Materials Science and Engineering A 527 (2010) 4136–4145 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea A multi-scale characterization of deformation twins in Ti6Al4V sheet material deformed by simple shear W. Tirry a,c,∗ , F. Coghe a , S. Bouvier b , M. Gasperini b , L. Rabet a , D. Schryvers c a Royal Military Academy, Renaissancelaan 30, 1000 Brussels, Belgium b LPMTM-CNRS, University Paris 13, 99, av. J.B. Clément, 93430 Villetaneuse, France c EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium article info Article history: Received 30 July 2009 Received in revised form 5 March 2010 Accepted 11 March 2010 Keywords: Ti6Al4V Deformation twinning TEM EBSD Optical microscopy Simple shear abstract Ti6Al4V sheet material is subjected to simple shear deformation with strain ratio’s of 10%, 30% and 50%. Optical microscopy, transmission electron microscopy and electron backscatter diffraction techniques are applied to study the presence and morphology of deformation twins. Only the  10 12  type of twins seems to be present with a volume fraction below 1%. These  10 12  twins show a high density of basal stacking faults of the ...ABABACAC... type identified using atomic resolution transmission electron microscopy. A resolved shear stress analysis shows that twins most often occur on those planes with the highest resolved shear stresses, but that the starting texture is not beneficial for the occurrence of twins. It is further suggested that a transitory strain hardening regime observed around 530 MPa might be related with the onset of twinning. © 2010 Elsevier B.V. All rights reserved. 1. Introduction In the case of pure titanium, which has a hexagonal close packed (hcp) crystal structure at room temperature, the lowest value of the critical resolved shear stress (CRSS) is found on the prism planes therefore favouring slip on these planes. However, this mecha- nism cannot accommodate for deformations having a component in the c-direction of the hexagonal unit cell. In order to accom- modate such a component there is the possibility of having slip or twinning on the pyramidal planes and as such arbitrary deforma- tions can be accommodated [1,2]. Whereas commercially pure Ti indeed exhibits deformation twinning as an important deforma- tion mechanism, it seems that in the case of biphased Ti6Al4V, one of the most popular Ti alloys, twinning is suppressed [3]. Never- theless, the largest fraction of this alloy still has the hcp crystal structure, called the alpha phase, with in addition the presence of the body centered cubic (bcc) beta phase. Without consider- ing all the possible different variant phases (alpha, alpha ′ , alpha ′′ ), which can be encountered in this alloy depending on the thermo- mechanical treatment, it is believed that mainly the addition of Al is responsible for the suppression of twinning mechanisms [3–5] which might be replaced by <c + a> slip on the pyramidal planes ∗ Corresponding author at: Royal Military Academy, COBO, Renaissancelaan 30, 1000 Brussels, Belgium. Tel.: +32 2 742 64 16, fax: +32 2 742 6412. E-mail address: wim.tirry@ua.ac.be (W. Tirry). or extra deformation taking place in the beta phase [3]. However, most studies reporting the suppression of deformation twins due to increased Al content are performed in the case of monotonic deformations and moderate strains and strain rates; when the Al content is higher than approximately 5 at.%, twinning is almost fully suppressed. An example illustrating that twins might still occur is reported in [5], showing that in case of cyclic deformation of Ti–5 at.%Al single crystals deformation twinning is active. In addi- tion, most works reporting the suppression of deformation twin mechanisms in case of Ti6Al4V are for moderate strain ratios and rates [3–7], and are not focussed on more severe deformations and higher strain rates in which the situation might be different. A similar remark can be made for other alpha/beta alloys with differ- ent alloying elements [7,8]. When considering studies describing the microstructural changes in more severely deformed Ti6Al4V the presence or lack of deformation twins is often only mentioned without a dedicated study [9–11]. A dedicated study, specifically focussed on the occurrence of deformation twinning, like identification of the twin types, their morphology and twin volume fraction will provide useful information related to their importance concerning the exhib- ited mechanical behaviour and texture change. In case of ECAE deformed Ti6Al4V a study dedicated to the importance of deforma- tion twinning is reported in [12]. In this particular case  10 11  twins are introduced in large numbers and their presence is nec- essary to explain the texture change (the standard four-index notation for hexagonal crystals will be used throughout this paper). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.03.039