EFFECT OF Al-RICH OFF-STOICHIOMETRY ON THE YIELD STRESS OF BINARY Ni 3 Al SINGLE CRYSTALS D. GOLBERG{, M. DEMURA and T. HIRANO National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki, 305, Japan (Received 15 August 1997; accepted 19 November 1997) AbstractÐStoichiometric binary single-phase Ni 3 Al single crystals (Ni±25 at.%Al) and Al-rich single crys- tal-like Ni 3 Al alloys (Ni±26 at.%Al, Ni±27 at.%Al) have been successfully produced using the ¯oating zone method (FZ). The Al-rich crystals consisted of single crystalline Ni 3 Al matrix and NiAl precipitated phase. The compressive yield stress of the as-grown crystals was measured at 293±1273 K for loading axes between [001] and [011]. All the crystals exhibited the yield stress anomaly. Compared to Al-rich crystals, stoichiometric Ni 3 Al exhibited higher peak temperature of the yield stress anomaly, lower yield stress below the peak temperature, but higher yield stress above the peak temperature. These features are thought to result from the combined eect of Ni 3 Al o-stoichiometry and NiAl second phase on the yield stress. The critical resolved shear stress (CRSS) on primary octahedral (111)[1 01] slip system in the yield stress anomaly region was found to be orientation-independent for stoichiometric Ni 3 Al, but strongly orien- tation-dependent in Al-rich crystals. Thus, the Schmid law is obeyed in stoichiometric Ni 3 Al single crystals, whereas it is violated in o-stoichiometric crystals. # 1998 Acta Metallurgica Inc. 1. INTRODUCTION Ni 3 Al shows anomalously positive temperature dependence of the yield stress [1, 2]. Since o-stoi- chiometric Ni 3 Al exists on both sides of the stoi- chiometric composition [3], it can be expected that mechanical properties depend on the alloy stoichi- ometry. So far, there have been only a few studies on the eect of Ni 3 Al o-stoichiometry on the mechanical properties at various temperatures [4±7]. In addition, all of them were carried out on poly- crystalline specimens because of diculty in grow- ing binary single crystals. This diculty is due to the peritectic reaction during solidi®cation, liquid + NiAl 4 Ni 3 Al, which easily causes grain nucleation. It is dicult to suppress this nucleation by the Bridgman technique. Thus, all the single crystals grown to date have contained ternary ad- ditions or have deviated from stoichiometry towards the Ni-rich side in order to avoid the peri- tectic reaction [8±17]. Therefore, the mechanical properties of binary stoichiometric Ni 3 Al single crystals and the eect of o-stoichiometry on the yield stress have never been elucidated. Compared to the Bridgman technique, the ¯oat- ing zone (FZ) method has an advantage in single crystal growth of incongruent compounds, such as Ni 3 Al. We have undertaken systematic work on unidirectional solidi®cation of binary Ni 3 Al using this method [18±20]. Normally, the grown alloys have columnar-grained structures with a high per- centage of low angle and low S boundaries [21]. Recently, we have found that large single crystals of binary stoichiometric Ni 3 Al can be grown occasion- ally by strictly controlling the dimensions of the ¯oating zone [22±24]. In this study we applied this method to compare the mechanical properties of stoichiometric and Al-rich crystals. The present paper reports the eect of Al-rich o-stoichiometry on the compressive yield stress between 293±1273 K for loading axes between [001] and [011]. 2. EXPERIMENTAL Stoichiometric (Ni±25Al, hereafter at.%) and Al- rich (Ni±26Al, Ni±27Al) alloys were arc-melted using 99.9999% Al and 99.97% Ni. All ingots were remelted 8±10 times to achieve chemical homogen- eity and cast into rod-shape ingots to prepare a starting material for single crystal growth. The FZ procedure was the same as we previously reported [18]. During the growth the seed and the feed rods were rotated at 24 rpm in opposite direc- tions to reduce the temperature gradient and to achieve forced convection in the molten zone. Crystals of length 160±170 mm and diameter 10± 12 mm were grown at the growth rate of 25 mm/h. Longitudinal and transverse sections of the as- grown crystals were examined on the as-polished and etched surfaces with an optical microscope. Marble reagent (2.5 g CuSO 4 +25 cm 3 HCl + 30 cm 3 H 2 O) was used for etching. The Al-content was determined by the wet chemical method in the central parts of the crystals. The results agreed Acta mater. Vol. 46, No. 8, pp. 2695±2703, 1998 # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 1359-6454/98 $19.00 + 0.00 PII: S1359-6454(97)00476-X {Address for correspondence: National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki, 305, Japan. 2695