Microstructural influences on superplasticity in Ti-6AI-4V
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The strong dependence of the superplastic behavior of metals and alloys on g r a i n size has been demonstrated, and it is now well known that a fine g r a i n size is normally a requirement for superplasticity. However, the microstructure of certain alloy systems such as Ti-6A1-4V cannot always be adequately characterized by a single p a r a m e t e r such as g r a i n s i z e . In two-phase o~-fl alloys such as Ti-6A1-4V, other microstructural parameters such as volume fractions of the two phases, g r a i n aspect ratio, g r a i n size distribution and crystallographic texture may also influence superplasticity. For example, if " g r a i n switching" is an important deformation mechanism in superplastic flow as suggested by Ashby and Verall, then factors such as g r a i n aspect r a t i o and r a n g e of g r a i n sizes would be expected t o have an effect on superplastic behavior. In this s t u d y , these microstructural features were determined for s e v e r a l different heats of Ti-6A1-4V, and the corresponding superplastic properties were evaluated in t e r m s of t h e i r fully characterized microstructure. The flow s t r e s s as a function of strain r a t e , strain rate sensitivity exponent (m) as a function of s t r a i n rate and total elongation on properties were found t o be strongly influenced by microstructural parameters such as g r a i n aspect ratios, g r a i n size and g r a i n size distribution.
T H E strong dependence of the superplastic behavior of metals on g r a i n size has been adequately demonstrated numerous times, 1"4 and it is now well known that a fine g r a i n size is a primary requirement for superplasticity. However, the microstructure of certain alloy systems cannot always be adequately c h a r acterized by a single parameter such as g r a i n s i z e . Two-phase ~-fi titanium alloys are an example of such systems and microstructural parameters such as uniformity of g r a i n size, volume fraction of ot and fi phases, g r a i n aspect r a t i o and volume fraction t r a n s formed fl (or 5') are a m o n g the parameters which must be used for a complete description of factors which m i g h t influence superplasticity. Lee and Backofen1 have examined the b a s i c requirements for superplasticity in Ti-6A1-4V, and by holding samples for various t i m e s at elevated temperature have established that the increase in flow s t r e s s with increasing g r a i n size can be expressed as ~ L n , where c; is the flow stress, L the g r a i n size, and n lies between 0.9 and 1.23 at strain r a t e s (4) below 1.5 x 10-4 s-~. They suggest that the slope of ~1 in conjunction with a strain rate sensitivity (m) of ~0.6 indicates that the rate controlling process is essentially Newtonian viscous g r a i n boundary s h e a r . Lee and Backofen have also shown that the largest m values are associated with the smallest g r a i n sizes and it is probably safe to a s s u m e that the greatest elongation to fracture would also be associated with the finest g r a i n sizes. These correlations
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