Rapid irreversible exothermic amorphous-to-polycrystalline phase transformations have been observed in sputter deposited (In,Ga)Sb films. The transformation was triggered in approximately 10 −7 sec in a local region of the film using a sufficiently large energy pulse induced by, for example, a pulsed laser or a mechanical impulse. When the sum of the heat of crystallization released from a given domain and the energy supplied by external heating was sufficient to trigger a neighboring domain then the transformation propagated throughout the sample with an average velocity on the order of 200 cm/sec. The critical temperature for propagation was dependent upon the film composition, growth conditions, and thermal history. A qualitative model to explain these results is discussed.