Nanocrystalline material

Last updated January 05, 2024

A nanocrystalline (NC) material is a polycrystalline material with a crystallite size of only a few nanometers. These materials fill the gap between amorphous materials without any long range order and conventional coarse-grained materials. Definitions vary, but nanocrystalline material is commonly defined as a crystallite (grain) size below 100 nm. Grain sizes from 100 to 500 nm are typically considered "ultrafine" grains.

The grain size of a NC sample can be estimated using x-ray diffraction. In materials with very small grain sizes, the diffraction peaks will be broadened. 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.^[1]

Synthesis

Nanocrystalline materials can be prepared in several ways. Methods are typically categorized based on the phase of matter the material transitions through before forming the nanocrystalline final product.

Solid-state processing

Solid-state processes do not involve melting or evaporating the material and are typically done at relatively low temperatures. Examples of solid state processes include mechanical alloying using a high-energy ball mill and certain types of severe plastic deformation processes.

Liquid processing

Nanocrystalline metals can be produced by rapid solidification from the liquid using a process such as melt spinning. This often produces an amorphous metal, which can be transformed into an nanocrystalline metal by annealing above the crystallization temperature.

Vapor-phase processing

Thin films of nanocrystalline materials can be produced using vapor deposition processes such as MOCVD.^[2]

Solution processing

Some metals, particularly nickel and nickel alloys, can be made into nanocrystalline foils using electrodeposition.^[3]

Mechanical properties

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. For example, the elastic modulus has been shown to decrease by 30% for nanocrystalline metals and more than 50% for nanocrystalline ionic materials.^[5] This is because the amorphous grain boundary regions are less dense than the crystalline grains, and thus have a larger volume per atom, $\Omega$ . Assuming the interatomic potential, $U(\Omega )$ , is the same within the grain boundaries as in the bulk grains, the elastic modulus, $E\propto \partial ^{2}U/\partial \Omega ^{2}$ , will be smaller in the grain boundary regions than in the bulk grains. Thus, via the rule of mixtures, a nanocrystalline material will have a lower elastic modulus than its bulk crystalline form.

Nanocrystalline metals

The exceptional yield strength of nanocrystalline metals is due to grain boundary strengthening, as grain boundaries are extremely effective at blocking the motion of dislocations. Yielding occurs when the stress due to dislocation pileup at a grain boundary becomes sufficient to activate slip of dislocations in the adjacent grain. This critical stress increases as the grain size decreases, and these physics are empirically captured by the Hall-Petch relationship,

\sigma _{y}=\sigma _{0}+Kd^{-1/2},

where $\sigma _{y}$ is the yield stress, $\sigma _{0}$ is a material-specific constant that accounts for the effects of all other strengthening mechanisms, $K$ is a material-specific constant that describes the magnitude of the metal's response to grain size strengthening, and $d$ is the average grain size.^[6] Additionally, because nanocrystalline grains are too small to contain a significant number of dislocations, nanocrystalline metals undergo negligible amounts of strain-hardening,^[5] and nanocrystalline materials can thus be assumed to behave with perfect plasticity.

As the grain size continues to decrease, a critical grain size is reached at which intergranular deformation, i.e. grain boundary sliding, becomes more energetically favorable than intragranular dislocation motion. Below this critical grain size, often referred to as the “reverse” or “inverse” Hall-Petch regime, any further decrease in the grain size weakens the material because an increase in grain boundary area results in increased grain boundary sliding. Chandross & Argibay modeled grain boundary sliding as viscous flow and related the yield strength of the material in this regime to material properties as

\tau ={\bigg (}L{\frac {\rho _{L}}{M}}{\bigg )}{\bigg (}1-{\frac {T}{T_{m}}}{\bigg )}f_{g},

where $L$ is the enthalpy of fusion, $\rho _{L}/M$ is the atomic volume in the amorphous phase, $T_{m}$ is the melting temperature, and $f_{g}$ is the volume fraction of material in the grains vs the grain boundaries, given by $f_{g}=(1-\delta /d)^{3}$ , where $\delta$ is the grain boundary thickness and typically on the order of 1 nm. 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 $d$ = 10 nm for BCC and FCC metals.^[4]

