Phase transformation of /spl beta/-FeSi/sub 2/-based alloys under in-situ high-temperature and high-pressure X-ray diffraction measurements

Abstract

The dynamic behaviour of phase transformation in /spl beta/-FeSi/sub 2/ based thermoelectric materials Fe/sub 1-x/M/sub x/Si/sub 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 /spl alpha/+/spl epsi/ eutectic structures in the rapidly solidified alloys transform to /spl beta/ phase (/spl alpha/-Fe/sub 2/Si/sub 5/ + /spl epsi/-FeSi /spl rarr/ /spl beta/-FeSi/sub 2/) in a lower temperature range below 700/spl deg/C. The formed /spl beta/ phase decomposes reversely to /spl alpha/ and /spl epsi/ phase in a higher temperature range above 950/spl deg/C. The transformation temperatures T/sub 1/(/spl alpha/+/spl epsi//spl beta/) 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 T/sub 1/ and T/sub 2/ (/spl beta//spl rarr//spl alpha/+/spl epsi/). 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.