Definitions vary, but nanocrystalline material is commonly defined as a crystallite (grain) size below 100 nm.
This broadening can be related to a crystallite size using the Scherrer equation (applicable up to ~50 nm), a Williamson-Hall plot,[1] or more sophisticated methods such as the Warren-Averbach method or computer modeling of the diffraction pattern.
The crystallite size can be measured directly using transmission electron microscopy.
Methods are typically categorized based on the phase of matter the material transitions through before forming the nanocrystalline final product.
Solid-state processes do not involve melting or evaporating the material and are typically done at relatively low temperatures.
Nanocrystalline metals can be produced by rapid solidification from the liquid using a process such as melt spinning.
Thin films of nanocrystalline materials can be produced using vapor deposition processes such as MOCVD.
[3] Nanocrystalline materials show exceptional mechanical properties relative to their coarse-grained varieties.
Because the volume fraction of grain boundaries in nanocrystalline materials can be as large as 30%,[4] the mechanical properties of nanocrystalline materials are significantly influenced by this amorphous grain boundary phase.
Thus, via the rule of mixtures, a nanocrystalline material will have a lower elastic modulus than its bulk crystalline form.
This critical stress increases as the grain size decreases, and these physics are empirically captured by the Hall-Petch relationship, where
is a material-specific constant that describes the magnitude of the metal's response to grain size strengthening, and
The maximum strength of a metal is given by the intersection of this line with the Hall-Petch relationship, which typically occurs around a grain size of
In nanocrystalline samples of low-melting point metals (i.e. aluminum, tin, and lead), the grain size of the samples was observed to double from 10 to 20 nm after 24 hours of exposure to ambient temperatures.
[5] Although materials with higher melting points are more stable at room temperatures, consolidating nanocrystalline feedstock into a macroscopic component often requires exposing the material to elevated temperatures for extended periods of time, which will result in coarsening of the nanocrystalline microstructure.
Thus, thermally stable nanocrystalline alloys are of considerable engineering interest.
Experiments have shown that traditional microstructural stabilization techniques such as grain boundary pinning via solute segregation or increasing solute concentrations have proven successful in some alloy systems, such as Pd-Zr and Ni-W.[7] While the mechanical behavior of ceramics is often dominated by flaws, i.e. porosity, instead of grain size, grain-size strengthening is also observed in high-density ceramic specimens.
[8] Additionally, nanocrystalline ceramics have been shown to sinter more rapidly than bulk ceramics, leading to higher densities and improved mechanical properties,[5] although extended exposure to the high pressures and elevated temperatures required to sinter the part to full density can result in coarsening of the nanostructure.
The large volume fraction of grain boundaries allows for a significant diffusional flow of atoms via Coble creep, analogous to the grain boundary sliding deformation mechanism in nanocrystalline metals.
A variety of techniques show potential in this respect, such as spark plasma sintering[9] or ultrasonic additive manufacturing,[10] although the synthesis of bulk nanocrystalline components on a commercial scale remains untenable.