Due to the large amount of interfacial energy associated with a large volume fraction of grain boundaries, nanocrystalline metals are thermally unstable. 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]

Nanocrystalline ceramics

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 associated with nanocrystalline materials causes interesting behavior in ceramic systems, such as superplasticity in otherwise brittle ceramics. 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. Because the diffusional creep rate scales as $d^{-3}$ and linearly with the grain boundary diffusivity, refining the grain size from 10 μm to 10 nm can increase the diffusional creep rate by approximately 11 orders of magnitude. This superplasticity could prove invaluable for the processing of ceramic components, as the material may be converted back into a conventional, coarse-grained material via additional thermal treatment after forming.^[5]

Processing

While the synthesis of nanocrystalline feedstocks in the form of foils, powders, and wires is relatively straightforward, the tendency of nanocrystalline feedstocks to coarsen upon extended exposure to elevated temperatures means that low-temperature and rapid densification techniques are necessary to consolidate these feedstocks into bulk components. 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.

Related Research Articles

<span class="mw-page-title-main">Sintering</span> Process of forming and bonding material by heat or pressure

Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing process used with metals, ceramics, plastics, and other materials. The nanoparticles in the sintered material diffuse across the boundaries of the particles, fusing the particles together and creating a solid piece.

In physics and materials science, plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the material itself. In engineering, the transition from elastic behavior to plastic behavior is known as yielding.

In materials science, superplasticity is a state in which solid crystalline material is deformed well beyond its usual breaking point, usually over about 400% during tensile deformation. Such a state is usually achieved at high homologous temperature. Examples of superplastic materials are some fine-grained metals and ceramics. Other non-crystalline materials (amorphous) such as silica glass and polymers also deform similarly, but are not called superplastic, because they are not crystalline; rather, their deformation is often described as Newtonian fluid. Superplastically deformed material gets thinner in a very uniform manner, rather than forming a "neck" that leads to fracture. Also, the formation of microvoids, which is another cause of early fracture, is inhibited. Superplasticity must not be confused with superelasticity.

A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains.

In materials science, creep is the tendency of a solid material to undergo slow deformation while subject to persistent mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods and generally increase as they near their melting point.

In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to slide over each other at low stress levels and is known as glide or slip. The crystalline order is restored on either side of a glide dislocation but the atoms on one side have moved by one position. The crystalline order is not fully restored with a partial dislocation. A dislocation defines the boundary between slipped and unslipped regions of material and as a result, must either form a complete loop, intersect other dislocations or defects, or extend to the edges of the crystal. A dislocation can be characterised by the distance and direction of movement it causes to atoms which is defined by the Burgers vector. Plastic deformation of a material occurs by the creation and movement of many dislocations. The number and arrangement of dislocations influences many of the properties of materials.

<span class="mw-page-title-main">Superhard material</span> Material with Vickers hardness exceeding 40 gigapascals

A superhard material is a material with a hardness value exceeding 40 gigapascals (GPa) when measured by the Vickers hardness test. They are virtually incompressible solids with high electron density and high bond covalency. As a result of their unique properties, these materials are of great interest in many industrial areas including, but not limited to, abrasives, polishing and cutting tools, disc brakes, and wear-resistant and protective coatings.

In materials science, work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. Work hardening may be desirable, undesirable, or inconsequential, depending on the context.

Precipitation hardening, also called age hardening or particle hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some steels, stainless steels, and duplex stainless steel. In superalloys, it is known to cause yield strength anomaly providing excellent high-temperature strength.

