What is Amorphous Metal?

Metal glass is also called amorphous alloy. It has the advantages of metal and glass, and overcomes their respective disadvantages. If the glass is brittle, it is not ductile. The strength of metallic glass is higher than that of steel, and its hardness is higher than that of high-hard tool steel, and it has a certain degree of toughness and rigidity. Therefore, metallic glass is praised as the King of Glass that does not break or smash.

The appearance of metallic glass can be traced back to the 1930s. Kramer first reported that metallic glass was prepared by vapor deposition. In 1950, metallurgists learned to mix a certain amount of metal, such as nickel and zirconium. Crystals were produced. In 1960, Klement and Duwez et al of California Institute of Technology used quenching technology to prepare metallic glass. When thin layers of alloys are cooled at a rate of one hundred degrees Celsius per second, they form metallic glass. But because they require rapid cooling, they can only be made into thin strips, wires, or powders.

Recently, scientists have mixed elements of four to five atoms of different sizes to form a variety of metallic glasses, such as stripes. Varying the size of the atoms causes it to mix to form glass and become tougher. One of the uses of these new alloys is commercially used to make golf club heads.

Most metals crystallize when they cool, arranging their atoms into a regular pattern called a crystal lattice. But if crystals do not appear, the atoms will be randomly arranged and become metallic glass.

The atoms of ordinary glass are also randomly arranged, but it is not a metal. Metal glass is not transparent, it has unique mechanical and magnetic properties, and it is not easy to break and deform. It is the ideal material for transformers, golf clubs and other products.

The current production of metallic glass is thinner and thinner, because the metal will crystallize quickly when it cools, so it needs to be frozen very quickly. John Nacho, a researcher at Johns Crane Kinsey University in the United States, is studying how to produce metal glass with super strength, elasticity and magnetic characteristics, but relatively large pieces. This new metal will remain solid without crystallization at high temperatures, which will be suitable for making engine parts and military weapons.

Metal glass made of iron is a very good magnetic substance, and because it becomes soft after heating, it is easy to be cast into finished products of different shapes.

Seen in the picture, He Nacho uses the induction furnace to quickly melt the metal mixture into metallic glass.

With support from the National Science Foundation and the US Army Research Agency, Hufnagel has set up a laboratory to test new alloys. He sought to create an alloy metallic glass that would remain solid and not crystalline at high temperatures, making it a useful material for engine parts. The material can also be used in military applications such as armor-piercing shells. Unlike most crystalline metal cannonballs, which change from a flat shape to a mushroom shape after impact, Hufnagel believes that the sides of the metallic glass warhead will turn and give the best penetration of sharpened projectiles.

It is difficult to make thick, bulky-shaped metallic glass, because most metals suddenly crystallize when cooled. In the manufacture of glass, the metal must harden because the crystal lattice changes when it is formed. From pure metals such as copper Nickel creates glass, which will cool at a rate of one trillion degrees Celsius per second.

In traditional crystalline materials, the atoms are periodically arranged into a crystal lattice, and the crystal lattice is defective, such as dislocations and stacking faults. The energy required for these defect movements is relatively low, making the macro plastic deformation of the crystal easier to achieve. So what is the plastic deformation mechanism of metallic glass without lattice structure?

From a macro perspective, the deformation characteristics of metallic glass are closely related to temperature. When the temperature is close to the glass transition point or even higher, each part of the material participates in deformation under the action of external force, which appears as a viscous flow, which is called uniform deformation. When the temperature is much lower than the glass transition point, metallic glass tends to show non-uniform deformation, and the deformation area is concentrated in a small area with a size of 10-50 nm. This deformation area is called a shear band. Since the glass transition temperature point of general metallic glass is much higher than room temperature, localization of deformation is the main feature of metallic glass deformation at room temperature, and it has received widespread attention. The highly localized deformation occurs only in the shear band. After the shear band is formed, it will expand rapidly without constraints, eventually leading to brittle fracture of the material. This is why metallic glass does not have macroplasticity at room temperature, and solving this problem is a key part of promoting the application of metallic glass. Many researchers have made hard efforts in this direction. In order to increase plasticity, some people use the method of preparing composite materials, and some use the introduction of residual stress or other processing methods. In 2007, Liu Yanhui and others from the Institute of Physics of the Chinese Academy of Sciences reported on Science that they developed metallic glass with a large compressive plasticity at room temperature, which can be bent into a certain shape like pure copper and pure aluminum, thereby further leading a large number of related Research work. However, the problem of room-temperature macroplasticity of metallic glass has not been solved, especially the tensile plasticity that everyone expects has not been obtained, and the academic community is looking forward to new progress.

Microscopically, deformation involves local atomic rearrangement of the material. To study the origin of deformation from this perspective, there are currently two mainstream theoretical models, namely the "free volume" model and the "shear transition zone" model. The free volume model was first proposed by Cohen and Turnbull to explain the problem of glass transition, and was later used by Spaepen to understand the deformation of glass. This model believes that the deformation of metallic glass is achieved by the transition movement of a single atom, and that each atom occupies a certain percentage of free volume at any position, and where there is a lot of free volume, the atomic transition movement is easy to achieve; Where atomic transitions are difficult to achieve. In the absence of external force, the probability of the atom's transition in all directions is equal, but under the condition of external force, the atom tends to transition in a certain direction, which causes deformation in the direction of stress. However, since the free volume itself is a vague concept, and it is difficult to imagine that the transition of a single atom can respond to the stress given by the outside world, the basis of the free volume model is very insecure. However, it provides a very intuitive concept to understand deformation, and it is very simple, so it has a very broad influence on workers in the glass field. The shear transition zone model is a more classic and well-known model, developed from the analogy of soap bubble valve by Argon et al. They believe that the deformation of metallic glass is not caused by the transition of a single atom at the micro level, but is caused by the shear motion of atomic clusters composed of several atoms relative to the matrix. Called the "shear transition zone", local plastic deformation accumulation in the shear transition zone eventually leads to macro-scale deformation. Based on the above model, many deformation phenomena of metallic glass can be explained, such as localization of shear bands at low temperatures, uniform rheology at high temperatures, and so on. However, because the shear transition model treats the local shear transition as a single event, that is to say, this processing method ignores the interaction between the different basic units of deformation, and it also causes some experimental phenomena that cannot be explained, such as stress Sawtooth wave phenomenon on the strain curve, etc. Recent research has carried out a detailed analysis of this sawtooth wave behavior, and found that the shear band dynamics of brittle metallic glass is characterized by chaotic behavior, while ductile metallic glass can evolve to a self-organized critical state. These results show that the shear band motion of amorphous alloys during the deformation process is relatively complicated, and the interaction between multiple shear bands and the coordinated motion need to be considered ^[1] .