The effect of (Ti + Al): V ratio on the structure and oxidation behaviour of TiAlN/VN nano-scale multilayer coatings

Nano-scaled multilayered TiAlN/VN coatings have been grown on stainless steel and M2 high speed steel substrates at U-B = - 85 V in an industrial, four target, Hauzer HTC 1000 coater using combined cathodic steered arc etching/unbalanced magnetron sputtering. X-ray diffraction (XRD) has been used to investigate the effects of process parameters (Target Power) on texture evolution (using texture parameter T*), development of residual stress (sin(2) psi method) and nano-scale multilayer period. The composition of the coating was determined using energy dispersive X-ray analysis. The thermal behaviour of the coatings in air was studied using thermo-gravimetric analysis, XRD and scanning electron microscopy. The bi-layer period varied between 2.8 and 3.1 nm and in all cases a {1 1 0} texture developed with a maximum value T* = 4.9. The residual stress varied between -5.2 and -7.4 GPa. The onset of rapid oxidation occurred between 628 and 645 degreesC depending on the (Ti+Al):V ratio. After oxidation in air at 550 degreesC AlVO4, TiO2 and V2O5 Phases were identified by XRD with the AlVO4, TiO2 being the major phases. The formation of AlVO4 appears to disrupt the formation of Al2O3 which imparts oxidation resistance to TiAlN based coatings. Increasing the temperature to 600 and 640 degreesC led to a dramatic increase in the formation of V2O5 which was highly oriented (0 0 1) with a plate-like morphology. At 640 degreesC there was no evidence of the coating on XRD. Increasing the temperature to 670 degreesC led to further formation of AlVO4 and a dramatic reduction in V2O5. (C) 2003 Elsevier B.V. All rights reserved

The use of Fe-30% Ni and Fe-30% Ni-Nb alloys as model systems for studying the microstructural evolution during the hot deformation of austenite

The development of physically-based models of microstructural evolution during thermomechanical processing of metallic materials requires knowledge of the internal state variable data, such as microstructure, texture, and dislocation substructure characteristics, over a range of processing conditions. This is a particular problem for steels, where transformation of the austenite to a variety of transformation products eradicates the hot deformed microstructure. This article reports on a model Fe-30wt% Ni-based alloy, which retains a stable austenitic structure at room temperature, and has, therefore, been used to model the development of austenite microstructure during hot deformation of conventional low carbon-manganese steels. It also provides an excellent model alloy system for microalloy additions. Evolution of the microstructure and crystallographic texture was characterized in detail using optical microscopy, X-ray diffraction (XRD), SEM, EBSD, and TEM. The dislocation substructure has been quantified as a function of crystallographic texture component for a variety of deformation conditions for the Fe-30% Ni-based alloy. An extension to this study, as the use of a microalloyed Fe-30% Ni-Nb alloy in which the strain induced precipitation mechanism was studied directly. The work has shown that precipitation can occur at a much finer scale and higher number density than hitherto considered, but that pipe diffusion leads to rapid coarsening. The implications of this for model development are discussed

Correlation of micromechanical property and microstructure of tribo-layers

Tribological contact often leads to surface deformation, resulting in a substantial increase in dislocation density and a considerable refinement in the microstructural scale. The extensive work hardening associated with this results in significant changes in the mechanical properties of the surface. It is not only the mechanical properties that change, but also the corrosion potential. In some cases, the surface changes enhance the wear resistance of the material. However, in other cases, higher wear rates are found with surface deformation that results in ultra-fine surface structures. Despite the importance of surface deformation, much is unknown about the mechanical properties of the wear induced surface layers. Nanoindentation provides useful information but does not give a good indicator of the ductility. The challenge is to test the mechanical properties of such a fine scale deformed structure. In this work, the micromechanical properties of Ti-6Al-4V worn surfaces after tribocorrosion testing were measured using an in-situ micropillar compression method in the chamber of a scanning electron microscope. Reciprocating tribocorrosion testing was undertaken in 25 vol % Bovine Serum Albumin (BSA) in phosphate-buffered saline (PBS) solution against an alumina counterface, with a load of 0.5N and a speed of 20 mm/s. Tests were conducted under Open Circuit Potential (OCP) conditions and at cathodic and anodic surface potentials, namely at +0.5V and −0.95V. The different test conditions resulted in different extents of surface deformation. This resulted in significant differences in the stress strain curves from the micropillar tests, both in terms of strength and ductility. The microstructure observed by subsequent TEM of the tested micropillars is correlated with the mechanical properties and the reasons for the different mechanical properties are discussed

Insights into tribofilm formation on Ti-6V-4Al in a bioactive environment: correlation between surface modification and micro-mechanical properties