<span class="mw-page-title-main">Grain boundary</span> Interface between crystallites in a polycrystalline material

In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material. Most grain boundaries are preferred sites for the onset of corrosion and for the precipitation of new phases from the solid. They are also important to many of the mechanisms of creep. On the other hand, grain boundaries disrupt the motion of dislocations through a material, so reducing crystallite size is a common way to improve mechanical strength, as described by the Hall–Petch relationship.

In metallurgy, selective leaching, also called dealloying, demetalification, parting and selective corrosion, is a corrosion type in some solid solution alloys, when in suitable conditions a component of the alloys is preferentially leached from the initially homogenous material. The less noble metal is removed from the alloy by a microscopic-scale galvanic corrosion mechanism. The most susceptible alloys are the ones containing metals with high distance between each other in the galvanic series, e.g. copper and zinc in brass. The elements most typically undergoing selective removal are zinc, aluminium, iron, cobalt, chromium, and others.

In materials science and engineering, the yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible and is known as plastic deformation.

<span class="mw-page-title-main">Recrystallization (metallurgy)</span> Process of forming new, defect-free crystal grains within a material

In materials science, recrystallization is a process by which deformed grains are replaced by a new set of defect-free grains that nucleate and grow until the original grains have been entirely consumed. Recrystallization is usually accompanied by a reduction in the strength and hardness of a material and a simultaneous increase in the ductility. Thus, the process may be introduced as a deliberate step in metals processing or may be an undesirable byproduct of another processing step. The most important industrial uses are softening of metals previously hardened or rendered brittle by cold work, and control of the grain structure in the final product. Recrystallization temperature is typically 0.3–0.4 times the melting point for pure metals and 0.5 times for alloys.

Nickel aluminide refers to either of two widely used intermetallic compounds, Ni₃Al or NiAl, but the term is sometimes used to refer to any nickel–aluminium alloy. These alloys are widely used because of their high strength even at high temperature, low density, corrosion resistance, and ease of production. Ni₃Al is of specific interest as a precipitate in nickel-based superalloys, where it is called the γ' (gamma prime) phase. It gives these alloys high strength and creep resistance up to 0.7–0.8 of its melting temperature. Meanwhile, NiAl displays excellent properties such as lower density and higher melting temperature than those of Ni₃Al, and good thermal conductivity and oxidation resistance. These properties make it attractive for special high-temperature applications like coatings on blades in gas turbines and jet engines. However, both these alloys have the disadvantage of being quite brittle at room temperature, with Ni₃Al remaining brittle at high temperatures as well. To address this problem, has been shown that Ni₃Al can be made ductile when manufactured in single-crystal form rather than in polycrystalline form.

Methods have been devised to modify the yield strength, ductility, and toughness of both crystalline and amorphous materials. These strengthening mechanisms give engineers the ability to tailor the mechanical properties of materials to suit a variety of different applications. For example, the favorable properties of steel result from interstitial incorporation of carbon into the iron lattice. Brass, a binary alloy of copper and zinc, has superior mechanical properties compared to its constituent metals due to solution strengthening. Work hardening has also been used for centuries by blacksmiths to introduce dislocations into materials, increasing their yield strengths.

<span class="mw-page-title-main">Grain boundary strengthening</span> Method of strengthening materials by changing grain size

In materials science, grain-boundary strengthening is a method of strengthening materials by changing their average crystallite (grain) size. It is based on the observation that grain boundaries are insurmountable borders for dislocations and that the number of dislocations within a grain has an effect on how stress builds up in the adjacent grain, which will eventually activate dislocation sources and thus enabling deformation in the neighbouring grain as well. By changing grain size, one can influence the number of dislocations piled up at the grain boundary and yield strength. For example, heat treatment after plastic deformation and changing the rate of solidification are ways to alter grain size.

Severe plastic deformation (SPD) is a generic term describing a group of metalworking techniques involving very large strains typically involving a complex stress state or high shear, resulting in a high defect density and equiaxed "ultrafine" grain (UFG) size or nanocrystalline (NC) structure.

