Asim Tewari

Effect of gravity on three-dimensional coordination number distribution in liquid phase sintered microstructures

Abstract Gravity affects microstructural evolution when a liquid phase is present during sintering. The effect of gravity on the three-dimensional coordination number distribution of tungsten grains in liquid phase sintered heavy alloy specimens is quantitatively characterized. A combination of montage serial sectioning, digital image processing, and unbiased stereological sampling procedures is used to estimate the coordination number distribution in three-dimensional microstructures. The microgravity environment decreases the mean coordination number. However, hardly any isolated grains are observed in the specimens, liquid phase sintered in a microgravity environment. The effect of microgravity on the coordination numbers mainly resides in its effect on the mean coordination number. In all specimens, there is a strong correlation between grain size and coordination number, which can be expressed as [D c / D ] 2 =C/C 0 where C0 is the mean coordination number, D the global average size of the tungsten grains, and Dc the average size of only those grains which have coordination number C.

MODELING OF NON-UNIFORM SPATIAL ARRANGEMENT OF FIBERS IN A CERAMIC MATRIX COMPOSITE

Abstract In the unidirectional fiber reinforced composites, the spatial arrangement of fibers is often non-uniform. These non-uniformities are linked to the processing conditions, and they affect the properties of the composite. In this contribution, a recently developed digital image analysis technique is used to quantify the non-uniform spatial arrangement of Nicalon fibers in a ceramic matrix composite (CMC). These quantitative data are utilized to develop a six parameter computer simulated microstructure model that is statistically equivalent to the non-uniform microstructure of the CMC. The simulated microstructure can be utilized as a RVE for the micro-mechanical modeling studies.

Quantitative Characterization of Spatial Arrangement of Micropores in Cast Microstructures

Abstract Spatial arrangement of micropores is an important geometric attribute of cast microstructures. The spatial arrangement of porosity is determined by the solidification processing parameters, and it affects the mechanical properties of cast components. Quantitative information of the spatial arrangement of micropores is useful for modeling cast microstructures. In this contribution, a digital image analysis based experimental technique is presented for unbiased quantitative characterization of the spatial arrangement of microporosity in terms of the statistical descriptors such as the first, the second, and the third nearest neighbor distance distribution of the pores observed in metallographic sections of cast micro-structures.

Estimation of three-dimensional grain size distribution from microstructural serial sections

Abstract Particle/grain size distribution is an important attribute of microstructures. However, direct estimation of three-dimensional particle/grain size distribution has never been performed in opaque material microstructures due to lack of a suitable serial sectioning technique. In this contribution, application of a montage-based efficient serial sectioning technique is presented for direct unbiased estimation of three-dimensional grain size distribution. The technique is used for estimation of three-dimensional grain size distribution of tungsten grains in a liquid phase-sintered (LPS) microstructure. It is shown that the montage-based serial sectioning technique is very efficient. It yields 25–100 times larger high-resolution three-dimensional serially sectioned volume as compared to that obtained from the same number of serial sections. The larger reconstructed microstructural volume provides sufficient number of particles/grains for a reliable estimation of detailed size distribution.

Application of Three-Dimensional Digital Image Processing for Reconstruction of Microstructural Volume from Serial Sections

Three-dimensional digital image processing is useful for reconstruction of microstructural volume from a stack of serial sections. Application of this technique is demonstrated via reconstruction of a volume segment of the liquid-phase sintered microstructure of a tungsten heavy alloy processed in the microgravity environment of NASAsspace shuttle, Columbia. Ninety serial sections (approximately one micrometer apart) were used for reconstruction of the three-dimensional microstructure. The three-dimensional microstructural reconstruction clearly revealed that the tungsten grains are almost completely connected in three-dimensional space. Both the matrix and the grains are topologically co-continuous, although the alloy was liquid-phase sintered in microgravity. Therefore, absence of gravity did not produced a microstructure consisting of discrete isolated W grains uniformly dispersed in the liquid Ni-Fe alloy matrix at the sintering temperature.