We document that in unstressed, undeformed, samples of pure iron containing silica inclusions precipitated by melt deoxidation, the iron matrix and a small fraction of the silica inclusions are locally separated at room temperature by a void. Thermal cycling, picnometry, dilatometry, and nano-holotomography experiments are combined to demonstrate that those voids originate from silica inclusion/iron matrix interfacial debonding during cooldown from elevated processing temperatures, resulting from the volumetric expansion that accompanies the austenite-to-ferrite phase transformation.