Modelling of Recrystallization Nucleation at Hard Inclusions

During plastic deformation, a metallic structure is deformed inhomogeneously near hard inclusions. Hence both the materials strengthening and recovery and thus softening depends on the local position. There are thus high gradients of point defects, such as vacancies and interstitials, of the dislocation density and hence of the strain energy. Those gradients govern the diffusional flow, whose pile-up influences the climbing of edge dislocations, i.e. recovery and materials softening, respectively.

Staphylococcus aureus-Fibronectin Interactions with and without Fibronectin-Binding Proteins and Their Role in Adhesion and Desorption

ABSTRACT Adhesion and residence-time-dependent desorption of two Staphylococcus aureus strains with and without fibronectin (Fn) binding proteins (FnBPs) on Fn-coated glass were compared under flow conditions. To obtain a better understanding of the role of Fn-FnBP binding, the adsorption enthalpies of Fn with staphylococcal cell surfaces were determined using isothermal titration calorimetry (ITC). Interaction forces between staphylococci and Fn coatings were measured using atomic force microscopy (AFM). The strain with FnBPs adhered faster and initially stronger to an Fn coating than the strain without FnBPs, and its Fn adsorption enthalpies were higher. The initial desorption was high for both strains but decreased substantially within 2 s. These time scales of staphylococcal bond ageing were confirmed by AFM adhesion force measurement. After exposure of either Fn coating or staphylococcal cell surfaces to bovine serum albumin (BSA), the adhesion of both strains to Fn coatings was reduced, suggesting that BSA suppresses not only nonspecific but also specific Fn-FnBP interactions. Adhesion forces and adsorption enthalpies were only slightly affected by BSA adsorption. This implies that under the mild contact conditions of convective diffusion in a flow chamber, adsorbed BSA prevents specific interactions but does allow forced Fn-FnBP binding during AFM or stirring in ITC. The bond strength energies calculated from retraction force-distance curves from AFM were orders of magnitude higher than those calculated from desorption data, confirming that a penetrating Fn-coated AFM tip probes multiple adhesins in the outermost cell surface that remain hidden during mild landing of an organism on an Fn-coated substratum, like that during convective diffusional flow.

Mechanism Based Unified Creep Model Incorporating Damage

Although it is often said that diffusional flow creep is out of the practical engineering applications, the need for a unified model capable to account for the resulting action of both diffusional and dislocation type creep is justified by the increasing demands of reliable creep design for very long lives (exceeding 100.000h), high stress-low temperatures and high temperature-low stress regimes. In this paper, a unified creep model formulation, in which the change of the creep mechanism has been accounted for through an explicit dependence of the exponent n on stress and temperature, has been proposed. The model has been also extended incorporating damage processes, characteristics of creep stage IV, adopting a time independent damage formulation proposed by the authors. An application example of the proposed approach to high purity aluminum is given.

An Alternative Constitutive Equation for Superplasticity

Different models have been put forward to explain superplasticity. Most of the models predict the independency of activation energy (Q) on stress. Superplasticity is observed in region II of creep curve (logε Vs σ/E). The most commonly considered mechanism for superplastic flow involves Grain Boundary Sliding (GBS), and it is necessary for an accommodation process to accompany GBS. The accommodation process might be grain boundary migration, recrystalisation, diffusional flow or some dislocation slip process. But the Arrhenius type of equation given by Becker gives the dependency of activation energy on stress. Here in this work we have considered this equation and relation between Q and σ is found out using genetic algorithm. The present model development studies the parameter optimization, where parameters appearing in the stress and energy relationship equation e.g. relationship between Q(σ) and σ for the Q(σ) equation given in present work as well as pre-exponential factors are optimized with the objective function being the error minimization of model predicted values and experimental data of strain are available from open literature.

Size effects of nanoindentation creep

The size effects on indentation creep were studied on single-crystal Ni3Al, polycrystalline pure Al, and fused quartz samples at room temperature. The stress exponents were measured by monitoring the displacement during constant indentation loads after correction for thermal drift effects. The stress exponents were found to exhibit a very strong size effect. In the two metals Al and Ni3Al, the stress exponent for very small indents is very small, and for Al, this even approaches unity, suggesting that linear diffusional flow may be the controlling mechanism. The stress exponents in these two metals rise rapidly to over 100 as the indent size gets larger, indicating a rapid change of the dominating mechanism to climb-controlled to eventually glide-controlled events. In fused quartz, the stress exponent also exhibits a sharply rising trend as the indent size increases. The stress exponent is also close to unity at the smallest indents studied, and it rises rapidly to a few tens as the indent size gets larger.