On the influence of tricritical point on the microstructural evolution in Fe-Ge system

Abstract

The synergism of ordering and clustering reactions can complicate the decomposition behaviour of solid solutions and hence microstructural evolution (1,2). In particular, the influence of multicritical points on the evolution of microstructure in systems exhibiting ordering and clustering reactions offer exciting possibilities in alloy design (3). The investigation of the Fe-Al system containing multicritical points by Allen and Cahn (4) provided an insight into the microstructural selection processes in similar systems. These authors proposed a set of rules to determine the transformation pathways. The rules have recently been confirmed by computer simulations which have taken into account the kinetics of the process (5) as well as the experimental work on Fe-Si and Fe-Be systems (6,7). More specifically, these investigations established that an alloy quenched below the tricritical point in the two-phase field decomposes first by a continuous ordering of the high temperature less-ordered phase followed by a spinodal decomposition of the ordered phase. This latter process is conditional to the existence of a homogeneous ordered phase and hence is often termed as a conditional spinodal. Finally, the second (less ordered) phase renucleates inside tim ordered phase to yield the final microstructure. The aim of the present communication is to demonstrate that the microstructural selection in the Fe-Ge system also obeys similar pathways. Unlike Fe-Si alloys, there are very few studies of the Fe-Ge system.