JOURNAL OF MATERIALS SCIENCE 27 (1992) 5939 5947 Tensile behaviour of Ni3AI + B intermetallic compound: influence of cold deformation and annealing T. S. SRIVATSAN, S. ANAND, S. SRIRAM* Department of Mechanical Engineering, The University of Akron, Akron, OH 44325, USA T. S. SUDARSHAN Materials Modification Inc., PO Box 481 7, Falls Church, VA 22044, USA Nickel aluminide, intermetallic compound Ni3AI, is a promising structural material on account of its high strength at elevated temperatures. The influence of cold deformation on the tensile behaviour of an Ni3AI alloy containing zirconium and boron is presented. The undeformed material, in the as-cast condition, was subjected to varying levels of cold deformation ranging from 11.4%-61.4%, and tensile tests performed. The tensile properties and fracture behaviour of the cold-deformed material are compared with undeformed material to highlight the influence of cold deformation on strength, ductility and fracture behaviour. Tensile tests were performed on cold-deformed plus annealed samples and properties compared with the cold- deformed counterpart in order to elucidate the influence of annealing on tensile behaviour. The intrinsic effects of cold deformation and annealing on microstructure, tensile properties and fracture behaviour are highlighted. 1. Introduction Rapid advances in aerospace technology have created a need for a new generation of structural materials. Ordered intermetallic compounds constitute a unique class of metallic materials which have shown promise for replacing conventional nickel-based superalloys. These compounds have been recognized for their superior strength coupled with superior resistance to creep, fatigue and corrosion resistance at elevated temperatures. The tendency for chemical ordering in these intermetallic compounds reduces atom mobility at elevated temperatures resulting in good structural stability and resistance to high-temperature deforma- tion [1]. The NiA1 and NiaA1 ordered intermetallic compounds of nickel and aluminium have, in recent years, been the subject of increasing research activity because of their excellent oxidation resistance, fairly high melting temperatures and relatively low densities which make them particularly attractive for potential use as aircraft engine materials. The Ni3A1 intermetallic compound is of interest for use at elevated temperatures, primarily because its yield strength actually increases rather than decreases with an increase in temperature [2-4]. However, use of these nickel aluminides, in polycrystalline form, as an engineering material is rendered difficult on ac- count of their low ductility and poor fracture-related properties at ambient temperature [5-9]. The low ductility and tendency for brittle intergranular frac- ture was a major obstacle that limited the develop- ment of polycrystalline Ni3A1 alloys as engineering materials. The inferior ductility of the intermetallic compound was attributed to concurrent and com- peting influences of [10-15]: (a) a low cohesive strength of grain boundaries; (b) embrittling effects of impurities at the grain boundaries; (c) a tendency to fail by brittle intergranular fracture mode; and (d) a low initial density of mobile dislocations. In general, techniques for improving the low ductility, inadequate fracture toughness and poor fracture-re- lated properties of the ordered intermetallic com- pound, Ni3A1, fall into two broad categories: (i) microstructural modifications through a refine- ment in grain size and ensuing difficulty in grain boundary crack initiation and propagation, (ii) compositional modifications through methods of enhancing the probability of (1 1 1) slip by lowering the antiphase energy and/or improving the cohesive strength of grain boundaries. This also facilitates enhancing the ductility of polycrystalline Ni3A1 specif- ically at cryogenic temperatures. The segregation of impurities to grain boundaries weakens them and promotes premature fracture be- fore macroscopic yield can occur. However, recent *Presentaddress: A. C. T. ConsultingInc., Canton, OH 44718, USA. 0022-2461 9 Chapman &Hag 5939