Microstructurally stable nanocrystalline alloys

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Microstructurally stable nanocrystalline alloys are alloys that are designed to resist microstructural coarsening under various thermo-mechanical loading conditions. [1] [2]

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Many applications of metal materials require that they can maintain their structure and strength despite very high temperatures. Efforts to prevent deformations from long term stress, referred to as creep, consist of manipulating alloys to reduce coarsening and migration of individual grains within the metal. [3]

The small size of individual metal grains provides high interfacial surface energy which is what prompts coarsening, the increase in grain size, and eventually metallic softening. [4] Nanocrystalline creep is considered to follow the Coble creep mechanism, the diffusion of atoms along grain boundaries at low stress levels and high temperatures. One method used to reduce coarsening, is by employing an alloy in which one component has good solubility with another. Since grain size decreases with high solute concentration, the rate of coarsening is slowed until inconsequential. [4]

Copper and 10% atomic tantalum nanocrystalline alloy

In 2016, researchers at the Arizona State University and the United States Army Research Laboratory reported a microstructurally stable nanocrystalline alloy made of copper and 10% atomic tantalum (Cu–10 at% Ta). [3] [2] This microstructurally stable nanocrystalline alloy demonstrated high creep resistance under an applied stress and temperature ranges 0.85 to 1.2% of the shear modulus and .5-.64Tm respectively, the steady creep rates were consistently less than 10−6 s−1. [2]

This stability was credited to the mechanistic creep process and the alloy’s core–shell-type structures. The scientists determined that the copper alloy creep occurred in dislocation climb areas under levels of relatively larger stress, claiming that any diffusion creep occurring was negligible. The core–shell-type nanostructures prevented coarsening by securing grain boundaries, a mechanism known as Zener pinning. In these structures more interfacial bonding interactions were possible, increasing strength. Oxide-dispersion strengthened (ODS) ferritic alloys16 and molybdenum alloys17’s great strength and ductility were also credited to these nanostructures. [2]

Nickel and 13% tungsten nanocrystalline alloy

In 2007, a nickel (Ni) and tungsten (W) nanocrystalline alloy was reported to have resistance to coarsening. Experimental data reported that the alloy coarsened to 28 nm from its original grain size of 20 nm after 30 minutes of exposure to heat of 600 degrees Celsius. This growth was then compared to the coarsening rate of an individual grain of Ni placed in heat of 300 degrees Celsius for 30 minutes. [4]

Tungsten and 20% titanium nanocrystalline alloy

In 2012, a tungsten (W) and 20% titanium (Ti) nanocrystalline alloy after a week of exposure to heat of 1100 degrees Celsius in an argon atmosphere was claimed by the researchers to have displayed no change in grain size from the initial 20 nm. Meanwhile, the unalloyed W under the same conditions exhibited a final size on the micrometer scale. [5] Another reviewer describes the coarsening of the W-Ti alloy to be a 2 nm size increase from the original 22 nm. [4] The authors attribute the microstructural stability to a complex chemical arrangement. [5] [4] The nanocrystalline metallic grains were made via a high-energy ball mill method.

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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.

<span class="mw-page-title-main">Creep (deformation)</span> Tendency of a solid material to move slowly or deform permanently under mechanical stress

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 increases as they near their melting point.

<span class="mw-page-title-main">Solid oxide fuel cell</span> Fuel cell that produces electricity by oxidization

A solid oxide fuel cell is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic electrolyte.

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">Superalloy</span> Alloy with higher durability than normal metals

A superalloy, or high-performance alloy, is an alloy with the ability to operate at a high fraction of its melting point. Key characteristics of a superalloy include mechanical strength, thermal creep deformation resistance, surface stability, and corrosion and oxidation resistance.

<span class="mw-page-title-main">Microstructure</span> Very small scale structure of material

Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material can strongly influence physical properties such as strength, toughness, ductility, hardness, corrosion resistance, high/low temperature behaviour or wear resistance. These properties in turn govern the application of these materials in industrial practice.

<span class="mw-page-title-main">Ceramic engineering</span> Science and technology of creating objects from inorganic, non-metallic materials

Ceramic engineering is the science and technology of creating objects from inorganic, non-metallic materials. This is done either by the action of heat, or at lower temperatures using precipitation reactions from high-purity chemical solutions. The term includes the purification of raw materials, the study and production of the chemical compounds concerned, their formation into components and the study of their structure, composition and properties.

