The non-cubic lattice of rapidly quenched packet martensite
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A N D P. G. W I N C H E L L
The positions of 200, 020 and 002 peaks of f e r r o u s m a r t e n s i t e have been m e a s u r e d individually by a p r e c i s i o n X - r a y d i f f r a c t o m e t e r technique. Martensite with 18 wt pct Ni and 0.1, 0.2, 0.3,and 0.4 wt pct C was f o r m e d by s a l t - w a t e r quenching from p r e o r i e n t e d austenite s i n g l e - c r y s t a l s l i c e s about 0.5 mm thick. The v a r i a t i o n s in X - r a y peak positions i n t e r p r e ted as changes in l a t t i c e p a r a m e t e r yield to the f i r s t approximation: A a / a o = - 0.005 [wt pet C], A b / a o = - 0.016 [wt pct C], A c / a o = - 0.037 [wt pet C]
for carbon contents between 0.1 and 0.4. F o r such carbon contents the m a r t e n s i t e has packet or mixed p a c k e t - l e n t i c u l a r morphology; consequently packet m a r t e n s i t e is not cubic as s o m e t i m e s claimed. Instead its lattice is n e a r l y like that of l e n t i c u l a r m a r t e n s i t e with t h e " a " and " c " p a r a m e t e r s v a r y i n g with carbon content as p r e v i o u s l y o b s e r v e d for t e t ragonal m a r t e n s i t e , and the " b " p a r a m e t e r unequal to the " a " . The significance of the inequality of the " a " and " b " p a r a m e t e r s is unclear, but that inequality a p p e a r s to be c h a r a c t e r i s t i c of some plate and l e n t i c u l a r m a r t e n s i t e .
F E R R O U S m a r t e n s i t e f o r m e d at high t e m p e r a t u r e s is l a t h - s h a p e d with p a r a l l e l laths c l u s t e r e d in packets, and lower t e m p e r a t u r e m a r t e n s i t e is plate shaped.' Fig. 1, due in e s s e n c e to Magee and Davies, ~ shows the r e l a t i o n s h i p of morphology to composition in i r o n nickel carbon m a r t e n s i t e s . Several r e p o r t s have indicated that the packet m a r t e n s i t e is cubic even when the alloy contains as much as 0.5 pct C (by weight*). ~'2 *All compositions are in wt pct.
Evidence for cubic s y m m e t r y of such m a r t e n s i t e has been obtained f r o m X - r a y diffraction p a t t e r n s f r o m r a n d o m l y oriented p o l y c r y s t a l s . In these p a t t e r n s , the t e t r a g o n a l i t y d e t e r m i n a t i o n is b a s e d on the resolution of two c l o s e l y spaced r a t h e r b r o a d Bragg peaks. Their r e s o l u t i o n or lack of r e s o l u t i o n is often v e r y difficult to o b s e r v e . This difficulty is n a t u r a l l y i n c r e a s e d when another s t r u c t u r e of i n t e r m e d i a t e t e t r a g o n a l i t y or an o r t h o r h o m b i c l a t t i c e o c c u r s producing an intense t h i r d line between the two that were supposed p r e s e n t under the assumption of s i m p l e t e t r a g o n a l i t y . E x p e r i m e n t a l o b s e r v a t i o n of such b r o a d s u p e r i m p o s e d powder p a t t e r n peaks could r e s u l t in the false conclusion that the l a t t i c e is cubic. Since t e m p e r i n g after quenching is known t
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