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Large single crystal segments of aluminum were produced by strain anneal, deformed in tension, and s e c t i o n e d to produce t e n s i l e s p e c i m e n s with a x e s p a r a l l e l to the o r i g i n a l t e n s i l e axis and with o t h e r o r i e n t a t i o n s . The s p e c i m e n s w e r e a n n e a l e d to produce a polygonized s u b s t r u c ture. T h e i r c r i t i c a l s h e a r s t r e s s in t e n s i o n was d e t e r m i n e d . The c r i t i c a l s h e a r s t r e s s was shown to be the sum of a s u b s t r u c t u r e independent s t r e s s and a s t r e s s p r o p o r t i o n a l to the s q u a r e root of the a v e r a g e p r i m a r y subboundary m i s o r i e n t a t i o n o v e r the a v e r a g e p r i m a r y subboundary spacing. C o m p a r i s o n of the c r i t i c a l s h e a r s t r e s s of s p e c i m e n s cut f r o m the s a m e p a r e n t c r y s t a l with v a r y i n g t e n s i l e a x e s d e m o n s t r a t e d that the significant subboundary s p a c i n g i s that between p r i m a r y s u b b o u n d a r i e s along the a c t i v e s l i p plane of the s p e c i m e n . The t e s t s a l s o showed that a v e r a g e subboundary m i s o r i e n t a t i o n i s significant b e c a u s e it r e p r e s e n t s the a v e r a g e s p a c i n g of d i s l o c a t i o n s in the s u b b o u n d a r i e s . The r e s u l t s , i n t e r p r e t e d in t e r m s of c u r rent s u b s t r u c t u r e s t r e n g t h e n i n g t h e o r i e s , indicate that s l i p is p r o p a g a t e d a c r o s s p r i m a r y subb o u n d a r i e s by the a c t i v a t i o n of d i s l o c a t i o n s o u r c e s in the s u b b o u n d a r i e s .

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influence of s u b s t r u c t u r e on the m e c h a n i c a l p r o p e r t i e s of a l u m i n u m has been the subject of a n u m b e r of i n v e s t i g a t i o n s . 1-6 The r e s u l t s of these i n v e s t i g a t i o n s , h o w e v e r , have indicated that an a m b i g u i t y e x i s t s in i n t e r p r e t i n g the r e l a t i o n s h i p between s u b s t r u c ture c h a r a c t e r i s t i c s and m e c h a n i c a l p r o p e r t i e s . E x p e r i m e n t a l r e s u l t s have been i n t e r p r e t e d to show that the significant v a r i a b l e is a v e r a g e subboundary m i s o r i e n t a t i o n , a v e r a g e subboundary spacing, o r the d i s l o c a t i o n density a r i s i n g f r o m the s u b s t r u c t u r e . The m o s t significant p r o b l e m s which have been e n c o u n t e r e d during p r e v i o u s i n v e s t i g a t i o n s a r e : unc e r t a i n t y as to the c o m p o s i t i o n and s t r u c t u r e of the s u b b o u n d a r i e s , d i f f i c u l t i e s in obtaining a c c u r a t e m e a s u r e m e n t s of subboundary m i s o r i e n t a t i o n and s p a c i n g , the p r e s e n c e of s e v e r a l types of s u b b o u n d a r i e s , and l i m i t e d knowledge of the s i g n i f i c a n c e of subboundary orientation. The substructure may be controlled b