DETERMINATION OF THE AVRAMI EXPONENT FOR SOLID-STATE TRANSFORMATIONS FROM NONISOTHERMAL DIFFERENTIAL SCANNING CALORIMETRY

Many solid state transformations follow the Johnson-Mehl-Avrami kinetic equation, particularly crystallization of polymers, glasses and amorphous metals. The value of the Avrami exponent in this relationship, important for understanding the reaction mechanism, is normally determined by isothermal differential scanning calorimetry. A computer model has been developed to generate non-isothermal differential scanning calorimeter curves. This model was used to construct a function that consists of parameters easily derived from non-isothermal experiments and which is strongly dependent on the values of the Avrami exponent and is nearly independent of all other variables. Thus, from the values of peak temperature, peak height, activation energy and enthalpy, the value of the function may be calculated and the Avrami exponent read from a correlation curve. The validity of this procedure has been demonstrated using an experimental example of the crystallization of an amorphous metal alloy. The Avrami exponent determined by this procedure was in good agreement with the value of the exponent determined by the conventional isothermal method and resulted in model differential scanning calorimeter curves which closely agreed with those measured experimentally.