<span class="mw-page-title-main">Magnesium alloy</span>

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<span class="mw-page-title-main">Nanocrystalline material</span>

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 100–500 nm are typically considered "ultrafine" grains.

Nickel aluminide typically refers to the one of the two most widely used compounds, Ni3Al or NiAl, however is generally any aluminide from the Ni-Al system. These alloys are widely used due to their corrosion resistance, low-density and easy production. Ni3Al is of specific interest as the strengthening γ' phase precipitate in nickel-based superalloys allowing for high temperature strength up to 0.7-0.8 of its melting temperature. Meanwhile, NiAl displays excellent properties such as low-density (lower than that of Ni3Al), good thermal conductivity, oxidation resistance and high melting temperature. These properties, make it ideal for special high temperature applications like coatings on blades in gas turbines and jet engines. However, both these alloys do have the disadvantage of being quite brittle at room temperature while Ni3Al remains brittle at high temperatures as well. Although, it has been shown that Ni3Al can be made ductile when manufactured as a single crystal as opposed to polycrystalline. Another application was demonstrated in 2005, when the most abrasion-resistant material was reportedly created by embedding diamonds in a matrix of nickel aluminide.

A cryogenic treatment is the process of treating workpieces to cryogenic temperatures in order to remove residual stresses and improve wear resistance in steels and other metal alloys, such as aluminum. In addition to seeking enhanced stress relief and stabilization, or wear resistance, cryogenic treatment is also sought for its ability to improve corrosion resistance by precipitating micro-fine eta carbides, which can be measured before and after in a part using a quantimet.

A deformation mechanism, in geology, is a process occurring at a microscopic scale that is responsible for changes in a material's internal structure, shape and volume. The process involves planar discontinuity and/or displacement of atoms from their original position within a crystal lattice structure. These small changes are preserved in various microstructures of materials such as rocks, metals and plastics, and can be studied in depth using optical or digital microscopy.

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.

Oxide dispersion strengthened alloys (ODS) are alloys that consist of a metal matrix with small oxide particles dispersed within it. They have high heat resistance, strength, and ductility. Alloys of nickel are the most common but includes iron aluminum alloys.

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">Friction stir processing</span>

Friction stir processing (FSP) is a method of changing the properties of a metal through intense, localized plastic deformation. This deformation is produced by forcibly inserting a non-consumable tool into the workpiece, and revolving the tool in a stirring motion as it is pushed laterally through the workpiece. The precursor of this technique, friction stir welding, is used to join multiple pieces of metal without creating the heat affected zone typical of fusion welding.

Iron aluminides are intermetallic compounds of iron and aluminium - they typically contain ~18% Al or more.

Liquid phase sintering is a sintering technique that uses a liquid phase to accelerate the interparticle bonding of the solid phase. In addition to rapid initial particle rearrangement due to capillary forces, mass transport through liquid is generally orders of magnitude faster than through solid, enhancing the diffusional mechanisms that drive densification. The liquid phase can be obtained either through mixing different powders—melting one component or forming a eutectic—or by sintering at a temperature between the liquidus and solidus. Additionally, since the softer phase is generally the first to melt, the resulting microstructure typically consists of hard particles in a ductile matrix, increasing the toughness of an otherwise brittle component. However, liquid phase sintering is inherently less predictable than solid phase sintering due to the complexity added by the presence of additional phases and rapid solidification rates. Activated sintering is the solid-state analog to the process of liquid phase sintering.

Aluminium–scandium alloys (AlSc) are aluminum alloys that consist largely of aluminium (Al) and traces of scandium (Sc) as the main alloying elements. In principle, aluminium alloys strengthened with additions of scandium are very similar to traditional nickel-base superalloys in that both are strengthened by coherent, coarsening resistant precipitates with an ordered L12 structure. But Al–Sc alloys contain a much lower volume fraction of precipitates, and the inter-precipitate distance is much smaller than in their nickel-base counterparts. In both cases however, the coarsening resistant precipitates allow the alloys to retain their strength at high temperatures.

References

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  4. 1 2 3 4 5 Weertman, Julia R. (2012-08-24). "Retaining the Nano in Nanocrystalline Alloys". Science. 337 (6097): 921–922. Bibcode:2012Sci...337..921W. doi:10.1126/science.1226724. ISSN   0036-8075. PMID   22923568. S2CID   36593516.
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