Amorphous metal
An amorphous metal (also known as metallic glass, glassy metal, or shiny metal) is a solid
There are several ways in which amorphous metals can be produced, including
History
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The first reported metallic glass was an
In 1969, an alloy of 77.5% palladium, 6% copper, and 16.5% silicon was found to have critical cooling rate between 100 and 1000 K/s.
In 1976, H. Liebermann and C. Graham developed a new method of manufacturing thin ribbons of amorphous metal on a supercooled fast-spinning wheel.[6] This was an alloy of iron, nickel, and boron. The material, known as Metglas, was commercialized in the early 1980s and is used for low-loss power distribution transformers (amorphous metal transformer). Metglas-2605 is composed of 80% iron and 20% boron, has a Curie temperature of 646 K (373 °C; 703 °F) and a room temperature saturation magnetization of 1.56 teslas.[7]
In the early 1980s, glassy ingots with a diameter of 5 mm (0.20 in) were produced from the alloy of 55% palladium, 22.5% lead, and 22.5% antimony, by surface etching followed with heating-cooling cycles. Using boron oxide flux, the achievable thickness was increased to a centimeter.[clarification needed]
In 1982, a study on amorphous metal structural relaxation indicated a relationship between the specific heat and temperature of (Fe0.5Ni0.5)83P17. As the material was heated up, the properties developed a negative relationship starting at 375 K, which was due to the change in relaxed amorphous states. When the material was annealed for periods from 1 to 48 hours , the properties developed a positive relationship starting at 475 K for all annealing periods, since the annealing induced structure disappears at that temperature.[8] In this study, amorphous alloys demonstrated glass transition and a super cooled liquid region. Between 1988 and 1992, more studies found more glass-type alloys with glass transition and a super cooled liquid region. From those studies, bulk glass alloys were made of La, Mg, and Zr, and these alloys demonstrated plasticity even when their ribbon thickness was increased from 20 μm to 50 μm. The plasticity was a stark difference to past amorphous metals that became brittle at those thicknesses.[8][9][10][11]
In 1988, alloys of lanthanum, aluminium, and copper ore were found to be highly glass-forming. Al-based metallic glasses containing Scandium exhibited a record-type tensile mechanical strength of about 1,500 MPa (220 ksi).[12]
Before new techniques were found in 1990, bulk amorphous alloys of several millimeters in thickness were rare, except for a few exceptions, Pd-based amorphous alloys had been formed into rods with a 2 mm (0.079 in) diameter by quenching,[13] and spheres with a 10 mm (0.39 in) diameter were formed by repetition flux melting with B2O3 and quenching.[14]
In the 1990s new alloys were developed that form glasses at cooling rates as low as one kelvin per second. These cooling rates can be achieved by simple casting into metallic molds. These "bulk" amorphous alloys can be cast into parts of up to several centimeters in thickness (the maximum thickness depending on the alloy) while retaining an amorphous structure. The best glass-forming alloys are based on zirconium and palladium, but alloys based on iron, titanium, copper, magnesium, and other metals are also known. Many amorphous alloys are formed by exploiting a phenomenon called the "confusion" effect. Such alloys contain so many different elements (often four or more) that upon cooling at sufficiently fast rates, the constituent atoms simply cannot coordinate themselves into the equilibrium crystalline state before their mobility is stopped. In this way, the random disordered state of the atoms is "locked in".
In 1992, the commercial amorphous alloy,
By 2000, research in
In 2004, bulk amorphous steel was successfully produced by two groups: one at
In 2018 a team at SLAC National Accelerator Laboratory, the National Institute of Standards and Technology (NIST) and Northwestern University reported the use of artificial intelligence to predict and evaluate samples of 20,000 different likely metallic glass alloys in a year. Their methods promise to speed up research and time to market for new amorphous metals alloys.[21][22]
Properties
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Amorphous metal is usually an alloy rather than a pure metal. The alloys contain atoms of significantly different sizes, leading to low free volume (and therefore up to orders of magnitude higher viscosity than other metals and alloys) in molten state. The viscosity prevents the atoms moving enough to form an ordered lattice. The material structure also results in low shrinkage during cooling, and resistance to plastic deformation. The absence of grain boundaries, the weak spots of crystalline materials, leads to better resistance to wear[23] and corrosion. Amorphous metals, while technically glasses, are also much tougher and less brittle than oxide glasses and ceramics. Amorphous metals can be grouped in two categories, as either non-ferromagnetic, if they are composed of Ln, Mg, Zr, Ti, Pd, Ca, Cu, Pt and Au, or ferromagnetic alloys, if they are composed of Fe, Co, and Ni.[24]
Thermal conductivity of amorphous materials is lower than that of crystalline metal. As formation of amorphous structure relies on fast cooling, this limits the maximum achievable thickness of amorphous structures. To achieve formation of amorphous structure even during slower cooling, the alloy has to be made of three or more components, leading to complex crystal units with higher potential energy and lower chance of formation.
