Phase transformation of β-FeSi2-based alloys under in-situ high-temperature and high-pressure X-ray diffraction measurements

The dynamic behaviour of phase transformation in β-FeSi₂ based thermoelectric materials Fe_1-xM_xSi_2+y (M = Mn or Co) has been studied by using differential scanning calorimetry (DSC) and in situ high-temperature and high-pressure X-ray powder diffraction measurements (EDXRD) using synchrotron radiation. The investigated materials were powders prepared by rapid solidification (melt spinning). It´s found by DSC and EDXRD that the α+ε eutectic structures in the rapidly solidified alloys transform to β phase (α-Fe₂Si₅ + ε-FeSi → β-FeSi₂) in a lower temperature range below 700°C. The formed β phase decomposes reversely to α and ε phase in a higher temperature range above 950°C. The transformation temperatures T₁(α+εβ) is greatly influenced by the heating rate. By EDXRD measurements, it is found that high pressures have a great influence on the phase transformation temperature T₁ and T₂ (β→α+ε). Different doping elements (Mn, Co, or Al) have also great influence on the phase transformation temperature. EDXRD has simulated the process of hot uniaxial pressing.