Conclusiones

The effect of cross-slip on the formation of extrusions (29)

has been demonstrated by Avery, Miller and Backofen who fatigued two single crystals of high purity copper. The orientations of these crystals were such that the Schmid resolved shear stress factors for the primary slip system were nearly alike, but those for the cross-slip system were different, one being'* 10 times the other. Both crystals were fatigued beyond saturation, and the extrusions produced on the surface were removed by electropolishing. After an additional 50 cycles, the crystal with the higher Schmid

f a c t o r o n th e cr o s s - s l i p s y s te m sh o w e d m uch h i g h er e x t r u s i o n s th an th e o t h er c r y s t a l wh en e x a m in e d b y a t a p e r s e c t i o n i n g t e chn i q u e . A ls o , th e h e i g h t o f t h e e x tr u s i o n s w a s g r e a t e r th an c o u ld b e a c c o u n te d f o r b y a m e ch an is m i n v o l v i n g s t e a d y e x t r u s i o n d e v e lo p m e n t . I t h as a l s o b e e n sh own th a t f a t i g u e f a i l u r e i s e x t r e m e l y d i f f i c u l t t o p r o d u c e i n m a t e r i a l s wh ic h h a v e a h i g h r e s i s t a n c e t o c r o s s - s l i p , e . g . z i n c s i n g l e c r y s t a l s f a t i g u e d a t - 1 9 6 ° C ^ ^ , l i th i u m f l u o r i d e a n d m a g n e s iu m ( 3 2 ) ( 3 3 ) o x id e . A ld e n J h as c o m p a re d d y n a m ic r e c o v e r y i n z i n c a n d c a d m iu m a t lo w t e m p e r a t u r e s a n d fo u n d th a t c a d m iu m e a s i l y s a t u r a t e s a t a b o u t 2 0 0 l b / s q . i n . , wh er e a s th e f a t i g u e h ar d e n in g c u r v e f o r z i n c i s e s s e n t i a l l y l i n e a r u p t o 3 0 0 0 1 b / s q . i n . B o th m e t a ls a r e CPH an d h a v e a pr i m a r y s l i p s y s te m o f ( 0 0 0 1 ) £ 1 2 1 0 ] , b u t c a d m iu m c r o s s s l i p s o n ( 1 0 1 1 ) {71210J wh er e a s z i n c d o e s n o t . H o w e v e r , i t i s p o s s i b l e t o p r o d u c e f a t i g u e f a i l u r e i n z i n c a t lo w t e m p e r a t u r e ( s e e s e c t i o n 2.1.3.3.). I t s e e m s , th er e f o r e , th a t t h e a b i l i ty t o c r o s s - s l i p i s e x t r e m e l y i m p o r t a n t i n a l l o w i n g s a t u r a t i o n , a n d h en c e f a t i g u e f a i l u r e t o o c c u r , s i n c e i t h as b e e n m e n t io n e d p r e v i o u s l y th a t f a t i g u e cr a c k s do n o t fo r m i n f a t i g u e s l i p b a n d s u n t i l s a t u r a t i o n h as o c c u r r e d . Th e f a c t t h a t cr o s s ­ s l i p c a n b e i m p o r t a n t i n th e d e v e lo p m en t o f a s u r f a c e n o t ch p e a k t o p o gr a phy a l s o l e n d s s u p p o r t t o th e v ie w t h a t cr o s s ­ es s l i p p l a y s a n e s s e n t i a l p a r t i n th e f a t i g u e f a i l u r e m e ch an is m o f m o s t m e t a l s . Th e e f f e c t o f r e d u c in g s t a c k i n g f a u l t e n e r g y , a n d h en c e i n h i b i t in g c r o s s - s l i p h as b e e n d e m o n s t r a t e d b y M c G r a th

and T h u r s t o n ^ ^ who compared Cu, Cu-15/oZn and Cu-35%Zn fatigued at the same stress level. They found that slip bands became thinner and fatigue life increased greatly as the concentration of zinc (and the resistance to cross-slip) was increased. However, when zinc is added to copper,

mechanical properties, e,g. yield stress and U,T,S, are also (35)

altered. Later work' 7 has compared Cu-Al and Cu-Ni alloys. Aluminium and nickel additions have similar effects on the mechanical properties of copper but aluminium produces a greater reduction in stacking fault energy, and was shown to produce the better fatigue properties,

2.1.2.5, Pore Formation

Wood, Cousland and S a r g a n t ^ ^ used a taper sectioning technique on copper and 70/30 brass, and observed pore

formation on slip planes. Later during the tests, these pores linked Up to form fissures which were the embryo fatigue cracks. In another p a p e r b l o c k s of metal were seen to be loosened from the surface. Intrusions and extrusions were considered to be side effects which could not account for the primary source of damage, even though slip band cracks often seemed to be associated with surface extrusions.

(37)

Dover and Jones 7 used a technique of successive sectioning to build up a three-dimensional picture of the features near the surface in fatigued copper. The major conclusion of their work was that as successive surface layers were polished away, no new pores or cracks appeared below the surface. It was considered that the "pores" observed by Wood et al were in fact intrusions, which appeared to be pores because of Wood’s taper sectioning

technique. Irregular microcracks were seen linking up the slip band cracks below the surface. The slip band cracks had an irregular crack front, and split up into a series of tubular holes before disappearing.