Introduction to the preparation method of amorphous/nanocrystalline composite materials

Formation of amorphous alloys
Crystals are solids with regular geometric shapes formed through the crystallization process. They are formed by periodic arrangement of atoms or molecules according to a certain rule. Corresponding to a crystal, a solid with irregular arrangement of atoms or molecules, no periodicity and no symmetry is called amorphous. It is a structure with long-range disorder and short-range order. Due to the limitation of the ability to form amorphous, amorphous alloys mostly exist in low-dimensional forms such as powders, filaments and thin ribbons. The appearance of bulk amorphous alloys has opened the prelude to amorphous alloys. The main point of preparation is to inhibit the nucleation and growth of the crystal phase during the cooling process.

Preparation of amorphous alloy
At present, there are two main methods for preparing bulk amorphous alloys: direct solidification and powder consolidation. Direct solidification method mainly includes water quenching method, copper mold casting method, suction casting method, high pressure casting method and magnetic levitation smelting method; powder consolidation method is a method of pressing powder into a bulk amorphous alloy in the supercooled liquid phase region. The following are common methods for preparing bulk amorphous alloys:

01
The water quenching method is a common method for preparing bulk amorphous alloys by using the melt water cooling method, and has the advantages of simple process and easy control.

02
The copper mold casting method is a method in which the melt is cooled in a copper mold of a specific shape to obtain a bulk amorphous alloy. Due to the limited cooling rate of the copper mold, the size of the amorphous alloy is also limited to a certain extent.

03
The basic principle of the high-pressure casting method is to heat and melt the master alloy in the sleeve with a high-frequency induction coil under the protection of argon gas. Within a few milliseconds, the molten liquid is pushed into the water-cooled copper mold by hydraulically pushing the piston. Due to the extremely high compression molding speed, a higher cooling rate can be obtained, so the alloy is cooled to form a bulk amorphous alloy. At the same time, the high pressure makes the liquid alloy come into closer contact with the cavity, reducing casting defects such as looseness, and achieving a better net forming effect. For melts with higher viscosity, which are usually difficult to prepare bulk amorphous alloys, castings with complex shapes can also be prepared by this method.

04
The injection-suction casting method combines the advantages of copper mold casting, water cooling and suction casting, and is suitable for the preparation of a variety of bulk amorphous alloys. The advantage of this method is that high-frequency or medium-frequency induction heating of the master alloy is used, and magnetic stirring is added, which can accelerate the melting speed of the alloy and make the alloy composition more uniform. At the same time, the melting capacity of the alloy can range from several grams to several kilograms, which is suitable for smelting larger size amorphous alloy samples. In addition, the injection-suction casting method has a fast punching speed, which can significantly increase the cooling rate.

Amorphous powder metallurgy uses the characteristics of high viscous flow and easy forming of amorphous alloy powder in the supercooled liquid phase region, and can be consolidated and formed by a variety of pressure methods. Common powder metallurgy methods include: hot extrusion, hot pressing and spark plasma sintering (Spark Plasma Sintering, SPS), etc. Because the powder metallurgy method utilizes atomic diffusion and viscous flow, it is easier to prepare large-size net-shaped bulk amorphous alloy samples, which has become a method for preparing bulk amorphous alloys with both production and application prospects.

The consolidation of amorphous alloy powder is based on the preparation of amorphous alloy powder and the viscous flow in the supercooled liquid phase. The method of preparing amorphous alloy powder has long been mature; the atomic diffusion of the amorphous alloy powder in the supercooled liquid phase and the fragmentation of the oxide film on the powder surface under high pressure play a key role in the consolidation and formation of the amorphous alloy powder. The hot extrusion method is a method of encapsulating spherical amorphous alloy powder in a mold, heating the mold to a supercooled liquid phase zone and extruding according to set parameters, and finally obtaining a consolidated mass. Hot press sintering is another method of preparing bulk amorphous alloys. Usually, the amorphous alloy powder is put into the mold, and under the vacuum state, the appropriate temperature, pressure and time are loaded, and finally a sintered mass is obtained. Spark plasma sintering technology is an emerging powder sintering method. Compared with traditional sintering methods, its advantages are low sintering temperature, short holding time and fast heating rate. Therefore, it has strong advantages for the consolidation and molding of amorphous and nanocrystalline powders.