Amorphous metal

An amorphous metal is a solid metallic material, usually an alloy, with disordered atomic-scale structure. Most metals are crystalline in their solid state, which means they have a highly ordered arrangement of atoms. Amorphous metals are non-crystalline, and have a glass-like structure. But unlike common glasses, such as window glass, which are typically electrical insulators, amorphous metals have good electrical conductivity and can show metallic luster.
Amorphous metals can be produced in several ways, including extremely rapid cooling, physical vapor deposition, solid-state reaction, ion irradiation, and mechanical alloying. Small batches of amorphous metals have been produced through a variety of quick-cooling methods, such as amorphous metal ribbons produced by sputtering molten metal onto a spinning metal disk. The rapid cooling comes too fast for crystals to form and the material is "locked" in a glassy state. Alloys with cooling rates low enough to allow formation of amorphous structure in thick layers have been produced and are known as bulk metallic glasses. Batches of amorphous steel with three times the strength of conventional steel alloys have been produced. New techniques such as 3D printing, also characterised by high cooling rates, are an active research topic.

History

The first reported metallic glass was Au₇₅Si_25, produced at Caltech by Klement, Willens, and Duwez in 1960. This and other early glass-forming alloys had to be rapidly cooled to avoid crystallization. An important consequence of this was that metallic glasses could be produced in a few forms in which one dimension was small so that heat could be extracted quickly enough to achieve the required cooling rate. As a result, metallic glass specimens were limited to thicknesses of less than one hundred microns.
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, Liebermann and Graham developed a method of manufacturing thin ribbons of amorphous metal on a supercooled fast-spinning wheel. This was an alloy of iron, nickel, and boron. The material, known as Metglas, was commercialized in the early 1980s and became used for low-loss power distribution transformers. Metglas-2605 is composed of 80% iron and 20% boron, has a Curie temperature of and a room temperature saturation magnetization of 1.56 teslas.
In the early 1980s, glassy ingots with a diameter of were produced with an 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 increased to one centimeter.
In 1982, a study on amorphous metal structural relaxation indicated a relationship between the specific heat and temperature of ₈₃P₁₇. As the material was heated, the two properties displayed a negative relationship starting at 375 K, due to the change in relaxed amorphous states. When the material was annealed for periods from 1 to 48 hours, the properties instead displayed a positive relationship starting at 475 K for all annealing periods, since the annealing induced structure disappears at that temperature. 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 with ribbon thickness from 20 μm to 50 μm. The plasticity was a stark difference to past amorphous metals that became brittle at those thicknesses.
In 1988, alloys of lanthanum, aluminium, and copper ore were revealed to be glass-forming. Al-based metallic glasses containing scandium exhibited a record-type tensile mechanical strength of about.
Bulk amorphous alloys of several millimeters in thickness were rare, although Pd-based amorphous alloys had been formed into rods with a diameter by quenching, and spheres with a diameter were formed by repetition flux melting with B₂O₃ and quenching.
New techniques were found in 1990, producing alloys 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 alloys can be cast into parts several centimeters thick while retaining an amorphous structure. The best glass-forming alloys were based on zirconium and palladium, but alloys based on iron, titanium, copper, magnesium, and other metals are known. The process exploited a phenomenon called "confusion". Such alloys contain many elements such that upon cooling sufficiently quickly, constituent atoms cannot achieve an equilibrium crystalline state before their mobility is lost. In this way, the random disordered state of the atoms is "locked in".
In 1992, the commercial amorphous alloy, Vitreloy 1, was developed at Caltech, as a part of Department of Energy and NASA research of new aerospace materials.
By 2000, research in Tohoku University and Caltech yielded multicomponent alloys based on lanthanum, magnesium, zirconium, palladium, iron, copper, and titanium, with critical cooling rate between 1 K/s and 100 K/s, comparable to oxide glasses.
In 2004, bulk amorphous steel was successfully produced by a groups at Oak Ridge National Laboratory, which refers to their product as "glassy steel", and another at University of Virginia, named "DARVA-Glass 101". The product is non-magnetic at room temperature and significantly stronger than conventional steel.
In 2018, a team at SLAC National Accelerator Laboratory, the National Institute of Standards and Technology 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.