<span class="mw-page-title-main">Grain boundary sliding</span> Material deformation mechanism

Grain boundary sliding (GBS) is a material deformation mechanism where grains slide against each other. This occurs in polycrystalline material under external stress at high homologous temperature and low strain rate and is intertwined with creep. Homologous temperature describes the operating temperature relative to the melting temperature of the material. There are mainly two types of grain boundary sliding: Rachinger sliding, and Lifshitz sliding. Grain boundary sliding usually occurs as a combination of both types of sliding. Boundary shape often determines the rate and extent of grain boundary sliding.

A nanostructured film is a film resulting from engineering of nanoscale features, such as dislocations, grain boundaries, defects, or twinning. In contrast to other nanostructures, such as nanoparticles, the film itself may be up to several microns thick, but possesses a large concentration of nanoscale features homogeneously distributed throughout the film. Like other nanomaterials, nanostructured films have sparked much interest as they possess unique properties not found in bulk, non-nanostructured material of the same composition. In particular, nanostructured films have been the subject of recent research due to their superior mechanical properties, including strength, hardness, and corrosion resistance compared to regular films of the same material. Examples of nanostructured films include those produced by grain boundary engineering, such as nano-twinned ultra-fine grain copper, or dual phase nanostructuring, such as crystalline metal and amorphous metallic glass nanocomposites.

Microstructurally stable nanocrystalline alloys are alloys that are designed to resist microstructural coarsening under various thermo-mechanical loading conditions.

References

A. Inoue; K. Hashimoto, eds. (2001). Amorphous and nanocrystalline materials : preparation, properties, and applications. Berlin: Springer. ISBN 3540672710.

1 2 Anandkumar, Mariappan; Bhattacharya, Saswata; Deshpande, Atul Suresh (2019-08-23). "Low temperature synthesis and characterization of single phase multi-component fluorite oxide nanoparticle sols". RSC Advances. 9 (46): 26825–26830. doi: 10.1039/C9RA04636D . ISSN 2046-2069. PMC 9070433 .
↑ Jiang, Jie; Zhu, Liping; Wu, Yazhen; Zeng, Yujia; He, Haiping; Lin, Junming; Ye, Zhizhen (February 2012). "Effects of phosphorus doping in ZnO nanocrystals by metal organic chemical vapor deposition". Materials Letters. 68: 258–260. doi:10.1016/j.matlet.2011.10.072.
↑ Giallonardo, J.D.; Erb, U.; Aust, K.T.; Palumbo, G. (21 December 2011). "The influence of grain size and texture on the Young's modulus of nanocrystalline nickel and nickel–iron alloys". Philosophical Magazine. 91 (36): 4594–4605. doi:10.1080/14786435.2011.615350. S2CID 136571167.
1 2 Chandross, Michael; Argibay, Nicolas (March 2020). "Ultimate strength of metals". Physical Review Letters. 124 (12): 125501–125505. doi: 10.1103/PhysRevLett.124.125501 . PMID 32281861.
1 2 3 4 5 Gleiter, Herbert (1989). "Nanocrystalline materials". Progress in Materials Science. 33 (4): 223–315. doi: 10.1016/0079-6425(89)90001-7 .
↑ Cordero, Zachary; Knight, Braden; Schuh, Christopher (November 2016). "Six decades of the Hall–Petch effect – a survey of grain-size strengthening studies on pure metals". International Materials Reviews. 61 (8): 495–512. doi:10.1080/09506608.2016.1191808. hdl: 1721.1/112642 . S2CID 138754677.
↑ Detor, Andrew; Schuh, Christopher (November 2007). "Microstructural evolution during the heat treatment of nanocrystalline alloys". Journal of Materials Research. 22 (11): 3233–3248. doi:10.1557/JMR.2007.0403.
↑ Wollmershauser, James; Feigelson, Boris; Gorzkowski, Edward; Ellis, Chase; Gosami, Ramasis; Qadri, Syed; Tischler, Joseph; Kub, Fritz; Everett, Richard (May 2014). "An extended hardness limit in bulk nanoceramics". Acta Materialia. 69: 9–16. doi:10.1016/j.actamat.2014.01.030.
↑ Cha, Seung; Hong, Soon; Kim, Byung (June 2003). "Spark plasma sintering behavior of nanocrystalline WC–10Co cemented carbide powders". Materials Science and Engineering: A. 351 (1–2): 31–38. doi:10.1016/S0921-5093(02)00605-6.
↑ Ward, Austin; French, Matthew; Leonard, Donovan; Cordero, Zachary (April 2018). "Grain growth during ultrasonic welding of nanocrystalline alloys". Journal of Materials Processing Technology. 254: 373–382. doi: 10.1016/j.jmatprotec.2017.11.049 .

