An Anomalous Thermal Expansion Phenomenon Induced By Phase Transition Of Fe-Co-Ni Alloys

The thermal expansion and the phase transition of Fe-15.6 wt. % Co-12 wt. % Ni single-phase solid solution alloy were systematically investigated by thermal analysis experiments and molecular dynamics simulations. The coefficient of thermal expansion (CTE) was accurately measured in the temperature range of 300-1580 K. The eccentric changes of thermal expansion ranging from 900 to 1150 K were verified from the incomplete transformation of alpha-Fe phase to gamma-Fe phase by differential scanning calorimetry (DSC) and in situ X-ray diffraction experiments. The CTE of a-Fe phase increases nonlinearly from 9.29 x 10(-6) to 1.278 x 10(-5) K-1 in the range of 300-900 K, which is in good agreement with the results obtained by molecular dynamics simulation, whereas the CTE of alpha-Fe phase increases linearly from 2.024 x 10(-5) to 2.398 x 10(-5) K-1 in the range of 1150-1580 K. Meanwhile, the visual atomic positions at different temperatures indicate that thermal expansion is attributed to the anharmonic vibration and short-range diffusion of atoms when the temperature exceeds a certain value. Furthermore, the Curie temperature is determined as 725 K by the thermal expansion and DSC experiments. Additionally, the isothermal sections of the Fe-rich corner [Fe-5x wt. % Co-5y wt. % Ni(2 <= x + y <= 8)] in Fe-Co-Ni non-equilibrium ternary phase diagram at 300 K are derived by X-ray diffraction. Moreover, the CTE ranging from 300 to 1700 K of the Fe-rich corner in Fe-Co-Ni ternary phase diagram was predicted theoretically on the basis of the molecular dynamics method. Published by AIP Publishing.