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

Many solid state transformations follow the Johnson-Mehl-Avrami kineti c equation, particularly crystallization of polymers, glasses and amor phous metals. The value of the Avrami exponent in this relationship, i mportant for understanding the reaction mechanism, is normally determi ned by isothermal differential scanning calorimetry. A computer model has been developed to generate non-isothermal differential scanning ca lorimeter curves. This model was used to construct a function that con sists 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 va lue of the function may be calculated and the Avrami exponent read fro m a correlation curve. The validity of this procedure has been demonst rated using an experimental example of the crystallization of an amorp hous metal alloy. The Avrami exponent determined by this procedure was in good agreement with the value of the exponent determined by the co nventional isothermal method and resulted in model differential scanni ng calorimeter curves which closely agreed with those measured experim entally.