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[ReferenceA-1] 1 2 Anandkumar, Mariappan; Bhattacharya, Saswata; Deshpande, Atul Suresh (2019-08-23). "Low temperature synthesis and characterization of single phase multi-component fluorite oxide nanoparticle sols". RSC Advances. 9 (46): 26825–26830. doi: 10.1039/C9RA04636D . ISSN 2046-2069. PMC 9070433 .

[2] Jiang, Jie; Zhu, Liping; Wu, Yazhen; Zeng, Yujia; He, Haiping; Lin, Junming; Ye, Zhizhen (February 2012). "Effects of phosphorus doping in ZnO nanocrystals by metal organic chemical vapor deposition". Materials Letters. 68: 258–260. doi:10.1016/j.matlet.2011.10.072.

[3] Giallonardo, J.D.; Erb, U.; Aust, K.T.; Palumbo, G. (21 December 2011). "The influence of grain size and texture on the Young's modulus of nanocrystalline nickel and nickel–iron alloys". Philosophical Magazine. 91 (36): 4594–4605. doi:10.1080/14786435.2011.615350. S2CID 136571167.

[chandross-4] 1 2 Chandross, Michael; Argibay, Nicolas (March 2020). "Ultimate strength of metals". Physical Review Letters. 124 (12): 125501–125505. doi: 10.1103/PhysRevLett.124.125501 . PMID 32281861.

[gleiter-5] 1 2 3 4 5 Gleiter, Herbert (1989). "Nanocrystalline materials". Progress in Materials Science. 33 (4): 223–315. doi: 10.1016/0079-6425(89)90001-7 .

[6] Cordero, Zachary; Knight, Braden; Schuh, Christopher (November 2016). "Six decades of the Hall–Petch effect – a survey of grain-size strengthening studies on pure metals". International Materials Reviews. 61 (8): 495–512. doi:10.1080/09506608.2016.1191808. hdl: 1721.1/112642 . S2CID 138754677.

[7] Detor, Andrew; Schuh, Christopher (November 2007). "Microstructural evolution during the heat treatment of nanocrystalline alloys". Journal of Materials Research. 22 (11): 3233–3248. doi:10.1557/JMR.2007.0403.

[wollmershauser-8] Wollmershauser, James; Feigelson, Boris; Gorzkowski, Edward; Ellis, Chase; Gosami, Ramasis; Qadri, Syed; Tischler, Joseph; Kub, Fritz; Everett, Richard (May 2014). "An extended hardness limit in bulk nanoceramics". Acta Materialia. 69: 9–16. doi:10.1016/j.actamat.2014.01.030.

[9] Cha, Seung; Hong, Soon; Kim, Byung (June 2003). "Spark plasma sintering behavior of nanocrystalline WC–10Co cemented carbide powders". Materials Science and Engineering: A. 351 (1–2): 31–38. doi:10.1016/S0921-5093(02)00605-6.

[10] Ward, Austin; French, Matthew; Leonard, Donovan; Cordero, Zachary (April 2018). "Grain growth during ultrasonic welding of nanocrystalline alloys". Journal of Materials Processing Technology. 254: 373–382. doi: 10.1016/j.jmatprotec.2017.11.049 .

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