As temperatures change, the electrical resistivity of amorphous metals behaves very different than that of regular metals. While the resistivity in regular metals generally increases with temperature, following the
The alloys of
The superconductivity of amorphous metal thin films was discovered experimentally in the early 1950s by Buckel and Hilsch.[29] For certain metallic elements the superconducting critical temperature Tc can be higher in the amorphous state (e.g. upon alloying) than in the crystalline state, and in several cases Tc increases upon increasing the structural disorder. This behavior can be understood and rationalized by considering the effect of structural disorder on the electron-phonon coupling.[30]
Amorphous metals have higher tensile yield strengths and higher elastic strain limits than polycrystalline metal alloys, but their ductilities and fatigue strengths are lower.
Perhaps the most useful property of bulk amorphous alloys is that they are true glasses, which means that they soften and flow upon heating. This allows for easy processing, such as by
Thin films of amorphous metals can be deposited via
Applications
Commercial
Currently the most important application is due to the special magnetic properties of some ferromagnetic metallic glasses. The low magnetization loss is used in high efficiency transformers (amorphous metal transformer) at line frequency and some higher frequency transformers. Amorphous steel is a very brittle material which makes it difficult to punch into motor laminations.[34] Also electronic article surveillance (such as theft control passive ID tags,) often uses metallic glasses because of these magnetic properties.
A commercial amorphous alloy,
Ti-based metallic glass, when made into thin pipes, have a high tensile strength of 2,100 MPa (300 ksi), elastic elongation of 2% and high corrosion resistance.[35] Using these properties, a Ti–Zr–Cu–Ni–Sn metallic glass was used to improve the sensitivity of a Coriolis flow meter. This flow meter is about 28-53 times more sensitive than conventional meters,[36] which can be applied in fossil-fuel, chemical, environmental, semiconductor and medical science industry.
Zr-Al-Ni-Cu based metallic glass can be shaped into 2.2 to 5 by 4 mm (0.087 to 0.197 by 0.157 in) pressure sensors for automobile and other industries, and these sensors are smaller, more sensitive, and possess greater pressure endurance compared to conventional stainless steel made from cold working. Additionally, this alloy was used to make the world's smallest geared motor with diameter 1.5 and 9.9 mm (0.059 and 0.390 in) to be produced and sold at the time.[37]
Potential
Amorphous metals exhibit unique softening behavior above their glass transition and this softening has been increasingly explored for thermoplastic forming of metallic glasses.
