Scripta Materialia, Vol. 34, No. 12, pp. 1925-1930, 1996 Elsevier Science Ltd PII S1359-6462(96)00070-X Copyright Q 1996 Acta Metallurgica Inc. Printed in the USA. All rights resewed 1359~6462/96 $12.00 + .OO HIGH TENSILE ELONGATION OF p-NiAl SINGLE CRYSTALS AT 293 K V.I. Levit, I.A. Bul, J. Hu and M. J. Kaufman Department of Materials Science and Engineering, University of Florida, Gainesville, FL 3261 l-6400 (Received December 27,1995) (Revised January 26,1996) Introduction It is well established that both polycrystals and (“hard”) <OOl> oriented single crystals of NiAl show practically no ductility at room temperature (l-3). For “soft” (non-<OOl>) oriented single crystals, the elongations at ro,om temperature typically do not exceed l-2% (45). Higher room temperature ductilities have been achieved using various schemes such as heat treatment, microalloying, etc. (l-l 0). In one of the more controversial papers, Darolia et al. (1) presented single crystal data suggesting that small (< 1%) additions of iron, gallium or molybdenum enhance the room temperature tensile ductility of ~11 O> single crystals to 5-6%,; this spurned a large amount of work around the world aimed at elucidating the mech- anism(s) resporrsible for this effect. While there has been no apparent independent confirmation of this unexpected ductilization effect, various investigators have suggested and, in some cases, shown that the following factors are of extreme importance to the mechanical behavior of this compound: impurity con- tent, heat treatment (thermal vacancies), constitutional defects (due to deviations from stoichiometry), orientation, prestraining and surface condition (1-12). For example, prestraining at temperatures ranging from RT to lOOOK has been reported to increase the fracture toughness (6,l l), tensile ductility and yield stress (8) while pressurization led to a decreased yield stress (12) presumably by introducing new mobile dislocations and/or by unpinning existing ones. In a recent study, Noebe et al. (7) noted that, out of six specimens from a stoichiometric crystal, one exhibited 12.9% elongation whereas the other five had elongations between 1 and 2.4%. The purpose of this note is to illustrate that it is possible to achieve high (- 25%) RT tensile elonga- tions by optimiving these factors and to shed further light on the overall behavior of NiAl single crystals in soft orientations. ExDerimental Procedure A NiAl single crystal (28 mm diameter by 75 mm length) was grown in a high purity alumina crucible by the Bridgman technique in an atmosphere of purified argon. The starting charge was in the form of small ingots prepared by non-consumable arc melting. Care was taken to use high purity starting materials (99.98 Ni and 9!>.999 Al) and the chemical analysis of the crystal indicated that the purity was high and that the Ni/Al ratio was near the stoichiometric composition (Table 1). After growth, the crystal was annealed at 1573 K for 3 h and cooled inside the furnace at - 0.1 K/s. The orientation of the single crystal 1925