Deformation microstructures in titanium sheet metal
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of the imposed stress system and the crystallographic texture of the rolled sheet, which alters the distribution of the grain basal-plane poles with respect to the operative stress axes. In uniaxial tension parallel to the longitudinal direction, twins favored by 'c' axis compression are produced, while in the transverse direction twins favored by 'c' axis tension appear. In plane strain and biaxial tension the dominant stress is through-thickness compression, which produces twins favored by 'c' axis compression in nearly all cases. The alterations in twin orientation and numbers are associated with changes in stress-strain behavior. As twin volume fraction increases and twins are aligned more closely to the principal stress axis, the instantaneous work-hardening rate tends to stabilize at a nearly constant value over a large strain range.
THE nature of plastic deformation in titanium of commercial or higher purity has been the subject of debate for some time, especially concerning the role played by twinning. Polycrystalline metals require five independent shear systems to satisfy von Mises' criterion and undergo homogeneous strain at constant volume. Hexagonal metals such as cadmium, which has five independent slip modes, exhibit high ductility. ~ Pure titanium, which also has high ductility, has only four independent slip modes at ambient temperature, involving basal, prismatic and pyramidal slip. These all have a Burgers vector in the basal plane, and another mode is necessary to accommodate 'c' axis extension or compression. 2 Twinning has been proposed as the source of a fifth independent mode? The main objections to this have been twofold: firstly, it is argued that the total amount of strain available from twinning is limited to about 10 pct in a polycrystalline material; 4 and secondly, that twinning accommodates 'c' axis extension or compression, but not both. The former argument ignores secondary twinning inside twins and slip in reoriented twin volumes, and the second the existence of twins activated by both tension and compression in titanium and other hexagonal metals2 There have been six twinning systems reported for titanium 6,7,8 which are listed in order of increasing shear in Table I, taken from Arthey & Roberts. 9 Three are activated by 'c' axis tension, and three by 'c' axis compression. The tendency toward twinning in titanium is influS. MULLINS is Metal Forming Engineer, The APV Company, Crawley, West Sussex RH10 2QB, England. B.M. PATCHETT is NOVA Professor, Dept. of Mineral Engineering, The University of Alberta, Edmonton, Alberta T6G 2G6, Canada, Formerly Chief Metallurgist, The APV Company. Manuscript submitted December 10, 1979. METALLURGICAL TRANSACTIONS A
enced by purity, grain size and temperature. Orava et no twins in tensile tests at low temperatures, and Rodgers and Roberts l~ found very few in similar tests at amb
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