Ti40Cu36Pd14Zr10 is believed to be noncarcinogenic, is about three times stronger than titanium, and its
Mg60Zn35Ca5, rapidly cooled to achieve amorphous structure, is being investigated at Lehigh University as a biomaterial for implantation into bones as screws, pins, or plates, to fix fractures. Unlike traditional steel or titanium, this material dissolves in organisms at a rate of roughly 1 millimeter per month and is replaced with bone tissue. This speed can be adjusted by varying the content of zinc.[42]
Bulk metallic glasses also seem to exhibit superior properties like SAM2X5-630 which has the highest recorded elastic limit for any steel alloy, according to the researcher, essentially it has the highest threshold limit at which a material can withstand an impact without deforming permanently(plasticity). The alloy can withstand pressure and stress of up to 12.5 GPa (123,000 atm) without undergoing any permanent deformation, this is the highest impact resistance of any bulk metallic glass ever recorded (as of 2016) .This makes it as an attractive option for Armour material and other applications which requires high stress tolerance.[43][44][45]
Additive manufacturing
One challenge when synthesising a metallic glass is that the techniques often only produce very small samples, due to the need for high cooling rates. 3D-printing methods have been suggested as a method to create larger bulk samples. Selective laser melting (SLM) is one example of an additive manufacturing method that has been used to make iron based metallic glasses.[46][47] Laser foil printing (LFP) is another method where foils of the amorphous metals are stacked and welded together, layer by layer.[48]
Modeling and theory
Bulk metallic glasses have been modeled using atomic scale simulations (within the
One common way to try and understand the electronic properties of amorphous metals is by comparing them to liquid metals, which are similarly disordered, and for which established theoretical frameworks exist. For simple amorphous metals, good estimations can be reached by semi-classical modelling of the movement of individual electrons using the Boltzmann equation and approximating the scattering potential as the superposition of the electronic potential of each nucleus in the surrounding metal. To simplify the calculations, the electronic potentials of the atomic nuclei can be truncated to give a muffin-tin pseudopotential. In this theory, there are two main effects that govern the change of resistivity with increasing temperatures. Both are based on the induction of vibrations of the atomic nuclei of the metal as temperatures increase. One is, that the atomic structure gets increasingly smeared out as the exact positions of the atomic nuclei get less and less well defined. The other is the introduction of phonons. While the smearing out generally decreases the resistivity of the metal, the introduction of phonons generally adds scattering sites and therefore increases resistivity. Together, they can explain the anomalous decrease of resistivity in amorphous metals, as the first part outweighs the second. In contrast to regular crystalline metals, the phonon contribution in an amorphous metal does not get frozen out at low temperatures. Due to the lack of a defined crystal structure, there are always some phonon wavelengths that can be excited.[52][53] While this semi-classical approach holds well for many amorphous metals, it generally breaks down under more extreme conditions. At very low temperatures, the quantum nature of the electrons leads to long range interference effects of the electrons with each other in what is called "weak localization effects".[26] In very strongly disordered metals, impurities in the atomic structure can induce bound electronic states in what is called "Anderson localization", effectively binding the electrons and inhibiting their movement.[54]
See also
- Bioabsorbable metallic glass
- Glass-ceramic-to-metal seals
- Liquidmetal
- Materials science
- Structure of liquids and glasses
- Amorphous brazing foil
References
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Further reading
- Duarte, M. J.; Bruna, P.; Pineda, E.; Crespo, D.; Garbarino, G.; Verbeni, R.; Zhao, K.; ISSN1098-0121.
- Liu, Chaoren; Pineda, Eloi; Crespo, Daniel (2015). "Mechanical Relaxation of Metallic Glasses: An Overview of Experimental Data and Theoretical Models". Metals. 5 (2): 1073–1111. ISSN2075-4701.
External links
- Liquidmetal Design Guide
- "Metallic glass: a drop of the hard stuff" at New Scientist
- Glass-Like Metal Performs Better Under Stress Physical Review Focus, June 9, 2005
- "Overview of metallic glasses"
- New Computational Method Developed By Carnegie Mellon University Physicist Could Speed Design and Testing of Metallic Glass (2004) (the alloy database developed by Marek Mihalkovic, Michael Widom, and others)
- Telford, Mark (March 2004). "The case for bulk metallic glass". Materials Today. 7 (3): 36–43. .
- New tungsten-tantalum-copper amorphous alloy developed at the Korea Advanced Institute of Science and Technology Digital Chosunilbo (English Edition) : Daily News in English About Korea
- Amorphous Metals in Electric-Power Distribution Applications
- Amorphous and Nanocrystalline Soft Magnets
- Kumar, Golden; Neibecker, Pascal; Liu, Yan Hui; Schroers, Jan (26 February 2013). "Critical fictive temperature for plasticity in metallic glasses". Nature Communications. 4 (1): 1536. PMID 23443564.
- "New metallic glass material created by starving it of nuclei". newatlas.com. 8 December 2017. Retrieved 2017-12-09.
- Metallic glasses and those composites, Materials Research Forum LLC, Millersville, PA, USA, (2018), p. 336 "Metallic Glasses and Their Composites". www.mrforum.com.