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I. INTRODUCTION The anomalous increase of flow stress with increasing temperature has been observed in both single crystals and polycrystals of Ni3Al and Ni3Al-based compounds.1'2 Many theories have been introduced to rationalize this unusual mechanical behavior, and they may be classified into two groups3: (a) cross-slip models and (b) other models based on dislocation intersections. The prevailing cross-slip-pinning (CSP) model4"7 has been successful in explaining not only the positive temperature dependence of the critical resolved shear stress (CRSS) for the (111) [TOI ] primary slip, but also the orientation effect of the stress axis and the tension/ compression asymmetry observed in single crystals. According to the CSP model, cross-slipped "segments," in the form of a sessile configuration on the (010) plane,8 statistically distributed along the leading screw superpartial dislocation, act as the pinning points on it while gliding on the (111) plane,4 as shown schematically in Fig. 1 (a). The direct observation by in situ deformation in a transmission electron microscope (TEM) 9 revealed the apparent pinning action that is consistent with the CSP model. Some of the recent weak-beam TEM observations1011 indicated, however, that the so-called Kear-Wilsdorf (KW) "locks" proposed in the original cross-slip model12 seem to be the main barrier for the primary slip dislocations at room and intermediate temperatures. The formation of a KW lock by cross-slip and double cross-slip processes is described in Fig. 1 (b). The issue here is the nature of the pinning points: whether the extent of cross-slipping is on the order of the magnitude of Burgers vector,6'8 or is it much larger as is the case in the normal cross-slip process. The purpose of this article is to resolve the abovementioned issue by examining the effect of prestress at elevated temperatures on the tensile yield strength of Ni 3 Al at room temperature. It is expected that during the proposed prestressing and quenching experiment, J. Mater. Res. 3 (5), Sep/Oct 1988

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the number of cross-slipped segments is reversible because it is a function only of temperature and the stress state. On the other hand, if KW locks are developed in a sufficient number to cause the anomalous yield behavior, the room temperature yield strength measured after the prestressing and quenching should show some increase since the KW lock, as depicted in Fig. l(b), is considered to be an irreversible configuration. The experimental results of this work, which are consistent with the CSP model, will be discussed together with those available in the literature.

II. EXPERIMENTAL A Ni3Al alloy containing 23 at. % Al, 0.5 at. % Hf, and 0.1 at. % B, designated as IC-136, was prepared by arc melting and drop casting, using pure nickel and aluminum metals and Ni-4 wt. % B master alloy. The ingot was homogenized for 5 h at 1000 °C and then fabricated into 0.76 mm-thick sheet by repeated cold rolling with intermediate annealing at 1050 °C. Tensile specimens with a