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Amorphous Formation and Transformation

by:Catech      2023-04-10

Amorphous materials have similar structural characteristics to liquids. Also known as 'supercooled liquid'. It has two characteristics of long-range disorder, short-range order and metastable state. According to such characteristics, the key problems to be solved in the preparation of amorphous substances are as follows:

① Inhibit nucleation and growth in the melt, and maintain a liquid structure;

② Keep the amorphous metastable structure stable within a certain temperature range and not transform to the crystalline state;

③ Introduce or cause disorder in the crystalline solid, so that the crystalline state is transformed into an amorphous state. Amorphous state can be formed by rapid cooling of gas phase and liquid phase, and can also be formed directly in solid state (such as ion implantation, high-energy ion bombardment, high-energy ball milling, electrochemical or chemical deposition, solid-state reaction, etc.) .

The formation method of ordinary glass is to melt the raw materials at high temperature to form a melt, and then supercool (quickly cool) the melt to solidify into a glass body. The general cooling rate cannot convert the metal and alloy melt into an amorphous state, and a special preparation method must be adopted. The cooling rate must be extremely fast so that it has no time to crystallize and form an amorphous state. The cooling rate for pure metal to form amorphous state is above 1010K/s, and the cooling rate for alloy to form amorphous state is above 106K/s. After the 1970s, people began to use the melt spin quenching method (Melt Spinning) to prepare amorphous strips, that is, the high-temperature melt was sprayed onto the high-speed rotating cooling roller, and the melt was rapidly cooled at a speed of one million degrees Celsius per second. , so that the atoms in the metal have no time to rearrange, and the disordered structure is frozen, thus forming an amorphous alloy liquid phase. When the liquid phase crystallizes or enters the amorphous state during cooling, some properties change as shown in the figure. As the temperature decreases, the temperature range can be divided into three states: A, B, and C: in the A range, the liquid phase is in an equilibrium state; when the temperature drops below Tf and enters the B range, the liquid phase is in a supercooled state and occurs Crystallization, Tf is the equilibrium solidification temperature; if the cooling rate is so large that nucleation growth is too late to proceed and the temperature has cooled to the C range below Tg, the viscosity of the liquid phase will increase greatly, and atomic migration is difficult to proceed, and it is in a 'frozen' state, so crystallization The process is inhibited and enters the amorphous state, and the glass transition temperature is the glass transition temperature. It is not a thermodynamically determined temperature, but is determined by kinetic factors. Therefore, Tg is not constant. When the cooling rate is large, it is Tg1, such as cold If the cooling rate is reduced (still in the range of cooling rate that inhibits crystallization), Tg1 will be reduced to Tg2. The free energy of the amorphous state is higher than that of the crystalline state, so it is in a metastable state. It can also be seen that the volume (density) changes abruptly during liquid phase crystallization, but not during vitrification; but the specific heat capacity Cp is significantly greater than that during crystallization when the specific heat capacity Cp changes. The ability of an alloy to change from a liquid phase to an amorphous state depends on both the cooling rate and the alloy composition. The minimum cooling rate that can inhibit the crystallization process and achieve amorphization is called the critical cooling rate (Rc). According to the theoretical calculation of the crystallization nucleation conditions of pure metals such as Ag, Cu, Ni, and Pb, the minimum cooling rate must reach The amorphous state can only be obtained at 1012~1013K/s, which is difficult to achieve in the current quenching method of the melt, so the pure metal cannot form an amorphous state by quenching the melt; and the critical cooling rate of some alloy melts is relatively low. Low, generally below 107K/s, the amorphous state can be obtained by using the existing quenching method. In addition to the cooling rate, whether the alloy melt forms amorphous or not is also related to its composition. Different alloy systems have different abilities to form amorphous. In the same alloy system, usually only within a certain composition range can form amorphous.

Amorphous state transition, the liquid metal whose temperature is higher than or equal to the melting point Tm, its interior is in an equilibrium state. From an energy point of view, when the temperature is lower than the melting point Tm. Undercooling without crystallization, the free energy of the system will be higher than that of the corresponding crystalline metal, and it will be metastable. If the structural relaxation (or atomic rearrangement) time τ in the system is smaller than the reciprocal of the cooling rate dT/dt, the system still maintains internal equilibrium, so it is in a balanced metastable state. As the liquid metal system cools, its viscosity coefficient η or relaxation time τ will increase rapidly. When it increases to a certain value, τ is already so large that the system cannot reach equilibrium within a limited time, that is, in non-equilibrium metastable state. Counting from leaving the internal equilibrium point, it is called configuration freezing or amorphous transition. The change of heat enthalpy H, specific volume V and entropy S with temperature T when forming amorphous alloy.

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