Ti-6Al-4V has been used as a surgical implant material for a long time because of its combination of strength, corrosion resistance and biocompatibility. However, there remains much that is not understood about how the surface reacts with the environment under tribocorrosion conditions. In particular, the conditions under which tribofilms form and their role on friction and wear are not clear. To evaluate the complicated nature of the dynamic surface microstructural changes on the wear track, high resolution transmission electron microscopy (TEM), scanning transmission electron microscope (STEM) and electron energy loss spectroscopy (EELS) have been used to characterise the structure and chemical composition of the tribofilm. Detailed analysis of the formation and structure of the tribofilm and the metal surface deformation behaviour were studied as a function of applied potential and the role of proteins in the lubricant. For the first time, graphitic and onion-like carbon structures from wear debris were found in the testing solution. The presence of carbon nanostructures in the tribocorrosion process and the formation of the tribofilm leads to an improved tribocorrosion behaviour of the system, in particular a reduction in wear and friction. A detailed, quantitative, analysis of surface deformation was undertaken, in particular, the geometrically necessary dislocation (GND) density was quantified using precession electron diffraction (PET). A clear correlation between applied potential, tribofilm formation and the surface strain was established. Statement of significance: The formation of tribofilm and microstructure modification of the Ti-6Al-4V surface during tribocorrosion in a physiological environment is not fully understood. In particular, the correlation between microstructural changes and electrochemical conditions is not clear. This study presents a detailed investigation of the structure and chemical composition of tribofilms at the nanoscale during tribocorrosion tests in simulated body fluid and gives a detailed and quantitative description of the evolved surface structure. A clear correlation between applied potential, tribofilm formation and the surface strain was established. Moreover, particular attention is paid to the wear debris particles captured from the lubricating solution, including nanocarbon onion structures. The implications for tribocorrosion of the alloy in its performance as an implant are discussed

Individual effect of recrystallisation nucleation sites on texture weakening in a magnesium alloy: Part 2- shear bands

The entire recrystallisation process of a cold-rolled WE43 Mg alloy containing a high density of shear bands was tracked using a quasi-in-situ electron backscatter diffraction method. The results showed the resultant recrystallised texture arose principally from recrystallisation within shear bands rather than from deformation twins, which made a negligible contribution to the final texture. Only a weakened basal texture with scattered weak texture components was observed after annealing at 450 °C and 490 °C. This is in contrast to the widely reported “Rare Earth” textures, believed to come from shear band recrystallisation. The texture appeared during the nucleation of recrystallised grains and was preserved during subsequent grain growth. Simultaneous occurrence of precipitation on grain and twin boundaries during recrystallisation suppressed any potential preferential grain growth. When annealed at 545 °C, at which temperature both solute drag and Zener pinning were eliminated, a non-basal texture was produced after annealing due to orientated grain growth governed by different grain boundary mobility between basal and off-basal grains

Effect of deformation twinning on crystallographic texture evolution in a Mg–6.6Zn–0.2Ca (ZX70) alloy during recrystallisation

The entire recrystallisation sequence and associated crystallographic texture evolution of Mg-6.6Zn-0.2Ca (ZX70, wt.%) alloy were tracked using a quasi-in-situ electron backscatter diffraction (EBSD) method. The commonly observed grain boundary recrystallisation in conventional Mg alloys was largely restricted, since all the prior grain boundaries were pinned by intermetallic compounds distributed along grain boundaries. A typical “Rare Earth” (RE) texture was observed during the whole recrystallisation process. The RE texture appeared during the nucleation stage because of RE texture orientations introduced by recrystallisation from double twins. This texture was retained during the subsequent grain growth stage because of uniform grain growth. Recrystallised grains induced by a considerable amount of intermetallic compounds did not change the leading role of double twins on contribution to recrystallised texture. All results indicate that a small amount of Ca addition into a conventional Mg-Zn alloy system can significantly alter the recrystallised texture from basal texture to RE texture

Assistive technology assessment and planning for children with multiple disabilities in educational settings

A number of critical elements have been identified in assistive technology assessment and planning to optimise its integration into the educational environments of children and hence address their functional goals. These elements are as follows: adopting a collaborative think-tank team approach to which all educational team members contribute equally and where technology experts are consulted once the need for specific technical support or training is identified; involving the family by establishing mutual expectations and using effective communication strategies; and conducting in-depth assessment that identifies clear goals, includes task analysis within daily environments, examines the child-device interaction closely and investigates the resources available to implement assistive technology use. With this backdrop, this paper reviews existing assessments and proposes that the Lifespace Access Profile (LAP) (Williams et al 1993) and Lifespace Access Profile (Upper Extension) (LAPUE) (Williams et al 1994) satisfy many of the criteria for effective assessment and planning advocated in the literature