Eutectoid decomposition in Fe₂Si₅ based alloys with a small amount of 10(Pd, Pt) and 11(Cu, Ag, Au) group elements

Abstract

The addition of a small amount of Cu is effective in accelerating the α → β + Si eutectoid decomposition. Some elements (Pd, Pt, Ag and Au) that are expected to have similar chemical properties were added to an Fe₂Si₅ based alloys up to 1.0 at.% to examine whether a similar effect could be revealed. The microstructures, X-ray diffraction and differential thermal analysis (DTA) of slowly solidified or heat treated specimens were investigated in detail. The solubility of all containing elements into the α phase was less than 0.2 at.% for the slowly solidified specimen. The excess of the additive solidified as a phase of a eutectic with the Si phase. On the other hand, the solubility of these additives in the β phase was classified into two groups. The first group had higher solubility in the β phase than that in the α phase. The solubility of the other group was as low as that in the α phase. Pd and Au belonged to the former and Pt and Ag belonged to the later. The acceleration of the eutectoid decomposition was only observed in the former group but it was negligible in the later group. The existence of the eutectic melt at the annealing temperature was effective on the coarsening of the eutectoid structure but not essential for the acceleration. The eutectoid decomposition strongly depended on the solubility of these elements into the β phase.