elements. For both alloys, the tensile strength values increased drastically in the initial stage of ARB process. The tensile strength values of both alloys tended to saturate after the third ARB process cycle. The tensile elongation value greatly decreased by 1 cycle of ARB process due to the strain hardening. After the third cycle of ARB process, each alloy showed a gradual increase in tensile elongation due to the dynamic recovery. For PMC-90 alloy, the strength value is higher than that of OFC due to addition of the alloying elements.

Abstract: This paper describes experiments in which high purity aluminum (Al) and copper (Cu) single crystals of different crystallographic orientations were processed for one pass by equal-channel angular pressing (ECAP). The deformed structures were examined using optical microscopy (OM), orientation imaging microscopy (OIM) and transmission electron microscopy (TEM). The results for Al single crystals are compared with those for Cu single crystals.

Abstract: Two commercial Al alloys having different second phase particle distributions were subjected to severe plastic deformation (SPD) via equal channel angular pressing with or without subsequent cold rolling, and the effect of such SPD on the particle size distribution of the alloys was investigated. The particles larger than ∼ 3 μm were fragmented into several smaller ones by SPD. Contrarily, those smaller than ∼ 3 μm were hardly broken up by SPD but their distribution became more uniform. Along with these findings and the theoretical models for cavity nucleation at second phase particles, the cavitation behavior of ultrafine grained Al alloys during low temperature or high strain rate superplastic deformation was discussed.