Amorphous metal
Amorphous metals can be produced in several ways, including extremely rapid cooling, physical vapor deposition, solid-state reaction, ion irradiation, and mechanical alloying.
The rapid cooling (millions of degrees Celsius per second) comes too fast for crystals to form and the material is "locked" in a glassy state.
[3] Alloys with cooling rates low enough to allow formation of amorphous structure in thick layers (over 1 millimetre or 0.039 inches) have been produced; bulk metallic glasses.
An important consequence of this was that metallic glasses could be produced in a few forms (typically ribbons, foils, or wires) in which one dimension was small so that heat could be extracted quickly enough to achieve the required cooling rate.
In 1976, Liebermann and Graham developed a method of manufacturing thin ribbons of amorphous metal on a supercooled fast-spinning wheel.
[7] In the early 1980s, glassy ingots with a diameter of 5 mm (0.20 in) were produced with an alloy of 55% palladium, 22.5% lead, and 22.5% antimony, by surface etching followed with heating-cooling cycles.
[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, the two properties displayed a negative relationship starting at 375 K, due to the change in relaxed amorphous states.
Such alloys contain many elements (often four or more) such that upon cooling sufficiently quickly, constituent atoms cannot achieve an equilibrium crystalline state before their mobility is lost.
In 1992, the commercial amorphous alloy, Vitreloy 1 (41.2% Zr, 13.8% Ti, 12.5% Cu, 10% Ni, and 22.5% Be), was developed at Caltech, as a part of Department of Energy and NASA research of new aerospace materials.
[15] By 2000, research in Tohoku University[16] 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.
The absence of grain boundaries, the weak spots of crystalline materials, leads to better wear resistance[23] and lesscorrosion.
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.
[25] The atomic radius of the components has to be significantly different (over 12%), 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.
[33] Thin films of amorphous metals can be deposited as protective coatings via high velocity oxygen fuel.
[38] Such low softening temperature supports simple methods for making nanoparticlecomposites (e.g. carbon nanotubes) and bulk metallic glasses.
[41] Laser powder bed fusion (LPBF) has been used to process Zr-based bulk metallic glass (BMG)[42] for biomedical applications.
[citation needed] Mg60Zn35Ca5 is under investigation 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.
[46][47][48] 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.
Selective laser melting (SLM) is one example of an additive manufacturing method that has been used to make iron based metallic glasses.
[51] Bulk metallic glasses have been modeled using atomic scale simulations (within the density functional theory framework) in a similar manner to high entropy alloys.
As such, new bulk metallic glass systems can be tested and tailored for a specific purpose (e.g. bone replacement or aero-engine component) without as much empirical searching of the phase space or experimental trial and error.
Ab-initio molecular dynamics (MD) simulation confirmed that the atomic surface structure of a Ni-Nb metallic glass observed by scanning tunneling microscopy is a kind of spectroscopy.
At negative applied bias it visualizes only one soft of atoms (Ni) owing to the structure of electronic density of states calculated using ab-initio MD simulation.
