The Effect of Scandium on the Microstructure of Ti 3 Al
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THE EFFECT OF SCANDIUM ON THE MICROSTRUCTURE OF Ti3AI
PAUL R.MUNROE School of Materials Science and Engineering, University of New South Wales, P.O.Box 1, Kensington, NSW 2033, Australia.
ABSTRACT Using Pettifor Maps as a guide to alloying, scandium additions were made to Ti3 AI to try and transform the crystal structure of this intermetallic alloy from ordered hexagonal to ordered face centred cubic. Scandium was found to have a solid solubility limit of about 6.4at.% in Ti 3 AI and preferentially occupy the titanium sublattice. According to the A3B Pettifor Map, these are criteria which should effect a change in crystal structure, however no such transformation was observed. Scandium additions in excess of the solid solubility limit resulted in the formation of Sc2Al at grain boundaries and triple points. INTRODUCTION The ordered intermetallic compound Ti 3 AI has a low density, good high temperature strength and good oxidation resistance at elevated temperatures and is envisaged as a candidate material for aerospace applications. However, its commercial exploitation has been inhibited by its poor room temperature ductility and fracture toughness. Research aimed at improving the ductility and toughness of this material has focused upon the addition of strong 13stabilizers, such as niobium and molybdenum, to Ti 3 AI. Such alloys, when subject to appropriate thermomechanical treatment, may exhibit two-phase, ordered hexagonal plus body-centred cubic (a2+13), microstructures [1-5]. The ductile 13phase inhibits crack propagation and thus improves ductility [6]. A more novel approach to improving the ductility of Ti 3Al is to change the crystal structure, from an ordered hexagonal (or DO1 9) structure to the higher symmetry ordered face-centred cubic (or L1 2) structure, through selective alloying. Liu and his co-workers have shown that the DO 1 9-structured compound Co3V can be transformed to a L1 2 structure by alloying with iron, with significant improvements in ductility [7,8]. Furthermore, the tetragonal structured compound AI 3Ti can be transformed to a L12 structure through additions of transition metals and other elements [9-14], although it is worth noting that no significant increases in ductility have been achieved through this transformation [15]. It should be noted that transformations to more symmetrical crystal structures do not alone guarantee ductility. One theoretical basis for structural modification through selective alloying is the Pettifor Map [16]. These maps are semi-empirical representations of the crystal structures of alloy phases based upon a single parameter, the Mendeleev number, of the constituent elements. It has been demonstrated elsewhere that ternary additions with low Mendeleev numbers, which substitute on to the titanium sublattice of Ti 3AI and thus lower the average Mendeleev number of this sublattice, should effect a change in the crystal structure of Ti 3 AI from D0 1 9 to L12 [17,18]. Scandium was identified as an element, with a lower Mendeleev number than titanium, which c
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