Microporosity Fraction Research Articles

Microporosity reduction in modified RR2072 single crystal superalloys

The effect of the addition of grain boundary strengthening elements (carbon, hafnium) on the microporosity of an experimental single crystal superalloy RR2072 has been investigated. Quantitative examination showed that the volume fraction of microporosity decreased in the alloys modified by these additions, and the reduction was associated with MC phase/carbon distribution and/or high hafnium content. Excessive carbon (i.e. >0.05 wt-%), however, did not reduce microporosity as efficiently as expected. Detailed analysis has revealed that carbon atoms, whether they stay in MC carbides or in the matrix as interstitial atoms, cause lattice expansion and thus reduce volume contraction during solidification. This mechanism is responsible for microporosity reduction in the carbon bearing alloys. Based on this mechanism of interstitial induced lattice expansion, theoretical calculation is consistent with experimental results. In the carbon and hafnium bearing material, however, high hafnium is another important reason for the reduction of microporosity.

Microporosity prediction in aluminum alloy castings

A comprehensive methodology that takes into account solidification, shrinkage-driven interdendritic fluid flow, hydrogen precipitation, and porosity evolution has been developed for the prediction of the microporosity fraction and distribution in aluminum alloy castings. The approach may be used to determine the extent of gas and shrinkage porosity, i.e., the resultant microporosity which occurs due to gas precipitation and that which occurs when solidification shrinkage cannot be compensated for by the interdendritic fluid flow. A solution algorithm in which the local pressure and microporosity are coupled is presented, and details of the implementation methodology are provided. The models are implemented in a computational framework consistent with that of commonly used algorithms for fluid dynamics, allowing a straightforward incorporation into existing commercial software. The results show that the effect of microporosity on the interdendritic fluid flow cannot be neglected. The predictions of porosity profiles are validated by comparison with independent experimental measurements by other researchers on aluminum A356 alloy test castings designed to capture a variety of solidification conditions. The numerical results reproduce the characteristic microporosity profiles observed in the experimental results and also agree quantitatively with the experimentally measured porosity levels. The approach provides an enhanced capability for the design of structural castings.

Changes of the local pore space structure quantified in heterogeneous porous media by H1 magnetic resonance relaxation tomography

Magnetic resonance imaging and relaxation analysis are combined in a spatially resolved technique (relaxation tomography), which is able to quantify the parameters connected to the local structure in the internal regions of a porous material saturated by water, giving information on the pore space structure beyond the nominal instrumental resolution. Voxel-by-voxel longitudinal (T1) and transverse (T2) relaxation curves are acquired in order to obtain T1, T2 and S(0) maps, where S(0) is the extrapolation to zero time of the total equilibrium magnetization corrected for T2 decay. The proposed method permits evaluation of the porosity (ratio of pore space to total volume), at different length scales, from the sample to the voxel, not all achievable by traditional methods. More striking is its ability to describe how porosity is shared among different classes of surface-to-volume ratios of diffusion cells (the regions that the individual water molecules, starting at their particular positions, can experience by diffusion before relaxing). This is a consequence of the fact that relaxation times of water confined in a porous material can, under favorable circumstances, distinguish regions with the same local porosity but with different pore sizes and connections. So, parameters can be introduced, such as the microporosity fraction, defined as the fraction of the “micropore” volume with respect to the total pore volume, and several voxel average porosities, defined as the average porosities of the voxels characterized by particular classes of diffusion cells. Moreover, the imaging methods enable us to get all this information in a user-defined region of interest. The method has been applied to quantify changes in the structure of carbonate cores with wide distributions of pore sizes induced by repeated cycles of freezing and heating of the sample. With freezing, the microporosity fraction decreases significantly; the voxel average porosity of voxels with T1 shorter than for free water tend to decrease; and the distributions of porosity as functions of T1 show a trend, with much more signal with the T1 of free water, in accordance with the picture suggesting large vugs breaking, with fractures contributing to homogenizing the structure of the pore space and favoring coupling between neighboring pores.

Adsorption Characteristics of Coal-Based Activated Carbons. 1. Adsorption of Nitrogen and Carbon Dioxide

Non-activated carbon from coal contains ultrafine pores in which adsorption of nitrogen at 77 K is controlled by activated diffusion. Carbons activated with phosphoric acid or with steam contain a large fraction of microporosity. However, they do not contain ultrafine pores or pore constrictions in which activated diffusion takes place. The BET equation can be applied to calculate the surface area of microporous carbon when the monolayer capacity lies in the relative pressure range 0.05–0.30, the BET areas in such cases being comparable with those obtained from the αs-method. The Dubinin–Radushkevich (D–R) equation is the most suitable for the interpretation of CO2 adsorption at 298 K. Chemical and physical activation starts with new pore creation, further activation being associated with pore widening. Microporosity is more predominant in chemically-activated carbons.

Modeling of inverse segregation and porosity formation in directionally solidified aluminum alloys

A model has been developed which simulates inverse segregation and microporosity formation in directionally solidified alloys. Based upon a finite difference scheme, the model takes into account volume changes associated with density variations during solidification. The continuity equations for the mass, the solute, and the energy together with the Darcy equation describing the flow in the mushy zone are solved in a mixed Lagrangian-Eulerian representation. All nodal points within the liquid phase move with the fluid velocity, whereas nodes are fixed in space as soon as they are reached by dendrite tips. When the dendrite tips arrive at the end of the ingot, the remaining interdendritic liquid partially compensates for the solidification shrinkage occurring deeper within the volume. Since the size of the ingot remains fixed from that point on (absence of a purely liquid region), air (macroporosity) is introduced at the mesh points to satisfy the mass balance, starting from the top of the mushy zone. The formation of microporosity is also accounted for in the model through a calculation of local hydrogen segregation. Using this model, it is shown that inverse segregation decreases with increasing hydrogen content (or volume fraction of microporosity). The results of the simulation are compared with experimental results obtained on an Al-Cu alloy solidified under well-controlled directional conditions.