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

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.