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in the h o l d e r and a l l o w e d to r e a c h t h e r m a l e q u i l i b r i u m at s o m e t e m p e r a t u r e T, a f t e r which t h e y w e r e i n s t a n t l y " q u e n c h e d " into the liquid n i t r o g e n in the i n n e r d e w a r . Once e q u i l i b r i u m was r e a c h e d at - 196~ the change in l e v e l of the liquid n i t r o g e n in the i n n e r d e w a r was u s e d to c a l c u l a t e the h e a t a b s o r b e d , Q, noting that the h e a t of v a p o r i z a t i o n of liquid n i t r o g e n is 1.61 • 108 J / m 3 (38.56 c a l / c m 3 ) . F o r a given s p e c i m e n , c a l i b r a t i o n is c a r r i e d out a f t e r p r e c o o l i n g to - 196~ in which c a s e the change in n i t r o g e n l e v e l due to the s p e c i m e n v o l u m e alone i s d e t e r m i n e d . F o l l o w i n g the a b o v e p r o c e d u r e , a plot of Q v s . t e m p e r a t u r e was m a d e which showed a d i s c o n t i n u i t y o v e r the t e m p e r a t u r e i n t e r v a l of the a u s t e n i t e - m a r t e n s i t e t r a n s f o r m a t i o n . R e s u l t s f o r the o r d e r e d F e - P t a l l o y a r e shown in F i g . 2. The m a g n i t u d e of the d i s c o n t i n u i t y is a d i r e c t m e a s u r e of the t r a n s f o r m a t i o n l a t e n t heat, AQ, and the s l o p e s of the low and high t e m p e r a t u r e p o r t i o n s d e t e r m i n e the h e a t c a p a c i t y of the two p h a s e s , a s shown in Fig. 2. F o r the d i s o r d e r e d a l l o y the l a t e n t h e a t was found to b e 2640 J / K g (570 c a l / mole), in m a r k e d c o n t r a s t to the v a l u e of 385 J / K g (83 c a l / m o l e ) found f o r the o r d e r e d a l l o y . The r e a s o n for this difference is considered elsewhere. 2 This w o r k was s u p p o r t e d in p a r t b y the U. S. A r m y R e s e a r c h Office (Durham) and the A t o m i c E n e r g y C o m m i s s i o n , through the M a t e r i a l s R e s e a r c h L a b o r a t o r y at the U n i v e r s i t y of I l l i n o i s . 1. D. P. Dunne and C. M. Wayman: Met. Trans., 1973, vol. 4, pp. 137 and 147. 2. H. C. Tong and C. M. Wayman: unpublished research, University of Illinois, 1974.

a n a l y s i s of f r a c t u r e , a l s o c o n t a i n s a t h r e s h o l d c o n d i tion a s an i m p l i c i t c o n s e q u e n c e . The b a s i c a s s u m p t i o n of the m o d e l is that the c r a c k a d v a n c e p e r c y c l e , da/dN, is p o s t u l a t e d to extend o v e r a d i s t a n c e o v e r which the normal stress or strain exceeds a certain critical v a l u e , tTFF o r E F F . * This l e a d s to an e x p r e s s i o n f o r *Such a critical normal stress failure criterion is supported by recent observations of intergranular separation s6,6s'~ or cleavage type features s7 at low stress amplitudes.

incrementai crack advance per cycle,

da dN

(

ax ] "F + 1

- ""

[i]

w)-p*/2

w h e r e &o N is the a p p l i e d s t r e s s a m p l i t u d e , w is the width of the s p e c i m e n , and f ( a , w) is