Bulk Amorphous Alloys
An amorphous metal is a metallic material with a disordered atomic-scale structure.
In contrast to most metals, which are crystalline and therefore have a highly ordered arrangement of atoms, amorphous alloys are non-crystalline.
Materials in which such a disordered structure is produced directly from the liquid state during cooling are called "glasses", and so amorphous metals are commonly referred to as "metallic glasses" or "glassy metals".
In a crystal, the atomic-scale structure is highly regular, something like this:
Because a glass solidifies without crystallizing, it retains a much more disordered structure reminiscent of the liquid state:
if Bragg's Law is satisfied, then x-rays will be strongly diffracted and a large intensity will be recorded. Because of the highly ordered nature of a crystal, this will happen only at a few angles. So the diffraction pattern from a crystal consists of a few, sharp diffraction peaks
The disordered atomic-scale structure of a glass, on the other hand, leads to a diffraction pattern that has only a few, broad scattering features
The difference in structure can also be seen in the electron microscope. Shown below are two high resolution transmission electron microscope (HRTEM) images. In the image on the left, you can clearly see the highly ordered contrast associated with the periodicity of a crystal. In the image on the right, which is from a glass, there are no periodic features.
There are several other ways in which amorphous metals can be produced, including
physical vapor deposition,
solid-state reaction,
ion irradiation,
melt spinning,
and mechanical alloying.
Amorphous metals produced by these techniques are, strictly speaking, not glasses; however, materials scientists commonly consider amorphous alloys to be a single class of materials, regardless of how they are prepared.
In most alloys it is relatively easy for crystals to nucleate and grow. As a result, the earliest metallic glasses required very rapid cooling – around one million degrees Celsius per second – to avoid crystallization.
One way to do it is by single-roller melt spinning, as shown here:
In this process, the alloy is melted (typically in a quartz tube) by induction heating, and then forced out through a narrow nozzle onto the edge of a rapidly rotating chill wheel (typically made of copper). The melt spreads to form a thin ribbon, which cools rapidly because it is in contact with the copper wheel.
Melt Spinning
Play Video
Zr65Fe5Ni8Cu12Al10
Melt-spun ribbon
693K, 1.2ks
Transmission Electron Microscopy
Transmission Electron Microscopy
(a) melt-spun ribbon; (b) 705K, 1.2ks
Zr60Cu30-xPdxAl10 x = 0
Zr60Cu30-xPdxAl10 x = 10;
Bulk metallic glasses (BMG) are amorphous metals with critical cooling rates low enough to allow formation of amorphous structure in thick layers (over 1 millimeter).
Bulk Amorphous Metal Samples in the Lab
Background:
The first metallic glass was an alloy (Au80Si20) produced at Caltech by Pol Duwez in 1957. This and other early glass-forming alloys had to be cooled extremely rapidly (on the order of one megakelvin per second, 106 K·s-1) to avoid crystallization. An important consequence of this was that metallic glasses could only be produced in a limited number of forms, typically
Ribbons, Foils, or Wires
in which one dimension was small so that heat could be extracted quickly enough to achieve the necessary cooling rate. As a result, metallic glass specimens (with a few exceptions) were limited to thicknesses of less than one hundred micrometres.
In the 1990s, however, 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.
Soft magnetic bulk glassy alloys prepared by B2O3 flux melting and water quenching technique
In 2004, two groups succeeded in producing bulk amorphous steel, one at Oak Ridge National Laboratory, the other at University of Virginia. The Oak Ridge group refers to their product as "glassy steel".
The product is non-magnetic at room temperature and significantly stronger than conventional steel, though a long research and development process remains before the introduction of the material into public or military use.
The critical casting thickness versus the year in which alloys were discovered. Over 40 years, the critical casting thickness has increased by more than three orders of magnitude. (© 2003 Elsevier Ltd.)
Characteristics:
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 and corrosion. Amorphous metals, while technically glasses, are also much tougher and less brittle than oxide glasses and ceramics.
Thermal conductivity of amorphous materials is lower than of crystals. 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.
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 heat of mixing, inhibiting crystal nucleation and prolongs the time the molten metal stays in supercooled state
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 ("elastic") deformations than crystalline alloys.
Amorphous metals derive their strength directly from their non-crystalline structure, which does not have any of the defects (such as dislocations) that limit the strength of crystalline alloys.
Other Applications:
Before we get to other possible uses of bulk amorphous alloys, it’s a good idea to list good and bad properties of BMGs since their applications are directley related to these properties.
High strength and hardness: Good load bearing ability and wear resistance
High fracture toughness (for some alloys): Good resistance to fracture
High resiliance (ability to absorb elastic strain energy): Good for springs and "snap fit" components
Low mechanical damping: Also good for springs
Ability to be formed as a supercooled liquid: Makes it easy to produce complex shapes
Lack of microstructural features: Allows very small features to be produced
Advantages:
Disadvantages:
Zero ductility in tension: Metallic glasses, particularly large components, tend to fracture brittlely
Poor resistance to propagation of fatigue cracks: Susceptible to failure due to repeated loading
High cost relative to many common alloys
Cheaper, easier manufacturing:
There are good prospects for BMG materials whose properties
favor easier, cheaper processing for more common-place
applications. For example, a five-year project funded by the
Japanese government between 1997-2002 (Inoue
Supercooled Liquid Glass Project) reported the first bulk
glassy alloy with tensile strength over 2 GPa (higher than
that for Mg-, Pd-, and Zr-based alloys) with distinct plastic
elongation in a less expensive Cu-based alloy system. This
high-strength alloy can be formed by Cu mold casting.
Fe-based bulk glassy alloys produced by Cu-mold casting