This micrograph indicates the nulceation and growth mechanism in the crystallization of amorphous near-equiatomic NiTi films. The crystal nucleates homogenously inside the bulk of the film, and quickly consume most of the film thickness, and then grows laterally in a two-dimensional growth mode. Heterogeneous nucleation at an interface was not observed due to the composition shift at those locations caused by interfacial reaction.

Source: Xi Wang, Joost.J. Vlassak, Crystallization Kinetics of Amorphous NiTi Shape Memory Alloy Thin Films, Scripta Materialia, 54, 925-930 (2005). (see attachment for preprint of the paper)