Phase change materials (PCMs) are typically composed of antimony, tellurium, or germanium. PCMs are used in computer memory devices, which depend on the rapid crystallization of the amorphous phase. This rapid crystallization, necessary for technological applications, is challenging to observe experimentally. Recent developments of high-speed transmission electron microscopy (TEM) detectors, and in-situ calorimetry have made the imaging of crystallization and measurements of the enthalpy of crystallization possible. In this work, the crystal growth rate and nucleation rate of Ag₃Sb₄Sb₇₆Te₁₇ (AIST) and GeTe were measured with TEM, above the glass transition temperature, in a previously unexplored temperature regime near 500 K. Both these materials are well studied PCMs and tend to form large crystal grains, making observations of crystal growth more practical. High-frame-rate direct electron detectors were utilized to image the crystallization process. Crystallization was induced by in-situ heating of thin film PCM samples on microfabricated nanocalorimeters. Classical theories of nucleation and growth were combined with the Avrami equation to form an applicable model of crystallization. Both materials were found to crystallize at around 10^-4 m/s with a nucleation rate of 10¹⁵ 1/(m² s) at 500 K. The nucleation rate measurements were compatible with the model. Compatibility between the growth rate measurements and the model required non-physical mobilities, highlighting where additional efforts, both theoretical and experimental, are needed to understand crystallization near the glass transition temperature.