Where does grain collide?

A single crystallite inside a polycrystalline solid: a region where the crystal lattice keeps one continuous orientation, bounded by grain boundaries where that orientation abruptly changes. Hall (1951), with Petch, established that a metal's yield strength rises as its grains get smaller, the Hall-Petch relation, because boundaries block the dislocations that carry plastic flow. The referent is a feature of a material's microstructure, and the unit is one crystallite bounded by grain boundaries. Here smaller is stronger, and grain size is a diameter measured under a microscope, not a particle you could pick up. It shares the word with a sand grain and a seed, but this grain has no separate existence: it is a patch of shared orientation welded to its neighbours.[1]

Unit

One crystallite bounded by grain boundaries

Recovery means

Not applicable

Measured by

Mean grain diameter or ASTM grain-size number by micrograph, linear intercept, or electron backscatter diffraction

An individual clastic particle of sediment or rock, a discrete fragment produced by weathering and moved by wind, water, or ice. Wentworth (1922) fixed the grade scale still in use, sorting particles by diameter into clay, silt, sand, gravel, and larger classes, so that a deposit is described by the sizes of the grains that compose it. The referent is a real, separable particle, and the unit is one detrital grain characterised by its diameter. Grain size here records the energy of the current that carried and dropped the particle: finer grains settle in calmer water. It is a loose object you can sieve, unlike the crystallite fused into metal or the direction read off a plank, though all are called grains.[2]

Unit

One detrital particle characterised by its diameter

Recovery means

Not applicable

Measured by

Grain-size distribution by sieving, settling, or laser diffraction, reported on the phi or Wentworth scale

The granular texture of a developed image: the random fluctuation of density that makes a uniform tone look mottled, arising in film from the clumping of silver particles and, by extension, in digital pictures as image noise. Zweig (1956) formalised it, treating granularity through the autocorrelation of density fluctuations and the root-mean-square deviation measured through a scanning aperture. The referent is not any single particle but a statistical property of a whole patch, and the unit is the RMS density fluctuation over a defined aperture. Finer grain here means less visible noise and a cleaner image. It borrows the word from the sand and the crystallite, yet names a texture of many specks, a signal-to-noise quantity rather than an object.[3]

Unit

The RMS density fluctuation over a scanning aperture

Recovery means

Not applicable

Measured by

RMS (Selwyn) granularity from a microdensitometer trace at a stated aperture, and the density autocorrelation