Response of Ti microstructure in mechanical and laser forming processes

Microstructural deformation mechanisms present during three different forming processes in commercially pure Ti were analysed. Room temperature mechanical forming, laser beam forming and a combination of these two processes were applied to thick metal plates in order to achieve the same final shape. An electron backscatter diffraction technique was used to study the plate microstructure before and after applying the forming processes. Substantial differences among the main deformation mechanisms were clearly detected. In pure mechanical forming at room temperature, mechanical twinning predominates in both compression and tensile areas. A dislocation slip mechanism inside the compression and tensile area is characteristic of the pure laser forming process. Forming processes which subsequently combine the laser and mechanical approaches result in a combination of twinning and dislocation mechanisms. The Schmid factor at an individual grain level, the local temperature and the strain rate are factors that determine which deformation mechanism will prevail at the microscopic level. The final microstructures obtained after the different forming processes were applied are discussed from the point of view of their influence on the performance of the resulting formed product. The observations suggest that phase transformation in Ti is an additional microstructural factor that has to be considered during laser forming

High Energy Density Processing Of A Free Form Nickel-Alumina Nanocomposite

The development of a free form bulk Nickel reinforced Alumina matrix nano composites using Air Plasma Spray and laser processing has been presented. The process consumes less time and requires further minimal machining and therefore is cost effective. The relative differences in using APS over laser processing in development of bulk metal-ceramic nanocomposites have been discussed. The process intricacies involved during processing such as material specific mandrel selection, plasma-particle interaction are highlighted. Electroless coating has been used to uniformly disperse Nickel in alumina matrix as a source material. The electroless Ni coated alumina particles are subjected to both laser processing and Air Plasma Spraying with optimized parameters. Consolidation by laser processing could not be achieved as the laser beam was reflective to Nickel. On the other hand, APS Ni-alumina nanocomposite with a cylindrical shape of 1.2″ OD × 1″ ID × 1.5″ length has been fabricated with minimum or no surface defects. HRTEM pictures revealed the nanostructure retention thereby corroborating the fact that bulk nanostructures can be made using Air Plasma Spray. XRD analysis confirmed the phase transformation from alpha alumina to gamma alumina and oxidation of Ni to NiO. Subsequent reduction of NiO to metallic nickel using hydrogen atmosphere has also been demonstrated. Mechanical properties such as, hardness (1025 HV) and fracture toughness (5 MPa m1/2) for the nanocomposite are presented herein. Copyright © 2006 American Scientific Publishers All rights reserved

Nanoscale Electrolytic Switching in Phase-Change Chalcogenide Films

[GRAPHICS] Reversible polarity-dependent resistance (PDR) switching in phase-change (PC) films is feasible. Nanometer-scale crystalline marks are produced in amorphous Ge2Sb2+xTe5 films by electrical pulses through an AFM tip. In these marks, PDR switching is demonstrated with three orders of magnitude current contrast using less than 1.5 V. No current contrast between the crystalline marks in the high-resistance state and the amorphous if background is observed

Growth Rate Determination through Automated TEM Image Analysis: Crystallization Studies of Doped SbTe Phase-Change Thin Films

A computer-controlled procedure is outlined here that first determines the position of the amorphous-crystalline interface in an image. Subsequently, from a time series of these images, the velocity of the crystal growth front is quantified. The procedure presented here can be useful for a wide range of applications, and we apply the new approach to determine growth rates in a so-called fast-growth-type phase-change material. The growth rate (without nucleation) of this material is of interest for comparison with identical material used in phase-change random access memory cells. Crystal growth rates in the amorphous phase-change layers have been measured at various temperatures using in situ heating in a transmission electron microscope. Doped SbTe films (20 nm thick) were deposited on silicon nitride membranes, and samples with and without silicon oxide capping layer were studied. The activation energy for growth was found to be 3.0 eV. The samples without capping layer exhibit a nucleation rate that is an order of magnitude higher than the samples with a silicon oxide capping layer. This difference can be attributed to the partial oxidation of the phase-change layer in air. However, the growth rates of the samples with and without capping are quite comparable.

Monodomain strained ferroelectric PbTiO3 thin films: Phase transition and critical thickness study

This work demonstrates that instead of paraelectric PbTiO3, completely c-oriented ferroelectric PbTiO3 thin films were directly grown on (001)-SrTiO3 substrates by pulsed-laser deposition with thickness up to 340 nm at a temperature well above the Curie temperature of bulk PbTiO3. The influence of laser-pulse frequency, substrate-surface termination on growth, and functional properties were studied using x-ray diffraction, transmission electron microscopy, and piezoresponse force microscopy. At low growth rates (frequency <5 Hz) the films were always monodomain. However, at higher growth rates (frequency >8 Hz) a domains were formed for film thickness above 20–100 nm. Due to coherency strains the Curie temperature (Tc) of the monodomain films was increased approximately by 350 °C with respect to the Tc of bulk PbTiO3 even for 280-nm-thick films. Nonetheless, up to now this type of growth mode has been considered unlikely to occur since the Matthews-Blakeslee (MB) model already predicts strain relaxation for films having a thickness of only ~10 nm. However, the present work disputes the applicability of the MB model. It clarifies the physical reasons for the large increase in Tc for thick films, and it is shown that the experimental results are in good agreement with the predictions based on an earlier published monodomain model.