Formation of amorphous alloy
Crystal is a solid with regular geometry formed by the crystallization process, which is formed by the periodic arrangement of atoms or molecules according to certain rules. Corresponding to the crystal, the irregular arrangement of atoms or molecules, no periodicity and no symmetry of the solid is called amorphous. It is a long-range disorder and short-range order structure. Due to the limitation of glass forming ability, amorphous alloys exist in the form of powders, filaments and ribbons. The appearance of bulk amorphous alloy opens the prelude of amorphous alloy. The key point of preparation is to restrain the nucleation and growth of crystal phase during cooling.
Preparation of amorphous alloy
At present, there are two main preparation methods of bulk amorphous alloys: direct solidification method and powder consolidation method. The direct solidification method mainly includes water quenching, copper mold casting, suction casting, high pressure casting and magnetic levitation melting; the powder consolidation method is the method of pressing powder into bulk amorphous alloy in supercooled liquid region. The following are the common preparation methods of bulk amorphous alloys:
1. Water quenching is a common method to prepare bulk amorphous alloy by melt water cooling method. It has the advantages of simple technology and easy control.
2. Copper mold casting is a method to obtain bulk amorphous alloy by cooling the melt in a copper mold with a specific shape. Due to the limited cooling rate of copper mold, the size of amorphous alloy is limited to a certain extent.
3. The basic principle of high pressure casting is to melt the alloy in the sleeve by high frequency induction coil under argon protection. Within milliseconds, the molten liquid is pushed into the water-cooled copper mold by hydraulic push piston. Because of the high die pressing speed, the cooling rate can be obtained, so the alloy cooling forms large amorphous alloy. At the same time, the high pressure makes the liquid alloy contact with the cavity more closely, and the casting defects such as porosity are reduced, and the net forming effect is good. For the melt with high viscosity which is difficult to prepare bulk amorphous alloy, it is also possible to prepare castings with complex appearance.
4. Due to the advantages of copper mold casting, water cooling and suction casting, spray casting suction casting method is suitable for the preparation of a variety of bulk amorphous alloys. The advantage of this method is that the master alloy is heated by high frequency or medium frequency induction, and magnetic stirring is added, which can accelerate the melting rate of the alloy and make the alloy composition more uniform. At the same time, the melting amount of the alloy can be from several grams to several thousand grams, which is suitable for melting large-size amorphous alloy samples. In addition, the spray casting suction casting method has a fast punching speed, which can significantly improve the cooling rate.
The amorphous powder metallurgy method takes advantage of the high viscous flow of amorphous alloy powder in the supercooled liquid region and its easy forming characteristics, and can adopt a variety of pressure consolidation methods. Common powder metallurgy methods include hot extrusion, hot pressing and spark plasma sintering (SPS). Due to the use of atomic diffusion and viscous flow, the powder metallurgy method is easier to prepare large-size net formed bulk amorphous alloy samples, which has both production and application prospects.
Consolidation of amorphous alloy powder is based on the preparation of amorphous alloy powder and viscous flow in supercooled liquid region. The method of preparing amorphous alloy powder has long been mature; the atomic diffusion of amorphous alloy powder in supercooled liquid region and the breaking of oxide film on the surface of the powder under high pressure play a key role in the consolidation of amorphous alloy powder. The hot extrusion method is to encapsulate the spherical amorphous alloy powder in the mold, heat the mold to the supercooled liquid region and extrude according to the set parameters, and finally get the consolidated block. Hot pressing sintering is another method to prepare bulk amorphous alloy. Usually, the amorphous alloy powder is put into the mold, and the appropriate temperature, pressure and time are loaded in the vacuum state to obtain the sintered block. Spark plasma sintering (SPS) is a new method of powder sintering. Compared with the traditional sintering method, it has the advantages of low sintering temperature, short holding time and fast heating rate. Therefore, it has strong advantages for consolidation molding of amorphous and nanocrystalline powders.
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