Active eutectoid decomposition in Zr-3 wt.% Fe

Abstract

The eutectoid reaction is exhibited by many binary Zr and Ti base alloys. In many of these systems, the eutectoid compositions correspond to low levels of solute concentration, implying a large equilibrium volume fraction of eutectoid product, which consists of a mixture of the a (h.c.p.) phase and an appropriate intermetallic phase. In some of these eutectoid systems, called “active” eutectoid systems, the decomposition of the b (b.c.c) phase into a lamellar aggregate of two phases cannot be suppressed, in alloys of near eutectoid composition, even on rapid b quenching. Some of these alloy systems are: Ti-Cu, Ti-Ni, Zr-Ni, Zr-Cu and Zr-Fe [1–6]. It has been suggested that the active eutectoid decomposition would occur when the eutectoid temperature is high, the eutectoid composition is solute lean and the intermetallic phase resulting from the reaction is rich in the base metal. The non-equilibrium two phase aggregate formed on quenching often comprises lamellae of the a phase and of a metastable phase which is structurally similar but not identical to the appropriate equilibrium intermetallic phase. On subsequent aging at temperatures below the eutectoid temperature, a cellular reaction may occur and the fine lamellar structure may be replaced by a coarser structure. This reaction may be the result of incomplete partitioning of the solute during the rapid eutectoid reaction and also of the tendency to minimize the interfacial energy by decreasing the interlamellar interfacial area [7]. Active eutectoid decomposition and martensite formation can be competitive processes in these alloys [4]. For compositions away from the eutectoid composition a large supercooling may generally be needed to initiate active eutectoid decomposition and this supercooling may be sufficient to bring about martensitic transformation as well. In near eutectoid alloys, however, the degree of supercooling needed for triggering active eutectoid decomposition is generally too small to induce martensitic transformation. The eutectoid decomposition can occur rapidly because the diffusion distances are small, interface diffusion can occur and solute partitioning can be incomplete. The Zr-rich side of the binary Zr-Fe phase diagram shows the eutectoid reaction: b 3 a 1 Zr3Fe. The eutectoid composition is 4 at.%Fe (;2.5 wt.%Fe) and the eutectoid temperature is 730°C. The Zr3Fe phase, which is the intermetallic phase richest in Zr in this system, has a base centred orthorhomic structure (Re3B type structure, space group Cmcm), with a 5 0.332 nm, b 5 1.095 nm and c 5 0.881 nm [8,9]. The next Zr enriched intermetallic phase in this system is the Zr2Fe phase which has a Al2Cu type structure which is body centred tetragonal with a 5 0.636 nm and c 5 0.582 nm [8,9]. In the work reported here, the microstructural features associated with the active eutectoid decomposition of a near eutectoid alloy (Zr-3 wt.%Fe) on b quenching were examined. The effects of aging after b quenching, and of furnace cooling from the b phase field, on the microstructure of the alloy were also studied. The microstructural characterization was carried out using optical microscopy, scanning Pergamon Scripta Materialia, Vol. 40, No. 6, pp. 723–728, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/99/$–see front matter PII S1359-6462(98)00491-6