Introduction, application and development of metallic glass

 What is metallic glass?

The general glass composition is usually silicate or silicon oxide, which is characterized by the arrangement of atoms in transparent, brittle and amorphous phases. The general metal composition is single crystal or polycrystalline, which is characterized by opacity and toughness. Figure 1 illustrates the arrangement of single crystal, polycrystalline and amorphous.

Single crystal	Polycrystalline	Amorphous

Figure 1. Illustration of single crystal, polycrystalline and amorphous

A single crystal is a crystal in which atoms are regularly arranged in a space lattice; a polycrystal is a crystal in which a variety of single crystals are arranged in a space lattice; an amorphous does not have any long-range order (Note 1) arrangement structure.The essence of metallic glass is metal, but its crystal phase is different from the single crystal and polycrystalline in the normal state, and on the contrary it is the same as the amorphous glass, so it is called metallic glass.

How to make metal glass？

The traditional preparation method is to heat the metal so that the temperature exceeds the crystalline phase, generally more than 1000 degrees Celsius, and then inject it into the mold, and quickly cool, so that the atoms can not be arranged into a single crystal or polycrystal in an orderly manner during solidification. Metallic glass that produces an amorphous phase. However, this method is easy to damage the mold, and the metal at 1000 degrees Celsius is easy to splash, resulting in product defects and reducing yield. Therefore, how to improve the heating and cooling methods has become a problem to reduce the cost of manufacturing metallic glass.

Characteristics of metallic glass

        The bonding between the metal glass atoms is still a metal bond, so it retains the characteristics of the metal, such as: opacity, high toughness, etc., and because the atoms are arranged in an amorphous structure, the internal defects caused by the original crystallization are improved, such as between the grains. Problems such as grain boundaries or poor alignment within the grain make it have higher yield strength and hardness than metal, and have the advantages of corrosion resistance, wear resistance, and good fatigue.

       Metal glass also has excellent plasticity. In the process of heating the metallic glass into a liquid state, it will first reach the glass transition temperature (Tg) and crystallization temperature (Tx). The temperature is between Tg and Tx, also known as the supercooled liquid phase region, and the relationship between phase transition and temperature is shown in Figure 2. At this time, it will be in a state similar to liquid metal, and the plasticity is extremely high. It can not be achieved by blow molding to form an ordinary metal. Although the material cost is higher, the processing cost is equivalent to that of plastic, and its strength is more general. Steel is twice as high.

Figure 2. Schematic diagram of supercooled liquid phase

Application of metal glass

Bulk metallic glass has excellent mechanical properties, strength and hardness are several times higher than steel and titanium; resistance to permanent deformation is also better than ordinary metals. It is often used in golf clubs, and its performance in transmitting impact force to golf balls is better than that of titanium alloys. The metallic glass ball head can transmit about 90%, and the titanium alloy is only about 70%. In addition, magnetic metal glass can also be used in the field of electronic technology, which has the advantages of high magnetic permeability and low iron loss. It has been widely used in sensors and transformer cores.

Metal glass application restrictions

Although the characteristics of the amorphous phase bring many advantages to the metal glass and increase many application fields, because of its glass characteristics, its ductility performance is poor, it is easily deformed under high stress, and it will cause shear bands in the crystal and break quickly. This is a major bottleneck in the application of metal glass blocks. On the other hand, due to the technical limitations of heating and cooling, the volume of metallic glass is currently unable to manufacture large metallic glass blocks, which also limits the possibility of other applications.

Metal glass development trend

The unique advantages of metallic glass have gained the attention of domestic and foreign industries and academia. The development of its processing has emerged as a method of high-speed heating and cooling, which can reach the process temperature in milliseconds and then cool to an amorphous phase in milliseconds of metal. In addition, the application of thin film has also been developed, which is prepared by sputtering, pulsed laser evaporation, etc. This method can retain the advantages of block materials, and can reduce the fracture caused by stress, and has low friction surface characteristics , But there is still much room for development of this technology. There have been examples of applications in the biomedical, semiconductor, and micro-electromechanical industries.

references:

[1] Chun-Hway Hsueh, "Development and Application of Metallic Glass", NTU Alumni Bimonthly No.98(2015)
[2] He Zhenyang, Ye Mingcang, "Metallic Glass", Science Online (2009)
[3] China Steel Research Institute , "Application of Metal Glass in Life", Read01(2019)
[4] Sally's compilation report , "Sturdy, Durable, and Low-Cost, New Technology for Metallic Glass", Epoch Times(2011)