The combined effect of superficial nanocrystallisation by SMAT (Surface Mechanical Attrition Treatment) followed by plasma nitriding on the mechanical properties of a medical grade austenitic ...stainless steel was studied. SMAT conditions were optimised to enhance nitrogen diffusion. Experimental observations (energy dispersive X-ray spectroscopy profiles, cross-sectional optical micrographs, phase analysis by X-ray diffraction and micro-hardness profiles) show that polishing away a very thin layer after SMAT and before nitriding significantly improves nitrogen diffusion into the substrate, yielding a 50% thicker nitrided layer. Possible causes for this improvement are discussed.
► The formation of a nanocrystalline surface layer improves the nitrogen diffusivity. ► It yields a 50% thicker nitrided layer. ► This layer is 5 times harder than the 316L substrate. ► This result is obtained by adding a polishing step between SMAT and nitriding.
Composite materials comprising a porous Ti2AlC matrix and Al 6061 alloy were fabricated by a current-activated pressure assisted melt infiltration process. Coarse, medium and fine meso-structures ...were prepared with Al alloy filled pores of differing sizes. Materials were subjected to uniaxial compressive loading up to stresses of 668 MPa, leading to the failure of specimens through crack propagation in both phases. As-fabricated and post-failure specimens were analysed by X-ray microscopy and electron microscopy. Quasi-static mechanical testing results revealed that compressive strength was the highest in the fine structured composite materials. While the coarse structured specimens exhibited a compressive strength of 80% relative to this. Reconstructed micro-scale X-ray tomography data revealed different crack propagation mechanisms. Large planar shear cracks propagated throughout the fine structured materials while the coarser specimens exhibited networks of branching cracks propagating preferentially along Al alloy-Ti2AlC phase interfaces and through shrinkage pores in the Al alloy phase. Results suggest that control of porosity, compensation for Al alloy shrinkage and enhancement of the Al alloy-Ti2AlC phase interfaces are key considerations in the design of high performance metal/Ti2AlC phase composites.
The interfaces between the stress-assisted diffusion bonded Ti3SiC2 and equiatomic NiTi, two distinct material systems that show pseudoelasticity were studied. The interfaces were formed in the ...800–1000°C temperature range, for 1, 5 and 10h under flowing argon. Bonding was observed in all the cases considered, except at 800°C after 1h. Morphology and reaction phases in the interface were characterized using scanning electron microscopy, elemental micro probe analysis and electron backscatter diffraction analysis. The interfacial structure formed between NiTi and Ti3SiC2 layers consists of NiTi/Ti2Ni/Ti5Si3/NiTiSi/Ti3SiC2. Diffusion of Si into NiTi from Ti3SiC2, and Ni from NiTi into reaction zone was found to be responsible for the formation of reaction layers in the interface and thus for bonding at these conditions. The overall reaction layer thickness grows following the parabolic kinetic law. Nano-indentation and Vickers micro hardness tests were carried out to investigate the mechanical properties of the interface. Nano-indentation showed that the elastic moduli of the phases in the interface are close to that of Ti3SiC2 while their hardness is higher than that of both Ti3SiC2 and NiTi. Artificially formed cracks through microindents were observed to be branched and propagated into Ti3SiC2 phase indicating good resistance against delamination.
Microstructure-sensitive design (MSD) is a novel mathematical framework that facilitates a rigorous consideration of the material microstructure as a continuous design variable in the engineering ...design enterprise Adams, B.L., Henrie, A., Henrie, B., Lyon, M., Kalidindi, S.R., Garmestani, H., 2001. Microstructure-sensitive design of a compliant beam. J. Mech. Phys. Solids 49(8), 1639–1663; Adams, B.L., Lyon, M., Henrie, B., 2004. Microstructures by design: linear problems in elastic–plastic design. Int. J. Plasticity 20(8–9), 1577–1602; Kalidindi, S.R., Houskamp, J.R., Lyons, M., Adams, B.L., 2004. Microstructure sensitive design of an orthotropic plate subjected to tensile load. Int. J. Plasticity 20(8–9), 1561–1575. MSD employs spectral representations of the local state distribution functions in describing the microstructure quantitatively, and these in turn enable development of invertible linkages between microstructure and effective properties using established homogenization (composite) theories. As a natural extension of the recent publications in MSD, we provide in this paper a detailed account of the methods that can be readily used by mechanical designers to construct first-order elastic–plastic property closures. The main focus in this paper is on the crystallographic texture (also called Orientation Distribution Function or ODF) as the main microstructural parameter controlling the elastic and yield properties of cubic (fcc and bcc) polycrystalline metals. The following specific advances are described in this paper: (i) derivation of rigorous first-order bounds for the off-diagonal terms of the effective elastic stiffness tensor and their incorporation in the MSD framework, (ii) delineation of the union of the property closures corresponding to both the upper and lower bound theories resulting in comprehensive first-order closures, (iii) development of generalized and readily usable expressions for effective anisotropic elastic–plastic properties that could be applied to all cubic polycrystals, and (iv) identification of the locations of readily available or easily processable ODFs (e.g. textures that are produced by rolling, drawing, etc.) on the property closures. It is anticipated that the advances communicated in this paper will make the mathematical framework of MSD highly accessible to the mechanical designers.
