Experimental and theoretical phase diagrams of the iron-rich Fe-Si-Ge ordering system

Abstract

The phase separation in iron-rich Fe-Si-Ge ternary alloys was investigated by means of transmission electron microscopy. The phase separations into A2 and D03 were recognized in several alloys in addition to the phase separation into B2 and D03 which has already been found in Fe-Si and Fe-Ge binary systems. On the basis of TEM observations, isothermal section diagrams are proposed for the iron-rich corner at 823, 873, 923 and 973 K. The A2 + D03 two-phase field exists in a band-shaped region at 823 K. With increase in temperature,the A2 + D03 field gradually shrinks and is eventually replaced at 973 Kwith the B2 + D03 field, which extends from both the Fe-Si and Fe-Ge binary sides. The phase diagrams were well reproduced by the calculations based on the so-called Bragg-Williams-Gorsky approximation, taking account of the chemical and magnetic pair interactions up to the second nearest neighbour. The effects of the magnetism on the equilibria are discussed.