Properties

Amorphous metal is usually an alloy rather than a pure metal. The alloys contain atoms of significantly different sizes, leading to low free volume in molten state. The viscosity prevents the atoms from moving enough to form an ordered lattice. The material displays low shrinkage during cooling, and resistance to plastic deformation. The absence of grain boundaries, the weak spots of crystalline materials, leads to better wear resistance and less corrosion. Amorphous metals, while technically glasses, are much tougher and less brittle than oxide glasses and ceramics. Amorphous metals are either non-ferromagnetic, if they are composed of Ln, Mg, Zr, Ti, Pd, Ca, Cu, Pt and Au, or ferromagnetic, if they are composed of Fe, Co, and Ni.
Thermal conductivity is lower than in crystalline metals. As formation of amorphous structure relies on fast cooling, this limits the thickness of amorphous structures. To form amorphous structure despite slower cooling, the alloy has to be made of three or more components, leading to complex crystal units with higher potential energy and lower odds of formation. The atomic radius of the components has to be significantly different, to achieve high packing density and low free volume. The combination of components should have negative mixing heat, inhibiting crystal nucleation and prolonging the time the molten metal stays in supercooled state.
As temperatures change, the electrical resistivity of amorphous metals behaves very different than that of regular metals. While resistivity in crystalline metals generally increases with temperature, following Matthiessen's rule, resistivity in many amorphous metals decreases with increasing temperature. This effect can be observed in amorphous metals of high resistivities between 150 and 300 microohm-centimeters. In these metals, the scattering events causing the resistivity of the metal are not statistically independent, thus explaining the breakdown of Matthiessen's rule. The fact that the thermal change of the resistivity in amorphous metals can be negative over a large range of temperatures and correlated to their absolute resistivity values was identified by Mooij in 1973, becoming Mooijs-rule.
Alloys of boron, silicon, phosphorus, and other glass formers with magnetic metals have high magnetic susceptibility, with low coercivity and high electrical resistance. Usually the electrical conductivity of a metallic glass is of the same low order of magnitude as of a molten metal just above the melting point. The high resistance leads to low losses by eddy currents when subjected to alternating magnetic fields, a property useful for e.g. transformer magnetic cores. Their low coercivity also contributes to low loss.
Buckel and Hilsch discovered the superconductivity of amorphous metal thin films experimentally in the early 1950s. For certain metallic elements the superconducting critical temperature T_c can be higher in the amorphous state than in the crystalline state, and in several cases T_c increases upon increasing the structural disorder. This behavior can be explained by the effect of structural disorder on electron-phonon coupling.
Amorphous metals have higher tensile yield strengths and higher elastic strain limits than polycrystalline metal alloys, but their ductilities and fatigue strengths are lower.
Amorphous alloys have a variety of potentially useful properties. In particular, they tend to be stronger than crystalline alloys of similar chemical composition, and they can sustain larger reversible deformations than crystalline alloys. Amorphous metals derive their strength directly from their non-crystalline structure, which does not have defects that limit their strength. Vitreloy is an amorphous metal with a tensile strength almost double that of high-grade titanium. However, metallic glasses at room temperature are not ductile and tend to fail suddenly and surprisingly when loaded in tension, which limits applicability in reliability-critical applications. Metal matrix composites consisting of a ductile crystalline metal matrix containing dendritic particles or fibers of an amorphous glass metal are an alternative.
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 injection molding, in much the same way as polymers. As a result, amorphous alloys have been commercialized for use in sports equipment, medical devices, and as cases for electronic equipment.
Thin films of amorphous metals can be deposited as protective coatings via high velocity oxygen fuel.