In‐situ transmission electron microscopy of the α ⟷ γ allotropic transformation in thin foil of iron

The α ⟷ γ allotropic transformation in vapor deposited thin foil of pure iron is investigated by in‐situ transmission electron microscopy. The α ⟷ γ transformation proceeds by following Bain's orientation relationship, which is {001} γ ∥ {001} α and 〈001〉 γ ∥ 〈011〉 α, indicating that the α ⟷ γ transformation is martensitic in nature. No eminent dislocation is introduced within the newly transformed α‐ or γ‐iron grains. The grain size of the re‐transformed α‐iron from the γ‐iron becomes extremely larger than that before α ⟷ γ transformation.