Ashok Gurumurthy

Montage Serial Sectioning: Some Finer Aspects of Practice

Serial sectioning as a probe of topological properties of solids has become increasingly important for quantitative materials characterization and provides accurate inputs for microstructure simulations. In recent years, large-area montage serial sectioning has gained traction, due to the high resolution of micrographs it produces and the large volumes it samples. But some finer aspects of the practice dealing with error detection and correction—necessary for accurate reconstruction of a microstructural volume—have not received much attention. This article describes the tools and techniques we developed to (1) correct shading in tiles that make up a montage and (2) detect metallographic errors in serial sectioning. These tools help reconstruct the most accurate three-dimensional microstructures for use in investigating the micromechanics of materials under a variety of conditions. They also help ensure accuracy in stereological measurements, in reconstruction of constituent morphologies for characterization or simulation, and in other applications of montage serial sectioning.

Meso-scale simulations of strain-induced reaction mechanisms in Ti/Al/B heterogeneous systems

We investigate the effects of bulk composition and reactant configuration on the strain-induced reaction behavior of Ti/Al/B powder mixture compacts under uniaxial stress loading conditions. Impact experiments reveal a varying degree of reactivity as a function of Al content in the ternary mixture. Impact simulations are conducted on both real and synthetic microstructures of the same composition and with a random distribution of particles. An increase in hot-spots and strain localization is observed to occur at higher impact velocities, contributing to greater reactivity. The synthetic structures behave similarly to the real structures providing a preliminary validation of the synthetic scheme. A number of stereological metrics were used to describe the microstructural evolution during simulation as a function of time.

Meso-scale heterogeneity effects on the bulk shock response of Ti+2B reactive powder mixtures

Highly heterogeneous reactive powder mixtures containing Ti+2B (Stoichiometric 1:2) are studied to ascertain the shock compression response and potential reaction behavior. The transit time through the pressed powder mixture compacts is monitored using poly-vinylidene fluoride (PVDF) stress gauges and used to compute a wave speed through the compact. The stress states at the back of the powder (measuring the state of the compacted and potentially transformed powder) are compared with thermodynamic mixture theories as well as meso-scale microstructure-based simulations to identify the onset of anomalous behavior which can be traced to highly exothermic reaction in this system. Shock compression experiments show highly dispersive wave fronts when measured from the back surface of the powder compact, which are compared with meso-scale simulations considering varying starting mixture states. These simulations also provide microstructure evolution parameters during shock compression which are stereologically e...

Three-Dimensional Metallography for Visualization, Characterization, Modeling, and Simulation of Three-Dimensional Microstructures

It is the central principle of materials science that processing governs microstructure and the microstructure influences the properties and performance of materials. Consequently, quantitative characterization and mathematical representation of microstructure are of considerable importance in materials science. Material microstructures are three-dimensional and, therefore, the attributes of three-dimensional microstructural geometry are of core interest. Nonetheless, as most of the materials are opaque, the microstructural observations are usually on the two-dimensional (2D) metallographic sections through the three-dimensional (3D) microstructural domains of interest. The microstructure observed in a metallographic section consists of intersections of the features in the 3D microstructure with the sectioning plane. Therefore, in a metallographic plane, the volumes (e.g., grains, voids, particles) in a 3D microstructure appear as areas, and the surfaces (e.g., grain boundaries, precipitate interfaces) appear as lines. Clearly, a 2D metallographic section does not contain all the information concerning the true 3D microstructural geometry. Consequently, development and applications of the techniques for reconstruction, visualization, and direct quantitative characterization of opaque 3D microstructures (i.e., 3D metallography) are of considerable interest.