Evolution of crystallographic texture during deformation of submicron grain size titanium
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Evolution of deformation texture in commercially pure titanium with submicron grain size (SMG) was studied using x-ray diffraction (XRD) and electron back scatter diffraction (EBSD) methods. The material was deformed by rolling at room temperature. The deformation mechanism was found to be slip dominated with a pyramidal ,c + a. slip system facilitating plastic deformation. No evidence of tensile or compressive twinning was detected, as generally seen in the case of titanium with conventional microcrystalline grain size. The absence of twinning and the propensity of the pyramidal ,c + a. slip system in the SMG Ti is attributed to the lack of coordinated motion of zonal partial dislocations that leads to twinning.
I. INTRODUCTION
Recent studies on the deformation behavior of ultrafine grain materials have revealed significant differences when compared to their coarse grain counterparts.1 Grain size is expected to play an important role in low stacking fault energy FCC as well as HCP metals and alloys that deform extensively by twinning.2 The commotion of twinning may lead to a change in deformation texture, particularly in HCP materials where more than one twin system may be activated.3 Texture is an important microstructural aspect of deformation, as it largely influences the mechanical properties of materials. The effect is more pronounced in the case of HCP materials due to the inherent anisotropy of the unit cell. Among HCP materials, titanium and its alloys have been investigated in greater detail due to their extensive use in aerospace and biomedical applications.4 The c/a ratio plays an important role in deciding the deformation mode in HCP materials, for example, (i) in materials with c/a , 1.632 (Ti, Zr, Be, etc.), a sufficient number of slip system is provided by prismatic and pyramidal slip; (ii) in materials with 1.63 , c/a , 1.73 such as Mg (c/a 5 1.624), only basal slip is available; and (iii) in materials with c/a . 1.73 (e.g., Zn, Cd), deformation is accommodated by basal slip and twinning.5 In titanium, with a c/a ratio of ~1.588, the deformation at room temperature is facilitated by prismatic, pyramidal ,c + a., and basal slip along with compressive and tensile twinning.2,6 The studies on evolution of deformation texture in titanium with grain size in the micron range3,7–9 revealed that the cold rolling texture does not develop monotonically with strain,7 therefore no single mechanism is operative throughout a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.70 J. Mater. Res., Vol. 26, No. 4, Feb 28, 2011
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the deformation. The entire deformation path is classified into twin dominated and slip dominated regimes. The interaction of twinning with normal dislocation slip plays an important role in determining the texture evolution in the low and intermediate strain regimes. However, these mechanisms and their regime of dominance are likely to be influenced by grain size. Submicron grain siz
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