Markus Rettenmayr

The electrochemical characteristics of native Nitinol surfaces.

The present study explored the avenues for the improvement of native Nitinol surfaces for implantation obtained using traditional procedures such as mechanical polishing, chemical etching, electropolishing and heat treatments for a better understanding of their electrochemical behavior and associated surface stability, conductivity, reactivity and biological responses. The corrosion resistance (cyclic potential polarization, open circuit potential and polarization resistance) of Nitinol disc and wire samples were evaluated for various surface states in strain-free and strained wire conditions. The surface response to tension strain was studied in situ. Surface chemistry and structure were explored using XPS and Auger spectroscopy and photoelectrochemical methods, respectively. It was found that the polarization resistance of the Nitinol surfaces varied in a range from 100 kOmega to 10 MOmega cm(2) and the open circuit potentials from -440 mV to -55 mV. The surfaces prepared in chemical solutions showed consistent corrosion resistance in strain-free and strained states, but mechanically polished and heat treated samples were prone to pitting. Nitinol surface oxides are semiconductors with the band gaps of either 3.0 eV (rutile) or 3.4 eV (amorphous). The conductivity of semiconducting Nitinol surfaces relevant to their biological performances is discussed in terms of oxide stoichiometry and variable Ni content. Such biological characteristics of Nitinol surfaces as Ni release, fibrinogen adsorption and platelets behavior are re-examined based on the analysis of the results of the present study.

Melting and remelting phenomena

Abstract Melting processes influence the microstructure evolution in metal alloys during casting and heat treatments. Melting is often treated as ‘inverse solidification’, which is only appropriate in a limited number of cases. In the present article, asymmetry between solidification and melting is reviewed in detail. The current state of the thermodynamic description of melting under diffusion control is outlined. Kinetic aspects that break the symmetry between solidification and melting that are discussed are solute partitioning, solute concentration gradients and solute transport in the involved phases. The view on nucleation of liquid and pre-melting phenomena, mostly of pure materials, based on experimental and theoretical work, is given. Emphasis is laid on aspects of melting with technical relevance, i.e. melting of alloys. Particularities of thermally and solutally controlled melting are introduced. Mechanisms that involve both melting and solidification simultaneously are capillary driven coarsening, temperature gradient zone melting and liquid film migration. The impact of these processes on microstructural evolution is discussed. Open questions concerning modelling and simulation of melting, namely, the interaction of different melting mechanisms and the role of the solid/liquid interface, are identified.

Deviation from local equilibrium at migrating phase interfaces

The effects of four causes of deviation from local equilibrium at migrating phase interfaces are illustrated with molar Gibbs energy diagrams and using the sharp interface model. The conditions during solidification and melting are compared. In particular, it is examined when the composition of a growing phase can move inside the two-phase field.

An extended numerical procedure for predicting microstructure and microsegregation of multicomponent alloys

A numerical model for the prediction of microstructure and microsegregation in multicomponent alloys during dendritic solidification using a finite difference scheme is presented. The main kinetic and thermodynamic effects that can influence microsegregation (solid state back diffusion, secondary dendrite arm coarsening, primary tip and eutectic undercooling and the thermodynamic correction of the interface concentrations) are accounted for. The liquid/solid phase equilibria in the thermodynamically stable and metastable range are calculated with thermodynamically formulated phase diagrams. For the calculation at high cooling rates non-equilibrium phase diagrams for multi-component alloys are assessed. The consideration of undercooling effects extends the applicability of the model to very low and very high cooling rates. The model thus covers the whole range of cooling conditions where dendritic solidification occurs. As compared with other models in the literature, the number of adjustable parameters is reduced to a minimum.

Effect of cooling rate on the solidification behaviour of Al-Fe-Si alloys

Abstract The effect of cooling rate on the solidification microstructure of an Al–Fe–Si alloy is investigated. Samples were solidified with cooling rates between 0.04 and 3.5 K s −1 . The microstructures are characterised in terms of eutectic volume fraction, dendrite arm spacing and size and shape of secondary phases. Eutectic volume fraction and dendrite arm spacing were also calculated using a microsegregation model. Predicted data are found to be in good agreement with the experimental observations. Further, the role of microsegregation on phase selection is discussed with reference to earlier studies on phase selection in these alloys. The concentration of the liquid at the onset of eutectic solidification is correlated with the selection of the secondary phases in the eutectic.

