Journal of AU.@~ AHD COMF~DUNDS ELSEVIER Journal of Alloys and Compounds 228 (1995) 105-111 A possible route to improving the ductility of brittle intermetallic compounds T.J. Davies, A.A. Ogwu Manchester Materials Science Centre, Manchester University/ UMlST, Grosvenor Street, Manchester M I 7HS, UK Received 9 January 1995; in final form 24 April 1995 Abstract Efforts to achieve ductility in a certain class of intermetallic compounds are described in terms of the experimental work performed and the empirical and quantitative quantum mechanical models used to achieve crystal structures likely to satisfy the Von Mises condition for plasticity. However, the current trend in the behaviour of these intermetallic compounds indicates that an underpinning electronic state requirement must be satisfied together with the Von Mises criterion for improved ductility to become achievable. It is suggested that a detailed understanding of this electronic factor may provide a way to improve the ductility via processes involving an alteration of the equilibrium electron distribution in some intermetallic compounds, possibly through the electroplastic effect. Keywords: Brittle intermetallics; Electronic structure; Charge density topology; Electroplastic effects 1. Introduction Intermetallic compounds are generally characterized by the following properties which make them suitable candidate materials for high temperature applications. (1) Strong bonding which does not generally de- grade with temperature. (2) Ordered structures with low self-diffusion lead- ing to good creep resistance. (3) High moduli of elasticity which decrease slowly with temperature. (4) Low specific density. (5) Good oxidation resistance. However, as a class of materials, many intermetallic compounds are brittle. This limited ductility has caused difficulty in fabrication and restricted the use of these materials in a range of fields including the aerospace industry; consequently, tremendous effort has been put into improving the ductility of inter- metallics. Excellent critical reviews of the current understanding of the properties of intermetallics have been reported in the literature by Paxton [1], Yamaguchi and Umakoshi [2], Varin and Winnica [3], Cahn [4] and Froes et al. [5]. The factors found to be responsible for the limited ductility are as follows. (1) A limited number of slip systems. 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSD1 0925-8388(95)01693-7 (2) A large slip vector. (3) Restricted cross-slip. (4) Difficulty in transmitting slip across grain boundaries. (5) Intrinsic grain boundary weakness associated with ordering energies in the intermetallics. 2. Empirical relationships between bonding and ductility in solids It is a general requirement of the Von Mises criter- ion [6] that a polycrystalline material should be able to undergo an arbitrary strain by slip without the nucleation of cavities at its grain boundaries only when five independent crystal slip systems are operative. It is now apparent, particularly in the case of the recent studies of transition metal aluminide intermetallics [1], that this crystallographic requirement of Von Mises must be accompanied by certain electronic structure requirements for arbitrary strain without the occurrence of cavity nucleation. Early empirical evi- dence for the existence of an interrelationship between electronic state and ductility can be found in the work of Schwab [7,8]. Schwab [7,8] suggested that a rela- tionship exists between the formation energy of dislo-