Halo Formation During Solidification of Refractory Metal Aluminide Ternary Systems

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THE formation of a halo or growth of a secondary phase around the primary phase and thereby precluding coupled growth is a common occurrence in off-eutectic alloy compositions.[1] One of the possible processing methods for the manufacture of components is via the liquid route, as it involves the extension of a well-established technology that permits components to be produced in a single-stage process. However in liquid–solid phase transformations, alloy compositions are restricted by the occurrence of appropriate phase equilibria that is dictated by thermodynamic as well as kinetic considerations. The latter specifically plays a key role in the evolution of phase morphologies for eutectic or near-eutectic compositions, such as in in situ composites, where fine-coupled eutectic structures are desirable over coarser dendritic structures (or halos) for superior mechanical properties.[2] Therefore, it becomes important to study the conditions under which the

N. D’SOUZA is with Rolls-Royce plc, PO. Box 31, Derby DE24 8BJ, UK; L.M. FEITOSA, and H.B. DONG are with the Department of Engineering, University of Leicester, Leicester, LE1 7RH, UK, Contact e-mail: [email protected] G.D. WEST is with the Warwick Manufacturing Group, University of Warwick, Coventry CV4 7AL, UK. Manuscript submitted September 19, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

fine-structure-coupled eutectic prevails over the coarser dendritic structures or halos during solidification. The aim of this paper is to investigate the formation of halos in some typical ternary systems. The refractory metal aluminide (RM-Al-X) systems are specifically chosen, since they have been considered as alternative materials for high-temperature applications. Typically, RM = Ta and Nb and X = Co, Fe. The high-temperature strength in these systems is derived through the presence of super-lattice precipitates, i.e., A2 + B2 or A2 + L21 phase mixtures, where A2 is bcc-disordered matrix, while the precipitates are super-lattice, i.e., B2, CsCl structure or L21, the Heusler phase,[3,4] while the role of Al is to impart oxidation resistance through the formation of a continuous layer of Al2O3.[5–7] The typical applications of these alloys are above 1000 C and finds use in structural applications. As an example, research investigating intermetallic Laves phase alloys has shown improved strength above 1273 K (1000 C).[8,9] In equiaxed solidification, halo formation has been interpreted on the basis of non-reciprocal nucleation.[10] This can be illustrated as follows—in Figure 1(a), b phase nucleates on a phase with undercooling, DTba, but a phase nucleation on primary b requires a greater undercooling, DTab. Therefore, when a hyper-eutectic alloy of nominal composition C2 is cooled from the liquid state, b phase forms from the liquid and on cooling to Tab, a phase nucleates at an undercooling, DTab. a phase grows around b phase as a halo. Subsequent growth of a phase returns the liquid

METALLURGICAL AND MATERIALS TRANSACTIONS A

b Fig. 1—Schematic illustration for