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Table I. Lattice Parameters of Orthorhombic Martensite

Alloy

a

Lattice Parameters, A b

c

Ti-4.0 Mo Ti-4.25 Mo- 0.25 Si Ti-4.0W

3.001 3.011 3.000

4.998 4.970 4.996

4.657 4.651 4.655

Tempering Behavior of Orthorhombic Martensite in Titanium Alloys J A M E S C. WILLIAMS AND BRIAN S. HICKMAN

IN

1958 B a g a r i a t s k i i e t a l . z r e p o r t e d that with i n c r e a s ing a l l o y content in a n u m b e r of quenched b i n a r y t i t a nium a l l o y s the n o r m a l l y o b s e r v e d hexagonal m a r t e n s i t e d i s t o r t e d to an o r t h o r h o m b i c s t r u c t u r e . They o b s e r v e d t h i s b e h a v i o r in T i - M o , T i - N b , Ti-W, T i - T a , and T i - R e s y s t e m s and s h o w e d that the o r t h o r h o m b i c d i s t o r t i o n i n c r e a s e d with i n c r e a s i n g a l l o y content. This p h a s e was s u b s e q u e n t l y r e p o r t e d by Brown e t a l . z in T i - N b a l l o y s who showed that it had a s t r u c t u r e s i m i l a r to t h a t of t~-U (Space Group Cmcm ) and that it was d e r i v e d f r o m a d i s t o r t e d h e x a g o n a l c e l l in which the c a x i s of the o r t h o r h o m b i c c e l l c o r r e s p o n d e d to the c a x i s of the hexagonal c e l l with the a and b axes c o r r e sponding to the o r t h o g o n a l a x e s of the hexagonal c e l l . In g e n e r a l , the p o s s i b i l i t y of the o c c u r r e n c e of t h i s s t r u c t u r e has been n e g l e c t e d by o t h e r w o r k e r s in this field. In the c o u r s e of a study of the o c c u r r e n c e , m o r phology, and c r y s t a l l o g r a p h y of the o r t h o r h o m b i c m a r t e n s i t e u n d e r t a k e n in this l a b o r a t o r y , s we have shown that it o c c u r s in a wide v a r i e t y of b i n a r y and t e r n a r y a l l o y s and a l s o in at l e a s t one c o m m e r c i a l t i t a n i u m alloy, n a m e l y T i - 6 A l - 2 S n - 4 Z r - 6 M o . 4 D u r i n g the c o u r s e of this study we have found that the t e m p e r i n g b e h a v i o r of t h i s type of m a r t e n s i t e is s t r i k i n g l y d i f f e r e n t f r o m that r e p o r t e d for h e x a g o n a l m a r t e n s i t e and it is the p u r p o s e of this note to d e s c r i b e and c o m m e n t on this t e m p e r i n g b e h a v i o r . The t e m p e r i n g study was done on t h r e e a l l o y s , n a m e l y T i - 4 at. pet Mo, T i - 4 . 2 5 at. p e t M o - 0 . 2 5 at. pet Si, (this a l l o y was used b e c a u s e in a n o t h e r i n v e s t i g a t i o n it was found that the addition of s i l i c o n tended to s t a b i l i z e the m i c r o s t r u c t u r e a g a i n s t r e v e r s i o n and s p o n t a n e o u s t r a n s f o r m a t i o n s in thin foil e l e c t r o n m i c r o s c o p y s a m p l e s ) and T i - 4 at. p c t W. The a l l o y s w e r e p r e p a r e d by s t a n d a r d a r c m e l t i n g t e c h n i q u e s , s e a l e d in e v a c u a t e d s i l i c a v i a l s , and h o m o g e n i z e d at 1000~ f o r ~ 1