Fatigue crack propagation resistance of beta-annealed Ti-6AI-4V alloys of differing interstitial oxygen contents
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INTRODUCTION
MATERIALS
THOUGH it is well known that interstitial oxygen can markedly affect the fracture toughness and uniaxial tensile properties of titanium alloys, the influence of oxygen content on fatigue crack propagation resistance in these alloys is poorly understood. Moreover, data available on this subject appear to be in disagreement. 1-4 For example, a decrease in fatigue crack propagation rates with increased oxygen content was reported for commercially pure s-titanium alloys. ~ On the other hand, an increase in fatigue crack propagation rates with increased oxygen was reported for titanium-oxygen alloys, 2 as well as for recrystallization annealed Ti-6AI-4V. ~ Further mixed results were obtained recently within a single study of Ti6AI-4V, 4 in the r e c r y s t a l l i z a t i o n - a n n e a l e d and b e t a quenched conditions, with s o m e e v i d e n c e f o r an inc r e a s e and o t h e r e v i d e n c e f o r a d e c r e a s e in growth r a t e s with i n c r e a s e d oxygen. To the a u t h o r s ' knowledge, h o w e v e r , no s y s t e m a t i c study of the effect of oxygen content has b e e n made f o r the b e t a - a n n e a l e d , Widmanst~itten m i c r o s t r u c t u r e , which has b e e n r e l a t e d to s u p e r i o r fatigue c r a c k p r o p a g at i o n r e s i s t a n c e in T i - 6 A 1 - 4 V . ~,s'~ A c c o r d i n g l y , the p u r p o s e of the p r e s e n t w o r k is to e x a m i n e fatigue c r a c k p r o p a g a t i o n b e h a v i o r in four b e t a - a n n e a l e d T i - 6 A 1 - 4 V p l a t e s with r e s p e c t i v e oxygen contents of 0.06, 0.11, 0.18 and 0.20 wt pct. F o r the alloy with 0.20 wt pct oxygen, it was r e p o r t e d in a p r e c e d i n g p a p e r s that fatigue c r a c k growth r a t e s f o r the a/fir o l l e d , m i l l - a n n e a l e d condition can be r e d u c e d by as m u c h as an o r d e r of magnitude with a b e t a anneal, owing p r i m a r i l y to a t r a n s i t i o n to s t r u c t u r e - s e n s i t i v e c r a c k growth in the W i d m a n s t ~ t t e n m i c r o s t r u c t u r e . It was found that the t r a n s i t i o n c o r r e s p o n d s to the point at which the r e v e r s e d p l a s t i c zone attains the a v e r a g e W i d m a n s t ~ t t e n p a c k e t s i z e , with the r e d u c tion in growth r a t e s below the t r a n s i t i o n a t t r i b u t a b l e to c r y s t a l l o g r a p h i c b i f u r c a t i o n in the Widmanst~itten packets. G. R. YODER and L. A. COOLEY are Metallurgists and T.W. CROOKER is Head, Fatigue Criteria Section, Metals Performance Branch, Material Science and Technology Division, Naval Research Laboratory, Washington, DC 20375. Manuscript submitted August 23, 1977. METALLURGICAL TRANSACTIONS A
AND PROCEDURES
The alloys studied were received in the form of rolled plate, with chemical analyses as given in Table I. Each alloy was subjected to the following beta anneal: ~ 0.5 h at 1311 K, cooled to RT + 2 h at 1005 K, cooled to RT. This heat treatment was performed in a vacuum
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