Interesting papers: part II

April 21, 2011

[1] Dissipated energy measurements as a marker of microstructural evolution: 316L and DP600

Connesson et al

The thermomechanical characteristics and, more specifically, the dissipative behavior of two steels (a DP600 and a 316L stainless steel) have been studied using infrared measurement methods. All dissipated energy measurements have been performed during traction–traction uniaxial tests in the elastic domain. It has been shown that the dissipated energy of these materials is dependent on the material plastic strain and could be used as a non-destructive criterion to monitor the material evolution during loading sequences. Different kinds of loading sequences have been tested, including uniaxial tensile tests, alternative traction–traction loadings and recovery periods to underline specific characteristics of the materials.

[2] Diffusive model of pore shrinkage in final-stage sintering under hydrostatic pressure

Kim et al

A grain-boundary-diffusion model is developed to understand the densification behavior of pores in the final stage of sintering under compressive hydrostatic pressure. From analysis of the diffusive model, the bulk viscosity, densification rate and shrinkage rate of pores are predicted for a dense matrix polycrystal containing spherical pores, and compared with the existing experimental results and models. A transition in the sintering mechanism is predicted from the different pore-size dependence of the shrinkage rate between the diffusive and the viscous flow models. The transition effect is experimentally confirmed by the appearance of a downward inflection in the size distribution of pores during sintering. The upward inflection observed experimentally in the cavity-size distribution after superplastic deformation is also explained by the transition of the mechanism.

[3] Phase-field analysis of a ternary two-phase diffusion couple with multiple analytical solutions

Heulens et al

Under certain conditions, there are multiple analytical solutions for the interface velocity in ternary two-phase diffusion couples. We have tackled this degeneracy problem by employing an isothermal phase-field model for diffusion couples that allows comparison with the analytical solutions. We find that, besides the three analytically predicted single-interface diffusion paths, a triple-interface diffusion path can also form. Furthermore, analysis of the phase-field simulation results shows that the Gibbs energies of both the bulk phases and the interfaces must be considered to determine uniquely the evolution of a diffusion couple, in case whether a single or multiple interfaces form.

[4] Atomistic study of the buckling of gold nanowires

Olsson and Park

In this work, we present results from atomistic simulations of gold nanowires under axial compression, with a focus on examining the effects of both axial and surface orientation effects on the buckling behavior. This was accomplished by using molecular statics simulations while considering three different crystallographic systems: left angle bracket1 0 0right-pointing angle bracket/{1 0 0}, left angle bracket1 0 0right-pointing angle bracket/{1 1 0} and left angle bracket1 1 0right-pointing angle bracket/{1 1 0}{1 0 0}, with aspect ratios spanning from 20 to 50 and cross-sectional dimensions ranging from 2.45 to 5.91 nm. The simulations indicate that there is a deviation from the inverse square length dependence of critical forces predicted from traditional linear elastic Bernoulli–Euler and Timoshenko beam theories, where the nature of the deviation from the perfect inverse square length behavior differs for different crystallographic systems. This variation is found to be strongly correlated to either stiffening or increased compliance of the tangential stiffness due to the influence of nonlinear elasticity, which leads to normalized critical forces that decrease with decreasing aspect ratio for the left angle bracket1 0 0right-pointing angle bracket/{1 0 0} and left angle bracket1 0 0right-pointing angle bracket/{1 1 0} systems, but increase with decreasing aspect ratio for the left angle bracket1 1 0right-pointing angle bracket/{1 1 0}{1 0 0} system. In contrast, it was found that the critical strains are all lower than their bulk counterparts, and that the critical strains decrease with decreasing cross-sectional dimensions; the lower strains may be an effect emanating from the presence of the surfaces, which are all more elastically compliant than the bulk and thus give rise to a more compliant flexural rigidity.

[5] Analysis by synchrotron X-ray radiography of convection effects on the dynamic evolution of the solid–liquid interface and on solute distribution during the initial transient of solidification

Bogno et al

In situ monitoring of the initial transient of directional solidification was carried out by means of synchrotron X-ray radiography. Experiments with Al–4 wt.% Cu alloy samples were performed on beamline ID19 of the European Synchrotron Radiation Facility (ESRF) in a dedicated Bridgman-type furnace. X-ray radiography enabled a detailed analysis of the evolution over time of the solid–liquid interface macroscopic shape in interaction with convection in the melt. Lateral solute segregation induced by fluid flow resulted in a significant deformation of the solid–liquid interface. The time-dependent velocity of the solidification front was determined at different abscissa values along the curved interface during the solidification process, from the growth phase with a smooth interface to the onset of morphological instability. Further, using a novel quantitative image analysis technique we were able to measure longitudinal solute profiles in the melt during the initial transient. Solutal length was then deduced as well as concentration in the melt, both at the interface and far away from it. The influence of convection on growth velocity and the characteristic parameters of the solute boundary layer are discussed, and a comparison with the Warren and Langer model is also presented.

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