Experimental study of the bcc-fcc phase transformations in the Fe-rich system Fe-Si at high pressures

In situ X-ray diffraction experiments at high pressure were carried out up to 8.9 GPa and 1100 degrees C to study phase transformations of iron and tw o iron-silicon alloys Fe0.91Si0.09 and Fe0.83Si0.17 For iron, the transform ation from the bcc phase to the fee phase was observed at pressures 3.8-8.2 GPa and temperatures that are consistent with previous in situ X-ray diffr action studies. Reversal of the transformation of iron was found to be sens itive to temperature; hysteresis of the transformation increased from 25 de grees C at 3.8 GPa to 100 degrees C at 7.0 GPa, primarily because the bcc-f cc phase boundary has a negative Clayperon slope. In the binary system Fe-S i, the observations of the present study indicate that the ferrite (bcc pha se)-stabilizing behavior of silicon persists at high pressures and that the maximum solubility of silicon in the fee phase increases with increasing p ressure: (1) the transformation from the bcc phase to the fee phase was obs erved in Fe0.91Si0.09 at 6.0, 7.4 and 8.9 GPa and the temperatures measured at the onset of the transformations were 300 degrees C higher than those i n iron at similar pressures, (2) the transformation rate in Fe0.91Si0.09 wa s extremely sluggish compared to that of iron, and (3) the bcc-fcc phase tr ansformation was not observed in Fe0.91Si0.09 at 4.7 GPa up to 1000 degrees C and in Fe0.83Si0.17 at 8.2 GPa and 1100 degrees C.