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A high nucleation density is clearly an important prerequisite for the development of nanocrystal dispersions during primary crystallization of metallic glass. A microstructure of 10²¹ m⁻³ crystals of 20 nm diameter in an amorphous matrix can be readily detected by TEM, but the net heat generation is too weak for detection by usual DSC methods. This highlights the need for caution in the interpretation of DSC data from amorphous samples. Upon approaching T_g, the enhancement of diffusivity promotes further nucleation and growth. However, the high nanocrystal density (10²²–10²³ m⁻³) results in rapid diffusion field impingement which arrests growth. This provides a mechanism to limit nanocrystal growth and maintain the high density of nanocrystals. By modeling the nanocrystal development in terms of diffusional growth including diffusion field impingement, a good accounting is obtained for the DSC exotherm …