| This thesis addresses the use of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model for better understanding of kinetics of crystallization of the beta-phase in sensitized aluminum alloys. Three acoustic parameters: longitudinal velocity, transverse velocity, and attenuation coefficient for longitudinal waves, are modeled as a function of time as the beta-phase volume fraction increases. The acoustic parameters were previously measured in the same laboratory with two ultrasonic techniques, Resonant Ultrasound Spectroscopy (RUS) and Pulse Echo (PE), on sensitized samples of Al5083-H116 and Al5456-H116. In this investigation, a comparative study of beta-phase kinetics in terms of the phase transformation rate constant, k, and Avrami exponent, n, was performed on the three acoustic parameters, to understand their evolution with the degree of sensitization. The values of n indicate a combination of 2D to 1D growth at the grain boundaries and at their intersections. The values of k grow non-linearly with the heat-treatment temperature; k is higher for the attenuation coefficient as compared with that for velocity. |