Modelling study on recrystallization, recovery and their temperature dependence in inhomogeneously deformed materials

A hybrid analytical/Monte Carlo model is set up for studying recovery and recrystallization at various annealing temperatures after inhomogeneous deformation. Equations are derived to describe the nucleation rate and the evolution of the stored energy as functions of local deformation, temperature and time. The analytical equations and a description of the inhomogeneous pre-deformation are introduced into a Monte Carlo model. The microstructure evolution is simulated for various temperatures and times. The characteristics of recrystallization after inhomogeneous deformation and the interaction between recrystallization and recovery are well reproduced and generated by the model. Due to the inhomogeneous pre-deformation, the recrystallized fraction varies strongly depending on the given processing conditions. The simulation calculations agree well with experiments for both morphological features and quantitative analysis.

In situ observation of surface oxide layers on medical grade Ni-Ti alloy during straining

Medical grade Ni-Ti alloys with shape memory or pseudo-elastic behavior exhibit good biocompatibility because of an electrochemically passive oxide layer on the surface. In this work, the mechanical stability of surface oxide layers is investigated during reversible pseudo-elastic deformation of commonly applied medical grade Ni-Ti wires. Surface oxide layers with varying thickness were generated by varying annealing times under air atmosphere. The thicknesses of the surface oxide layers were determined by means of Rutherford backscattering spectrometry. In situ scanning electron microscopy investigations reveal a damage mechanism, which is assumed to have a significant influence on the biocompatibility of the material. The conditions that lead to the appearance of cracks in the surface oxide layer or to the flaking of surface oxide layer particles are identified. The influence of the thickness of the surface oxide layer on the damage mode is characterized. The possible impact of the damaged surface oxide layer on the materials biocompatibility and the potentials to reduce or avoid the damage are discussed.

Quantification of the interaction between biomaterial surfaces and bacteria by 3-D modeling.

It is general knowledge that bacteria/surface interactions depend on the surface topography. However, this well-known dependence has so far not been included in the modeling efforts. We propose a model for calculating interaction energies between spherical bacteria and arbitrarily structured 3-D surfaces, combining the Derjaguin, Landau, Verwey, Overbeek theory and an extended surface element integration method. The influence of roughness on the interaction (for otherwise constant parameters, e.g. surface chemistry, bacterial hydrophobicity) is quantified, demonstrating that common experimental approaches which consider amplitude parameters of the surface topography but which ignore spacing parameters fail to adequately describe the influence of surface roughness on bacterial adhesion. The statistical roughness profile parameters arithmetic average height (representing an amplitude parameter) and peak density (representing a spacing parameter) both exert a distinct influence on the interaction energy. The influence of peak density on the interaction energy increases with decreasing arithmetic average height and contributes significantly to the total interaction energy with an arithmetic average height below 70 nm. With the aid of the proposed model, different sensitivity ranges of the interaction between rough surfaces and bacteria are identified. On the nanoscale, the spacing parameter of the surface dominates the interaction, whereas on the microscale the amplitude parameter adopts the governing role.

Advance in Orientation Microscopy: Quantitative Analysis of Nanocrystalline Structures

The special properties of nanocrystalline materials are generally accepted to be a consequence of the high density of planar defects (grain and twin boundaries) and their characteristics. However, until now, nanograin structures have not been characterized with similar detail and statistical relevance as coarse-grained materials, due to the lack of an appropriate method. In the present paper, a novel method based on quantitative nanobeam diffraction in transmission electron microscopy (TEM) is presented to determine the misorientation of adjacent nanograins and subgrains. Spatial resolution of <5 nm can be achieved. This method is applicable to characterize orientation relationships in wire, film, and bulk materials with nanocrystalline structures. As a model material, nanocrystalline Cu is used. Several important features of the nanograin structure are discovered utilizing quantitative analysis: the fraction of twin boundaries is substantially higher than that observed in bright-field images in the TEM; small angle grain boundaries are prominent; there is an obvious dependence of the grain boundary characteristics on grain size distribution and mean grain size.

Undercooling effects in microsegregation modelling

Numerical simulation of microsegregation phenomena is of continuous interest in solidification modeling and has reached a high level of sophistication. Existing models predict the type and amount of solidifying phases and some microstructural features in conventional castings of binary alloys with sufficient accuracy. For cooling rates exceeding those of technical casting processes, occurring e.g. in surface remelting or welding, undercooling phenomena need to be taken into account. Undercooling of the dendrite tip and delayed formation of interdendritic phases can reduce the amount of nonequilibrium phases significantly. Therefore an existing numerical microsegregation model was extended to very low and high cooling rates by incorporating dendrite tip and eutectic undercooling effects. In this paper it is investigated to what degree the differences of the undercooling models influence the results of microsegregation calculations, with the aim to select the appropriate description of undercooling for microsegregation modelling. As model systems an Al-Cr alloy are selected.