The importance of mechanical twinning in the stress-strain behavior of swaged high purity fine-grained titanium below 42
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E R E is disagreement in the literature concerning the existence and importance of deformation twinning in titanium at low temperatures. In 1966, O r a v a , Stone, and Conrad1 published an extensive set of tensile data for fine-grained commercial p u r i t y titanium. At that t i m e , they stated that a metallographic e x a m i n ation of t h e i r m a t e r i a l at X1000 failed t o r e v e a l the existence of twins even for strains beyond 10 pct at 77°K. This was attributed t o the fine g r a i n size and high imp u r i t y content of t h e i r material. Subsequently, J o n e s and Conrad2 reported that twinning was not evident even in the necked part of fine-grained titanium specimens deformed at 4.2°K. In a second paper3 by the same authors dealing with the deformation of titanium at subambient temperatures, the subject of twinning was not mentioned. It is stated in still another paper4 that no deformation twinning was observed in commercial p u r i t y A-70 r e g a r d l e s s of g r a i n size or strain. On the other hand, the e a r l y work of Rosi and Perkins 5 indicated that in coarse-grained iodide titanium specimens the same type of twins were found at 77°K as at room temperature, but that at bow temperatures a g r e a t e r portion of the deformation occ u r r e d by twinning. The paper of Kula and DeSisto6 showed the presence of deformation twinning in c o a r s e grained c o m m e r c i a l p u r i t y titanium specimens def o r m e d at 4.2°K. B u r r i e r , Amateau, and Steigerwald7 investigated titanium specimens of four t y p e s . Howe v e r , only t h e i r commercial p u r i t y , fine-grained (22 p) specimens, which were ductile at low temperatures (22 pct fracture strain at 77°K), need be considered s i n c e all the remaining fractured at 77°K at very s m a l l strains (order of 2 pct). Regarding the specimens which were ductile, they s t a t e that at A. M.GARDE and R. E. REED-HILL are GraduateStudent and Professor, respectively, Department of Metallurgical and Materials Engineering, University of Florida, Gainesville, Fla. Manuscript submitted March 4, 1971. METALLURGICAL TRANSACTIONS
-155°F (175°K) and below, thick twins were observed on the specimen surfaces. In addition to the above papers that confirm the existence of twinning at low temperatures, there are others that show that twinning not only occurs in titanium at low temperature, but has a significant effect on the mechanical behavior of this metal. Thus Rosi and Perkinss report that cylindrical tensile specimens of commercial purity titanium b e c a m e elliptical in cross-section after deformation. The observed ellipticity decreased with decreasing deformation temperature. This change in eccentricity requires a corresponding change in the deformation modes. In the normal commercial plate texture of titanium, there is a strong alignment of the basal planes of the grains parallel to the rolling plane. Tensile s p e c i m e n s cut f r o m such plates will invariably s h o w a p r o n o u n c e d elliptical cross-section w h e
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