Microstructure

  Microstructure refers of the microscopic description of the individual constituents of a material. The length scale is 100-1 micrometer, well above the atomic levels. That said, work has long been underway on nano-level microstructural control (although this really should be renamed nanostructure). The microstructure of a material (of which we can broadly classify into "metallic", "polymeric", "ceramic" and "composite") really is a study of the crystal structure of a material, their size, composition, orientation, formation, interaction and, ultimately, their effect on the macroscopic behaviour in terms of physical properties such as strength, toughness, ductility, hardness, corrosion resistance, high / low temperature behaviour, wearability, and so on, which in turn govern the application of these materials in industry and manufacture.

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The simplest microstructural observation is done with the naked eye. If one ever comes across a piece of galvanised steel, such as the casing of a lampost or road divider, one observes that the surface is not uniformly coloured, but is covered with a patchwork of interlocking polygons of different shades of the grey or silver. Each polygon (the most frequently occurring would be hexagons) is a single crystal of zinc adhering to the surface of the steel beneath. Zinc and lead are two common metals which form large crystals visible to the naked eye. The metallic atoms in each crystal are well-organised into one of 7 crystal lattice systems possible for metals (cubic, tetrahedral, hexagonal, monoclinic, triclinic, rhombohedral, orthorhombic); these systems dictate that the atoms are all lined up like points in a 3-D matrix. However, the direction of alignment of the matrices differ from crystal to adjacent crystal, leading to variance in the reflectivity of each presented face of the interlocked crstals on the galvanised surface. The next thing one should note would be the shape of each crystal: are they generally round or elongated? Symmetrical crystals are generally unstressed, unworked. They grow in all directions equally and were not subjected to deforming stresses either during or after. If elongated, are they all pointing the same way? What is the size distribution of the crystals? For large crystals, the ratio of crystal bulk to inter-crystal boundary (more properly, intergranullar boundary) is high. This indicates high ductility but correspondingly, lower strength (see "Hall-Petch Equation"), but a true study would take into quantitative account the relative strengths of the crystal and that of inter-crystal bonding.

More sophisticated microstructural examination would involve higher powered instruments: optical- and electron-microscopy, X-ray diffraction and so on, involving extensive pre-preparation of the material sample (cutting, microtoming, polishing, etching, vapor-deposition).

Nondestructive evaluation of microstructure for biological materials is a challenge and Computer Microtomography is the current solution. In fact, CMT can be used for the evaluation of microstructure of many other materials also. CMT can be very expensive though, and for research purposes, it is a necessity to generate a three-dimensional microstructure from two-dimensional cross-sectional images of the material. This is an area of active research and pursued right now by many scientists.

See also

• macrostructure