Stress Anisotropy of Stoichiometric NiAl Single Crystals

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STRESS ANISOTROPY OF STOICHIOMETRIC NiAI SINGLE CRYSTALS T. Takasugi, J. Kishino and S. Hanada Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980, Japan.

ABSTRACT The yield stress properties and the associated slip systems of stoichiometric NiAI single crystals were investigated in terms of crystal orientation, temperature and the deformation mode. The CRSS was, in a wide range of experimental conditions, higher in the sequence of {110}, {100) and (hkO slips. In all the crystal orientations studied, the CRSS in compression was higher than the CRSS in tension particularly at low temperatures. The tension-compression asymmetry on the CRSS was understood qualitatively as being due to the effect of the normal stress on the core structure of a dislocation.

INTRODUCTION In the B2-type NiAl which possesses a high ordering energy and deforms by slip, it is generally believed that the dislocation is not dissociated into any (super-) partials. However, some indications showing the strength anomaly, which has been generally postulated to be due to the dissociated dislocations, can be found. For example, the variation of the flow stress with temperature in the NiAI single crystals was not so monotonous at intermediate temperatures and also was very much dependent on the crystal orientation. Also, the NiAI single crystals exhibited significant elongation at ambient temperature by the addition of a small amount of Fe, Mo and Ga [1]. In addition, it was shown in the NiAI polycrystals that an asymmetry in the yield stress was shown with the compressive yield stress being higher than the tensile stress [2]. These results may be associated with the characteristic of the dislocation. In this study, the yield stresses of fairly stoichiometric NiAl single crystals were measured in terms of crystal orientation, temperature and the deformation mode whether tension or compression. Also, the activated slip systems were analyzed in the single crystals with various orientations. Based on these results, it is proposed that the yield stress anomaly observed in this study can be associated with the core effect of the dislocation.

EXPERIMENTAL PROCEDURES The just-stoichiometric NiAl single crystals used in this study were prepared by the Bridgman technique in the induction furnace using an alumina crucible. The analyzed chemical composition indicates that the NiAI single crystals were fairly stoichiometric (the deviation from stoichiometry was less than 0.4 at%). Both the tensile and the compressive specimens were taken from the same single crystal rod. The tensile specimens with lx2x13 mm gage dimensions and the compressive specimens with dimensions of approximately 2x2x5 mm were prepared in desired orientations (i.e. [001], [I I 1], [0 11 ] and [123]) using a wire-slitting and an electron discharge machine. Here, it is however noted that the compressive specimens with orientations [001] and [011] were deviated from their exact positions in a standard stereographic unit triangle. The faces of the tensile