On the unstable shear in fatigued β-annealed Ti-11 and IMI-685 alloys

The m i c r o s t r u c t u r e s r e su l t i ng f rom /3-processing or /3-annealing of a +/3 t i t an ium al loys have r ecen t ly been cons idered for room and elevated t e m p e r a t u r e appl ica t ions of ae rospace components . These m i c r o s t r u c t u r e s offer improved creep, 1 f r ac tu r e toughness and fatigue c rack propagat ion e p rope r t i e s over those of a 4,13 p roces sed and heat t r ea ted m i c r o s t r u c t u r e s . The potent ia l cost reduc t ion due to lower p r e s s load r e q u i r e m e n t s and be t t e r shape def in i t ions s'a make the m i c r o s t r u c t u r e s r e su l t i ng f rom s u p e r t r a n s u s forging an a t t r ac t ive choice for many eng inee r ing appl ica t ions . When cooled f rom above the /3transus t empe ra tu r e , the /3-phase t r a n s f o r m s into ~ -p l a t e l e t s separa ted by f i lms of/3. Between the a ma t r ix and the /3 f i lms , e lect ron mic roscopy r evea l s an in te r face phase, the na ture Of which was invest igated by Rhodes and Will iams.S The a p la te le t s can be a r r a n g e d in a basketweave mor phology 6 or in packets of s i m i l a r l y a l igned p la te le t s cal led colonies. The mean in te rcep t d i ame te r of the colonies may va ry in s ize f rom 0.01 to 0.25 mm. The tendency to t r a n s f o r m into large colonies of c~-platelets will i n c r e a s e with the dec rea se in the content of/3 s tab i l i z ing e lements , with the i n c r e a s e of the /3-solut ion t r e a t m e n t t e m p e r a t u r e and t ime, with the dec rea se of the amount of /3-work and with d e c r e a s i n g cooling r a t e s . The re fo re the la rge colony s t r uc tu r e is more l ikely to occur in th ick sec t ions of fi-worked or /3-annealed near -a t t i t an ium al loys like Ti-112 and IMI6857 (Fig. 1). An inves t iga t ion of the p r o p e r t y m i c r o s t r u c l u r e r e l a t i onsh ips in these two al loys showed that the s ize and the c rys ta l lograph ic o r ien ta t ion of the colon ies a re the con t ro l l ing e l emen t s in d e t e r m i n i n g prope r t i e s such as fatigue life and fatigue c rack growth ra te . The fatigue c racks in this m i c r o s t r u c t u r e tend to in i t ia te and propagate in s t ra igh t l ines a c r o s s the pla te le t colonies as shown in Fig. 1 for an IMI-685 fatigue c rack growth spec imen . This type of c rack ing is r e l a t ed to the in tense shear bands observed in the same spec imens ac ros s the colonies . 2'7 An inves t igat ion of fatigue c rack propagat ion in ex t r eme ly large colonies of T i l l s showed that in tense shear bands always precede the a c r o s s the colony fatigue f r a c t u r e . Unstable shear r e l a t ed f r ac tu r e s in monotonic loading have been observed in other /3 -processed Ti a l loys such as the T i -6A1-2Sn-4Zr -2Mo n e a r ~ al loy. 1~ It is be l ieved that this deformat ion mode is r e spons ib l e for the reduc t ion in the t ens i l e duct i l i ty of f l -annealed mic r o s t r u c t u r e s . The shear r e l a t ed f r ac tu r e domina tes the fatigue c rack in i t ia t ion (Location A in Fig . 1), as well as the fatigue c rack propagat ion (Location B), at low and in t e rmed ia t e s t r e s s in tens i ty r anges . The shear re la ted fatigue in i t i a t ion c rack in la rge colonies can cause a seve re life reduc t ion as was observed for /3-annealed Ti 6A1-4V. 9 On the other hand the shear