Faceted fracture in beta annealed titanium alloys
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T o b e t t e r understand the correlation between fatigue failure and p r i o r r - g r a i n size, in ~-annealed microstructures, the morphology of fatigue fracture facets which contain more than one a platelet colony in Ti-11 and IMI-685 titanium alloys was investigated. It was found that most a / ~ t r a c e s on the b a s a l fracture facets, intersect at a n g l e s approximately 50, 60 and 70 deg. T h e s e angles can be predicted by the B u r g e r s relation that allows platelet colonies t o have a common b a s a l plane within the same p r i o r B-grain. This commonality promotes s h e a r related faceted fracture through more than one colony and inc r e a s e s the effective colony s i z e . The increased effective colony size can c a u s e more fatigue c r a c k branching and, therefore, reduces fatigue c r a c k growth r a t e s in ~-annealed microstructures.
FACETED
fracture is a characteristic of a + fl titanium alloys worked or heat treated above the fltransus temperature. The fracture facets can be found both in monotonic and cyclic load failures ~-3 with fracture becoming more faceted as the colony size increases. The size of the fracture facets has been related 3'4 t o the size of the colonies of similarly aligned a-platelets. It was shown that fatigue c r a c k propagation in r-annealed Ti-6A1-4V%5 and high cycle fatigue strength of B-worked IMI-6852 are r e l a t e d t o the colony s i z e . Faceted fracture has been r e l a t e d to intense s h e a r activity a c r o s s a platelet colonies in b o t h monotonic~ and cyclic loading tests.Z'7 In the case of monotonic loading of Ti-6A1-2Sn-4Zr-2Mo,6 b a s a l slip was identified. In the case of cyclic loading, both b a s a l and prismatic slip were identified in IMI-6852'7 and Ti-111'7 alloys. In the latter alloy8 it was shown that faceted fatigue fracture followed on the same plane as the intense s h e a r bands. In m a t e r i a l containing l a r g e colonies of a platelets, the s h e a r related faceted fracture dominated the fatigue c r a c k initiation, propagation, and fast fracture. 2'8 Examination of facet orientation of l a r g e colony Ti-8AI-IMo-IV (Ref. 9) and IMI-685,~° by two surface t r a c e analysis and by electron channeling, respectively, showed that the facets were on the (0001) b a s a l plane. In addition t o the correlation of colony size to f r a c ture morphology, some correlation was reported between p r i o r E - g r a i n (PBG) size and fatigue behavior in B-annealed Ti-6A1-4V; ~,~ a d e c r e a s e in fatigue growth r a t e s with increasing PBG size was measured. The objective of the current work was t o determine the mechanism by which the faceted fatigue fracture in a + fl titanium alloys can be influenced by PBG size.
of similarly aligned and crystallographically oriented a platelets, separated by films of B-phase and an interface phase of a complex nature.~2 The size of the PBG (typically 0.1 t o 5 mm) will i n c r e a s e as the super transus annealing or working temperatures and annealing time i n c r e a s
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