A correlation method for determination of crystallization mechanism and activation energy of amorphous alloy
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I.
INTRODUCTION
CRYSTALLIZATION kinetics is an important subject in the research of amorphous alloys, and the crystallization activation energy is a significant part of crystallization kinetics. Usually, the Kissinger equation[1] is used for determining the crystallization activation energy by measuring the crystallization temperatures as a function of heating rates. Recently, during researching the effect of pressure on the crystallization rate of amorphous Zr70Cu30 alloy, it was unexpectedly seen that the crystallization activation energy obtained with the Kissinger equation resulted in quite a large departure of maximum crystallization rate from the rate measured. After further study, a new method was advanced, with which one can determine simultaneously the crystallization activation energy and mechanism much more coincident with reality than those obtained via the Kissinger equation. II.
EXPERIMENTAL
Ribbon-shaped samples were prepared with pure Zr and Cu by melt spinning. The ribbons are 2.5-mm wide and 0.02-mm thick. High pressure experiments for measurement of crystallization temperature and crystallization rate were performed in a piston-cylinder type installation of 20-mm i.d.[2] The apparatus was program controlled; therefore, the temperature and heating rate were adjustable. Measurement data, such as temperature, current, potential, etc., were recorded by a computer data acquisition system. Analysis and treatment of data and some figures were also done in this system. III.
resistance starts to decrease rapidly within a small temperature range when the temperature rises to around 360 7C. Crystallization temperature Tx or Tp was defined as the temperature at which the resistance starts to obviously decrease, or the change rate of relative resistance [(dr/dt)/(Rc 2 Ra)] is maximum. As shown in References 3 and 4, crystallization temperature is affected by pressure. The peak value of the relative change rate of resistance is regarded as the maximum crystallization rate, i.e., (dx/dt)max 5 [(dR/dt)p/(Rc 2 Ra)], where Rc or Ra is the resistance of full crystalline or full amorphous states, respectively. The maximum crystallization rate is also affected by pressure, as shown in Figure 2 and Table I. According to the Kissinger equation with Tp and heating rate F values, the dependence of crystallization activation energy DEp on pressure is shown in Figure 3(a). From the Kissinger reaction rate formula,[5] the maximum reaction rate formula was derived, as follows:
~dxdt !
max
5 (1 2 Xp) z
F z DEp R z T p2
[1]
where Xp is a reaction fraction (50.61 to 0.63) corresponding to maximum reaction rate. Letting Xp equal 0.62 and putting the relevant data into formula [1], we obtained the value of maximum crystallization rate under each pressure, respectively, as shown in Figure 2 and Table I. It can be seen that the maximum crystallization rates calculated from the Kissinger formula are much lower than the ones measured.
ANALYSIS OF RESULTS AND NEW METHOD ESTABLISHMENT
Figure 1 shows the variation of relati
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