Effects of Si Addition on the Martensitic Transformation, Structural and Thermodynamic Aspects in Fe_45−xMn₃₀Cr₁₀Co₁₅Si_x High Entropy Alloys

The effects of adding Si to Fe_45−xMn₃₀Cr₁₀Co₁₅Si_x high entropy alloys on martensitic transformation temperatures, structural parameters and strain energy is analyzed. Three different Si amounts are considered, 2, 4, and 6 atomic percent and results are compared with the alloy free of Si. It was verified using x-ray diffraction and energy dispersive spectroscopy that the alloys are substitutional solid solutions and completely homogeneous, in agreement with the prediction of the phenomenological theory of high entropy alloys. The phases present and their corresponding crystalline structure parameters were identified and measured. A thermally induced martensitic transformation FCC-HCP was found in all alloys. The addition of Si leads to a decrease in the paramagnetic to antiferromagnetic transition temperature of the FCC structure in a similar way as found in ternary Fe–Mn–Si alloys. However, it was found that the relative phase stability between the FCC austenite and the HCP martensite is strongly affected by the addition of Si. In fact, the critical martensitic transformation temperatures decrease 20 °C for each additional at% of Si, within the analyzed composition range. From the measurement of lattice parameters, it is shown that the volume change between FCC and HCP decreases after adding Si. The strain energy of the martensitic transformation is obtained and its contribution to the nucleation of HCP martensite is also discussed.