engleski

Aplication of numerical method for kinetic analysis on METGLAS 2826 MB crystallization

The crystallization kinetics of g-FeNiMo solid solution in metallic glass Metglass 2826 MB (Allied Co., USA) has been studied in isothermal conditions using differential scanning calorimetry (DSC). In despite with extensive work, the discrepancies exist among investigators regarding activation energy for this phase. The Johnson-Mehl-Avrami (JMA) equation was used to describe crystallization process. The kinetic analysis has been performed by recently developed numerical model. In order to describe the shape of the DSC peak, following function is proposed: f(t)=DELTAH(dalfa/dt)= DELTAHnk^n(t-tau)^(n-1)exp[-kn(t-tau};^n] as appropriate differential function of JMA equation. DELTAH is the total enthalpy difference between transformed and untransformed states, alpha is the degree of conversion, n is known as Avrami exponent, k is the rate constant and tau is related to the transient time. The experimental scans were fitted to the proposed model, and a very good match was attained for each curve. In such manner the rate constants and Avrami exponents were determined. For the purpose of method evaluation the classical, analytical method of data analysis has been applied too. The apparent activation energies were calculated on the basis of the both rate constant sets through Arrhenian dependence. Arrhenius plots for both constant rate sets revealed the linearity and enabled the calculation of activation energies. The obtained values were 280&plusmn ; 22 kJmol-1 and 296&plusmn ; 23 kJmol-l for analytical and numerical method, respectively. The obtained values are in agreement with the majority of results reported in literature and the differences between them are within the calculation error span. The Avrami exponents, obtained by the both methods, are consistent and yield between 1.75 and 1.95. Very good match between experimental and calculated curves and close similarity of calculated activation energies points out that recently proposed numerical model for kinetic analysis of DT A/DSC peaks, obtained in isothermal conditions, has been successfully applied. Therefore proposed numerical model, which involve no unnecessary mathematical transformation of experimental data, offer (at least) the same accuracy in less time.

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