Elevated temperature plastic anisotropy of Ti-6AI-4V plate
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The influence of temperature on the planar and n o r m a l anisotropy parameters (~r~ and R, respectively) for mill annealed, duplex annealed, and c r o s s r o l l e d Ti-6A1-4V plate was investigated from 25 t o 704°C (77 t o 1300°F). Both parameters were assessed in t e r m s of the plastic s t r a i n r a t i o (R), r a t i o of width to thickness strain at m a x i m u m load (~0.065 longitudinal strain) in tensile specimens oriented at 0, 45, and 90 deg to the r o l l ing direction, and correlated with texture and microstructure. With increasing temperature, plates characterized by alpha deformation type b a s a l plane textures exhibited significantly l a r g e r anisotropy variations than plate with a beta transformation type text u r e . This behavior was related t o the d e g r e e of textural randomness and t o a thermally induced transition in p r i m a r y deformation mode from twinning to slip. Depending on texture, the results strongly suggest that working temperature may be utilized advantageously t o a l t e r the plastic anisotropy of Ti-6A1-4V plate for improved formability in a given fabrication operation.
IN
addition t o composition and microstructure, crystallographic texture constitutes a c r i t i c a l d e s i g n factor for predicting and controlling the properties and deformation response of metallic materials. Emphasis in the p a s t has concentrated on developing random textures t o minimize plastic anisotropy effects; it is now well recognized, however, that significant improvements in mechanical performance can be achieved through texture control. Opportunities for realizing such benefits are particularly attractive for the hexagonal close packed metals because of t h e i r restricted slip behavior. T h e s e materials are capable of generating strong preferred orientations d u r i n g processing, and considerable attention has focused on exploiting the concomitant high levels of anisotropy developed in titanium alloys. Most of these efforts have centered on the texture strengthening of alpha and alpha-beta type compositions for pressure v e s s e l applications in which the b a s a l planes of the alpha phase are preferentially aligned parallel to the component surface. 1-s With this orientation, c r o s s sectional thinning of the m e m b r a n e is inhibited by the fact that the hcp unit cell does not possess a primary slip system that facilitates extension or contraction a l o n g the c - a x i s . Deformation, therefore, must be accommodated by the operation of the more difficult twinning or slip modes, thus providing increased resistance t o plastic flow under b i a x i a l loading conditions .9 An equally important aspect of texture control involves tailoring plastic anisotropy of a given alloy to a specific fabrication operation for optimum f o r m a bility. A c l a s s i c example of this endeavor is the improved deep drawing performance exhibited by sheet m a t e r i a l when the texture restricts thinning and minim i z e s the directionality in plastic flow within the plan
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