No abstract

No abstract

Slip in Ni3Al takes place primarily upon close-packed <111> planes by the motion of paired a/2 dislocations, although slip can also be initiated upon cube planes at temperatures above 700°K (1,2). Because the antiphase boundary energy is at a minimum on <001> (3), a net reduction of energy is possible if such paired dislocations cross-slip from octahedral onto cube planes. The likelihood of this cross-slip event plays an important role in the theories on the flow stress of Ni3Al (1,2). Observations reported previously (h) demonstrate that cross-slip takes place at 1030°K. This investigation represents a more thorough exploration of its temperature range of occurrence.

The formation of stacking faults upon aging a solution treated and quenched nickel-base alloy has been reported previously. More recent work has revealed a similar behavior in several γ’ precipitation hardened nickel-base alloys aged in the temperature range l400°-1650°F. The faults intersect both γ (nickel solid solution) and γ’ (Ni3Al solid solution) phases, and are of extrinsic type. Fault formation is attributed to a chemical segregation effect involving the climb of Frank partials, similar to that reported for austenitic steels.Fig. 1 shows extrinsic faults associated with a slip band in Udimet 700 lightly deformed at 70°F prior to aging. For the specified [011] normal to the foil, the slip trace is (11), and the faults lie in (111) and (11). Using the g · b = 0 or ± 1/3 (s = 0) criterion for dislocation invisibility, Figs, 1 (a-c) show that the outer partial for a (111) fault must be 1/6 [11] Shockley, or 1/3 [111] Frank.

Shear of Ni3Al crystals usually takes place by the motion of paired a/2<110> dislocations. The pairs are strongly coupled by an antiphase boundary such that their separation is less than 40Å. These are imaged as single dislocations having an a<110> Burgers vector. Thus g.b equals 2 for “in contrast“ imaging with lower order reflections such as g = 111 or 200. The dislocation image contrast under such conditions is shown in Fig. 1 for varying deviations w = sξg from the Bragg orientation. This dislocation was shown by the usual method of identification to have a Burgers vector b = a [011] and, other than the screw segment A, it is mixed in character. (The accurate straightness of the screw segment is due to relaxation of the paired dislocations upon (100). For this reason its image is not representative of that for g.b = 2).