The effect of orientation and sense of applied uniaxial stress on the morphology of coherent gamma prime precipitates in
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sibility of Y' shape change due to creep deformation. I Accordingly, the stress annealing was carried out at 22.5 ksi, 1750~ (about 0.8 of the absolute solidus temperature) and I00 hr of exposure so that the creep deformation of the specimen was less than I pct. These conditions were employed throughout this investigation, with all parameters held within 5 pct of their assigned values. In order to establish the equilibrium volume fraction of 7' at the test temperature, all specimens were soaked for 16 hr prior to loading. Both the tension and the compression specimens were subjected to a constant load. The resultant Y' morphology of each specimen was determined by sectioning on three orthogonal crystallographicplanes. The sectioned and polished specimens were then electroetched with a solution that contained: 87 P c t CH3OH, 8 pct H2SO4, 3 pct HNO3, and 2 pct Hr. A s t a n d a r d t w o - s t a g e r e p l i c a was then taken of the p r e p a r e d s u r f a c e s .
EXPERIMENTAL RESULTS The effects of and oriented stress on the coarsening morphology of ~' precipitates in Udimet700 single crystals will be established by comparing the microstructures obtained in the absence and presence of applied stress, under the same annealing conditions. A comparison will also be made with the previously determined oriented crystals stress annealed in tension and compression are shown in Figs. 4 and 5. Finally, the stress annealed morphology for , Fig. 6, neither tensile nor compressive stress annealing results in shape changes of the initial 7' cuboids; the resultant morphology is similar to crystals annealed in the absence of applied stress. A common feature of all the Y' morphologies, annealed with or without applied s t r e s s , is that the 7 ' p r e c i p i t a t e s a r e (100> o r i e n t e d . Fig. 7 s u m m a r i z e s the effects of s t r e s s o r i e n t a t i o n and s t r e s s s e n s e on the s t r e s s a n n e a l e d shape of the :~' p r e c i p i t a t e s , i n c l u d i n g the p r e v i o u s l y obtained (100) r e s u l t s . The dotted cubes in this f i g u r e show the r e l a t i o n s h i p between the i n i t i a l l y a l i g n e d cuboids and the (100>, (110}, and 0, i . e . , the c r y s t a l extends p a r a l l e l to (1002. F o r t e n s i l e loading (aA > 0) this r e s u l t s in a d e c r e a s e in f r e e e n e r g y , which a c c o r d i n g to Eq. [21] i s equal to (2/3)OAAr If we a s s u m e that v = v' we can e v a l u a t e AFB for t r a n s f o r m a t i o n s f r o m cuboids to the o t h e r p r e c i p i t a t e m o r p h o l o g i e s allowed by c o h e r e n c y s t r a i n s by applying Eqs. [10], [ 12a], [12b], [17], and [21]. The r e s u l t s of this e v a l u a t i o n and t h e i r c o m p a r i s o n with our e x p e r i m e n t a l o b s e r v a t i o n s a r e s u m m a r i z e d in T a b l e I for , and (111} s t r e s s a x e s , and both s e n s e s of s t r e s s . In Table I, the ~' m o r p h o l o g i e s a r e d e s c r i b e d by giving the o r i e n t a t i o
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