Thermal expansion of iron carbides, Fe₇C₃ and Fe₃C, at 297-911 K determined by in situ X-ray diffraction

The thermal expansion of Fe-carbides Fe₇C₃ and Fe₃C was measured above and below the ferromagnetic transition in a temperature range of 297-911 K by in situ X-ray diffraction enhanced by synchrotron radiation. The coefficient of thermal expansion was estimated as α = a₀ + a₁T. The results for paramagnetic phases are a₀ = 3.1 × 10^-5 K^-1 and a₁ = 1.2 × 10^-8 K^-2 for Fe₇C₃ and a₀ = 3.9 × 10^-5 K^-1 and a₁ = 1.2 × 10^-8 K^-2 for Fe₃C. The ferromagnetic phases show linear relations with V_298K = 371.9 ų, a₀ = 0.91 × 10^-5 K^-1 and a₁ = 0 for Fe₇C₃ and V_298K = 155.2 ų, a = 0.86 × 10^-5 K^-1 and a₁ = 0 for Fe₃C. Our obtained results indicate that the coefficients of thermal expansion, that were calculated by high-pressure equations of state for Fe-carbides, appeared to be high-precision and consistent with the data obtained at 0.1 MPa.