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From liquid to solid: How do nanocrystalline materials change in an instant?
Nanocrystalline material (NC material) is a polycrystalline material with a grain size of only a few nanometers. These materials fill the gap between amorphous materials without long-range order and traditional coarse-grained materials. Definitions of nanocrystalline materials vary, but materials with grain sizes below 100 nanometers are generally considered to be in this category, while those with grain sizes between 100 and 500 nanometers are considered "ultrafine" grains. The grain size of a nanocrystal sample can be estimated through X-ray diffraction. In materials with very small grain sizes, the diffraction peaks become broader.
Nanocrystalline materials are unique in that they bridge the gap between the smallest grain size and the greatest physical strength in structural materials.
Synthesis method
Nanocrystalline materials can be prepared using several methods, which are often classified based on the phases of matter the material goes through before forming the final nanocrystalline product.
Solid state processing
Solid-state processes do not involve melting or evaporating materials and typically occur at relatively low temperatures. Mechanical alloying, high energy ball milling, etc. are some examples.
Liquid processing
Nanocrystalline metals can be produced by rapid solidification from a liquid state, such as fusion spinning techniques, which typically produces an amorphous metal that can subsequently be converted into a nanocrystalline metal by heating above the crystallization temperature.
Gas phase treatment
Thin films of nanocrystalline materials can be produced by vapor deposition processes such as chemical vapor deposition.
Solution treatment
Certain metals, especially nickel and nickel alloys, can be made into nanocrystalline foils by electrodeposition.
Mechanical properties
Nanocrystalline materials exhibit superior mechanical properties compared to coarse-grained materials. Since the volume fraction of grain boundaries in nanocrystalline materials may be as high as 30%, this amorphous grain boundary has a significant impact on the mechanical properties of the material.
The excellent yield strength of nanocrystalline metals results from the strengthening of grain boundaries, which are extremely effective in preventing dislocation movement.
Nanocrystalline metal
The yield strength of nanocrystalline metals is mainly due to grain boundary strengthening, because grain boundaries very effectively block the movement of dislocations. As the grain size decreases, the critical yield stress increases.
Nanocrystalline ceramics
While the mechanical behavior of ceramics is usually dominated by defects such as porosity, grain size enhancement has also been observed in high-density ceramic samples. Additionally, nanocrystalline ceramics sinter more rapidly than coarse ceramics, resulting in higher density and superior mechanical properties.
Processing technology
Although the synthesis of nanocrystalline materials is relatively simple, there are still difficulties in converting them into large components. Prolonged high-temperature exposure leads to coarsening of nanocrystalline materials, thus requiring low temperature and rapid densification techniques to process these materials into bulk assemblies.
Currently, in the research of nanocrystalline materials, we have seen many potential technologies, such as the application of spark plasma sintering or ultrasonic additive manufacturing. However, how to promote these advanced nanocrystal technologies to commercial scale remains an important challenge in the future.
Nanocrystalline materials generally have low thermal stability, but their high-temperature resistance can be improved through alloying and other techniques.
In the process of exploring these materials, their liquid to solid state changes undoubtedly demonstrate the potential of nanotechnology. However, we need to think about what role future nanocrystal materials will play in other fields.
