User Contributed Dictionary

English

Noun

The fine structure of a material or tissue as revealed by microscopy.
The fine structure of a pure metal or alloy, as revealed by magnifications of 25X or greater.
In the context of "meteorology|oceanography": Fine-scale structure in such variables as temperature, salinity, velocity, etc.

Antonyms

italbrac fine structure: macrostructure

References

Extensive Definition

Microstructure refers of the microscopic description of the individual constituents of a material. The length scale is 100-1 micrometres, 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.

Methods

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 Strengthening), but a true study would take into quantitative account the relative strengths of the crystal and that of inter-crystal bonding.

Microscopy

When a polished flat sample reveals traces of its microstructure, its is normal to capture the image using macrophotography. More sophisticated microstructural examination involves higher powered instruments: optical microscopy, electron microscopy, X-ray diffraction and so on, some involving pre-preparation of the material sample (cutting, microtomy, polishing, etching, vapour-deposition etc). The methods are known colleectively as metallography as applied to metals and alloys, and can be used in modified form for any other material, such as ceramics, glasses, composites and polymers.

Nondestructive testing 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 by many scientists.

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