Papers by Keyword: Amorphisation

Abstract: Al-base and Fe-base powders have been amorphized by a high energy milling process in an Attritor miller. Microstructural evolution in powder particles has been analyzed by XRD, DSC, SEM and TEM. The conventional route of cold pressing and sintering applied to these powders does not result adequate to preserve their amorphous or nanometric character. An additional disadvantage of this route appears during the cold pressing stage, as a consequence of the insufficient green strength of the compacts, due to the high hardness of the milled powders. In order to avoid these difficulties a new consolidation technique, electrical resistance sintering (ERS), has been successfully employed. ERS consolidated compacts have been microstructurally characterized by optical microscopy and XRD, showing that compacts preserve their amorphous and/or nanometric character.

Abstract: We focus this work on multi-scale modeling of the ion-beam-induced amorphization and recrystallization in Si, although our scheme can be applied to other materials. We use molecular dynamics to study the formation mechanisms of amorphous regions. We have observed that along with energetic ballistic collisions that generate Frenkel pairs, low energy interactions can produce damage through the melting and quenching of target regions. By quantifying these results, we have developed an improved binary collision approximation model which gives a damage description similar to molecular dynamics. We have successfully applied our model to ion and cluster implantations. In order to define the energetic of defects in a more computationally efficient Kinetic Monter Carlo code, we have used molecular dynamics results related to the recrystallization behavior of local amorphous regions. The combination of all these simulation tools, molecular dynamics (fundamental studies of damage formation and recrystallization), improved binary collisions (including ballistic and melting-related damage) and Kinetic Monte Carlo (for efficient defect kinetics modeling during the implantation and the subsequent annealing), allows us to model the effect of ion mass, beam current and implant temperature on the amount and morphology of residual defects in Si.

Abstract: In the present study molecular dynamics simulations were carried out to investigate the deformation of pure FCC aluminum and diamond cubic silicon interfaces under shear stress. A second nearest-neighbor modified embedded atom method was used to describe the interactions between Al-Al, Si-Si and Al-Si atoms. The critical shear stress (CSS) was determined for various Al/Si and Al/Al interfaces with different alignments and orientations. Structural analyses show that the deformation is localized at approximately 10 Å thickness of the interface in Al. The critical shear stress of Al/Si interface was found to be significantly lower than the critical tensile stress due to the partial stick-slip in sliding. In addition, it has been proven that there is no explicit relationship between shear and tensile critical stresses, which is fundamentally different from isotropic materials, where the shear stress is about half of the tensile stress. The misorientation has a dramatic effect in reducing shear stress at Al/Al interfaces, but has no effect on CSS in Al/Si. As a result, it was shown that introducing Si improves the strength of the interface (and the composite material in general) for different grain orientations.

Abstract: Pure Al (99%) and pure Fe (99.5%) sheets were mutually stacked and severely deformed up to equivalent strain of 16 by the accumulative roll bonding (ARB) process in an attempt to achieve bulk mechanical alloying. The deformation was carried out at RT. The Al/Fe sheets ARB processed by 1 cycle showed a number of shear bands penetrating the stacked layers. The Fe layers, which were harder than the Al layers, were subdivided by the shear bands into diamond-shaped regions. Dissolution of Fe into Al was observed and a supersaturated solid solution was formed in the specimen ARB processed by 10 cycles. It was also found that local amorphization occurred at interface regions via formation of Al5Fe2 intermetallic compound.

Abstract: Precipitation-strengthened Cu-based alloys have limited use as structural materials at high temperatures due to precipitate coarsening and strength loss. We have recently shown that Curefractory metal alloys produced by various physical vapor deposition methods have stable, nanocrystalline microstructures and maintain their strength properties even when annealed at temperatures as high as 900 C for up to 100 hours. This paper presents discussions of how these alloys are processed and the resulting microstructures. X-ray and electron microscopy results will be presented to document the phase transformations that occur in these alloys and result in such exceptionally stable microstructures.

Abstract: 4H-SiC p-type MOS capacitors fabricated by wet oxidation of SiC preamorphized by nitrogen ion (N+) implantation have been investigated. The oxidation rate of the SiC layer preamorphized by high-dose N+ was much larger than that of crystalline SiC, allowing us to reduce the fabrication time of SiC MOS devices. We found that the presence of the surface amorphous SiC layer before the oxidation process did not influence the interface state density in MOS capacitors. Moreover, the shift of the flat-band voltage is not correlated to the amount of nitrogen in the oxide. On the contrary the density of interface states near the valence band edge increased according with the high concentration of the implanted N at the oxide–SiC interface, as in the case of dry oxidation reported by Ciobanu et al. The generation of positive charges due to the nitrogen embedded inside the oxide layer was smaller compared with dry oxidation. We discuss the difference between wet and dry oxidation for MOS capacitors fabricated with N+ implantation.

Abstract: The paper examines the phase evolution in blends consisting of different proportions of stainless steel (SS316) and Al (0, 25, 65 and 85 wt. %) powders during high-energy ball milling by x-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy. An attempt has also been made to study the mechanical property of the bulk samples obtained by hot pressing the ball milled powder blend at suitable a temperature and pressure. The results of microstructural changes and mechanical property and the ability of consolidation of the amorphous/nanocrystalline powders by high-pressure techniques to develop engineering components has been discussed and highlighted.

Abstract: Structure formation in TiNi-based shape memory alloys depending on deformation temperature (-196 °C to 400 °C) and pressure (4 to 8 GPa) under conditions of high-pressure torsion (HPT) was studied using TEM and X-ray diffraction methods. The tendency to form an amorphous structure depends on relative positions of the deformation temperature and Ms temperature. Isothermal martensitic transformation is observed in the Ti – 48.5 % Ni alloy as a result of 10-year keeping at RT after HPT. Increasing of pressure suppresses the tendency to form an amorphous structure. The upper deformation temperature limits for amorphous and nanocrystalline structures formation are determined. The thermomechanical conditions of the equal-channel angular pressing for obtaining actual nanocrystalline structure are recommended.

Abstract: Thermal- and stress-induced martensitic transformation was investigated on TiNi shape memory alloys subjected to severe plastic deformation (SPD) by cold rolling. TEM observation revelaed the sample is a mixture of nanocrystalline and amorphous after 40% cold rolling. DSC analysis suggested that the martensitic transformation was suppressed when the thickness reduction was over 25% reduction. Aging at lower temperatures (573 ~ 673 K, 3.6 ks) restores the phase transformations, but to a limited extent. The stress-strain curves of nanocrystalline/amorphous TiNi are characterized by absence of stress-plateau and small hysteresis.