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The amorphous formation and transformation

by:Transmart     2020-05-27
Amorphous materials because of its structural features similar to that of the liquid. Also known as 'supercooled liquid. It has a long-range disorder, short-range order, and in the metastable two characteristics. According to the characteristics of the preparation of amorphous material needs to solve the key problems are as follows: (1) inhibition of nucleation and growth, in the melt liquid structure; (2) make the amorphous structure of metastable stable in a certain temperature range, not to the crystalline conversion; (3) to introduce or cause disorder in the crystalline solid, the crystalline into amorphous. Amorphous cold quickly by the gas phase, liquid phase can be formed, can be directly in solid form ( Such as ion implantation, high-energy ion bombardment, the state high energy ball mill, the change of material performance with temperature electrochemical or chemical deposition, solid phase reaction, etc. ) 。 Common glass forming method, which is formed by high temperature melting materials will be melt, then will melt undercooling, Quench) Curing into vitreous body. General cooling speed cannot be metal and alloy melt into amorphous, must adopt special preparation methods, to achieve fast cooling speed to make it too late to crystalline and amorphous. Pure metal forming amorphous cooling rate is above 1010 k/s, the formation of amorphous alloy cooling rate of more than 106 k/s. Since the 1970 s, people began to use the melt spinning quenching quenching method ( 熔融纺丝) Preparation of amorphous strips, the high temperature melt into the high speed rotating cooling roller, melt at millions of degrees per second cooling rapidly, so that the atoms in the metal to rearrange, desultorily structure have been frozen, forming liquid phase occurs during cooling crystallization of amorphous alloys or into amorphous, some properties of change as shown. With the reduction of temperature can be divided into A, B, C three state temperature range: in A range of liquid phase equilibrium; When the temperature dropped below Tf in B range, the liquid phase in the supercooled state and crystallization, Tf is the equilibrium solidification temperature; Such as cold speed is ambassador nucleation growth and cold temperature is below Tg C range, the liquid viscosity increases greatly, atomic migration difficult, in the 'frozen' state, so the crystallization process is inhibited and into amorphous, handicrafts is the glass transition temperature, it is not a thermodynamic temperature is determined, but depends on the dynamic factors, so the Tg is not fixed, the cooling speed for Tg1 and reduce (such as cold speed Still in suppressing the cooling speed of crystallization range), , the Tg1 is reduced to Tg2. Is higher than the free energy of the amorphous crystalline state, so in the metastable state. It also can be seen when liquid crystal volume ( Density) Don't appear when mutations, and glass transition mutation; But when specific heat Cp in amorphous changes significantly greater than the crystallization of Cp. By liquid phase shift for the amorphous alloy, is determined by the cooling rate also depends on the alloy composition. Can inhibit the crystallization process of amorphous speed is called the critical cooling speed (xiao leng Rc) Of pure metals such as Ag, Cu, Ni, Pb crystallization nucleation conditions of theoretical calculation, the minimum cooling rate to reach 1012 ~ 1013 k/s to obtain amorphous, which in the current melt quenching method is hard to do, so the pure metal using melt quenching can't form amorphous; Some critical cooling velocity of the molten alloy is lower, generally below 107 k/s, using the existing quench method can obtain amorphous. In addition to the cooling speed, alloy liquid and forming of amorphous or not but also to the composition, different alloy system form amorphous ability is different, the same alloy system usually only within a certain ingredients can form amorphous. Amorphous transformation, the temperature of liquid metal is equal to or higher than the melting point Tm, its interior is in equilibrium. From the point of view of energy, when the temperature is lower than the melting point Tm. In the absence of crystallization undercooling, the system will be higher than the corresponding free energy of the amorphous metals, metastable. If the system structural relaxation ( Or atomic rearrangement) Time tau is smaller than the reciprocal of cooling rate dT/dT, the system still maintain internal balance, so the balance of the metastable. With cooling system of liquid metal, the viscous coefficient of eta or relaxation time tau will increase rapidly, when added to a certain value, the tau has very big, so that the system cannot achieve the equilibrium state in the limited time, namely in the non-equilibrium metastable. By leaving the internal balance, called configuration freeze or amorphous transformation. Formation of amorphous alloy thermal enthalpy and entropy H, volume V S along with the change of temperature T.
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