Summary
Focused ion beam micromachining provides a maskless and resistless technique for prototyping of structures from thermoplastic polymers, an example being the production of polystyrene ...microcantilevers with potential applications as micro/nanoelectromechanical systems sensors and actuators. The applicability of FIB technology is, however, often restricted by the damage created by high energy gallium ion bombardment and local beam heating, which can affect the desired properties and limit the minimum achievable size of the fabricated structure. To investigate the ion‐induced damage and determine the limitations of the technique for polymer nanofabrication, we have exposed thin polystyrene film to the ion beam at varying ion doses, ion energies and specimen temperatures. Ion doses ranging from 1016 to 1018 ions cm−2 show significant gallium implantation, redeposition of sputtered material and chemical degradation in the polymer. Raman results show that the local heating in polymer during milling is severe at room temperature, damaging the aromatic carbon bonding (C = C) in particular. These observations are supported by the results of a beam heating model and Monte Carlo simulations. The chemical degradation caused by local beam heating is found to be significantly reduced by cooling the specimen to −25°C during milling. This is consistent with observations that reversible and repeatable thermal actuation of a fabricated polystyrene–platinum microcantilever is only observed when the cantilever is prepared at low temperature milling. Using this cooling approach, polymer structures can be fabricated with dimensions as low as 200 nm and still retain a sufficient volume of material unaffected by the ion beam.
The main objective of this work is to understand the fundamental damage mechanisms involved in a SMAT treated CoCrMo alloy subjected to fatigue under rotating bending loads. For this purpose, ...different load amplitudes in rotating bending are imposed on cylindrical specimens, in as-machined and SMAT states. Different material characterizations are performed in order to determine which features play an important role in fatigue life of the studied alloy. Such characterizations are done via digital microscopy, electron backscatter diffraction (EBSD), roughness measurements and microhardness tests. The fatigue results presented in the form of S-N diagram show that SMAT with a moderate intensity (SMAT-2) can enhance the fatigue performance of the CoCrMo alloy, whereas SMAT with a higher intensity (SMAT-3) seems to deteriorate it. This fatigue life decrease is probably due to surface micro-cracks generated by an over-peening phenomenon.
A new computer code has been developed to automatically extract quantitative twin statistics from electron backscatter diffraction data. The new code is an improvement upon previous codes in that it ...handles materials of any crystal symmetry, type I, Type II and compound twins, and general stress states. Moreover, accuracy of the results has been greatly improved. In addition, twin statistics including number, area fraction, twin thickness and twinning dependencies on orientation, grain size and neighbourhood effects can be routinely analysed. The new code has been applied to scan data from deformed magnesium, zirconium and uranium, and can potentially be used for any twinning material for which reliable electron backscatter diffraction results can be obtained.
Polymer–metal microcantilever actuators have been fabricated using an innovative approach based on focused ion beam micromachining technology. The fabrication involves depositing a thin metal film ...onto the surface of the polymer and machining using the ion beam. The microcantilever created is then extracted and transferred to a desirable support using a micromanipulator. This approach demonstrates the potential for maskless and resistless prototyping of cantilevers that can be evaluated for use as MEMS/NEMS actuators. Nanometer-scale displacement of the resulting polystyrene–platinum bimorph microactuator with respect to temperature change is demonstrated via visual monitoring in a scanning electron microscope with a heating stage. The performance of the bimorph cantilever microactuators is verified using both analytical and finite element modeling.
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