. Twinning system selection in a metastable β-titanium alloy by Schmid factor analysis. Scripta Materialia, Elsevier, 2011, 64 (12) AbstractElectron backscattering diffraction and Schmid factor analysis were used to study the twinning variant selection in a Ti-25Ta-24Nb (mass%) metastable β-titanium alloy. The two twinning systems {1 1 2} 1 1 1 and {3 3 2} 1 1 3 were observed. For each system the Schmid factor was shown to be a relevant parameter to determine the activated variant.Moreover, selection between the two twinning systems depends on the crystallographic orientation of the grain with respect to the tensile direction.Keywords: Titanium alloy; Electron backscattering diffraction; Twinning; Schmid factor Metastable β-titanium alloys can be elaborated with fully biocompatible β-stabilizer elements such as Ta, Nb, Mo, etc. Their interesting mechanical properties make them good candidates for biomedical applications [1] and [2]. Theses alloys possess a non-ordered bcc structure and are subject to numerous deformation mechanisms: a stress induced martensitic transformation which can lead to a superelastic effect, slip and twinning [3] and [4].Twinning is a common deformation mechanism in materials exhibiting a low stacking fault energy in hcp, fcc or bcc structures. In bcc structures {1 1 2} 1 1 1 is a well-known twinning system, but other twinning systems such as {3 3 2} 1 1 3 have been observed in some metastable alloys subject to a martensitic transformation, like Fe-Ni-C or Fe-Be [5]. In metastable β-titanium alloys both twinning systems {1 1 2} 1 1 1 and {3 3 2} 1 1 3 have been observed, depending on the alloy composition [6] and [7]. Each twinning system can be activated in 12 different ways, termed variants in this paper.The Schmid law is commonly used to predict the activated slip system depending on the tensile direction compared with the crystallographic orientation of the crystal. Selection of the twinning variant can also be predicted by the Schmid law for materials exhibiting hcp or fcc structures, as reported in the literature [8] and [9]. Although it is well known that both slip and twinning occur in bcc titanium alloys, the objective of this paper was to confirm that twinning variant selection in each observed system obeys the Schmid law for a metastable β-titanium alloy (bcc). As several twinning systems can be observed in this kind of alloy a selection parameter between these twinning systems will also be established.The metastable β-alloy composition chosen for this study is Ti-25Ta-24Nb (mass%). The ingot was elaborated by cold crucible levitation melting (CCLM). It underwent homogenization annealing at 950 °C for 20 h, followed by a water quench. Each ingot was then cold rolled (CR = 90%), after which a recrystallization annealing was applied at 850 °C for 0.5 h, followed by a water quench in order to retain the β-phase microstructure at room temperature in its metastable state.Before the recrystallization annealing flat tensile test specimens with a section of 3 × 0.7 mm and a gage len...

In bcc metastable β titanium alloys, and particularly in superelastic alloys, a unique {332}⟨113⟩ twinning system occurs during plastic deformation. However, in situ synchrotron x-ray diffraction during a tensile test shows that the β phase totally transforms into α^{''} martensite under stress in a Ti-27Nb (at. %) alloy. {332}⟨113⟩_{β} twins are thus not formed directly in the β phase but are the result of the reversion of {130}⟨310⟩_{α^{''}} parent twins occurring in martensite under stress. The formation of an interfacial twin boundary ω phase is also observed to accommodate strains induced during the phase reversion.