Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea The room temperature tensile deformation behavior of thermomechanically processed β-metastable Ti-Nb-Ta-Zr bio-alloy: the role of deformation- induced martensite A. Maghsoudlou a , A. Zarei-Hanzaki a, ⁎ , H.R. Abedi a , A. Barabi a , F. Pilehva a , D. Dietrich b , T. Lampke b a Hot Deformation & Thermomechanical Processing Laboratory of High Performance Engineering Materials, School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran b Chemnitz University of Technology, Institute of Composite Materials and Surface Technology, D-09107 Chemnitz, Germany ARTICLEINFO Keywords: β-type Ti alloy Work hardening Deformation-induced martensite Twinning-induced plasticity ABSTRACT The present work was planned to investigate the microstructural evolution and mechanical properties of a metastable β-type titanium alloy composing of Ti–27.96Nb–11.97Ta–5.02Zr %wt (so-called TNTZ alloy), after applying a predetermined low-temperature thermomechanical processing (LTMP) cycle. The room temperature uniaxial tensile testing was utilized to evaluate the processed material flow behavior. To this end, the occurrence of any phase transformation and twinning-induced elasto-plasticity effects along with the work hardening be- havior of the experimental TNTZ alloy were thoroughly studied. A double yielding phenomenon was realized in the specimens subjected to cold rolling and subsequent short time annealing. The XRD analysis confirmed an increase in volume fraction of α″ martensite as a result of deformation-induced martensite transformation in the microstructure. Electron backscatter diffraction (EBSD) analysis revealed that low angle boundaries would form within grains holding near (001) texture and might well act as a preferred nucleation site to develop de- formation-induced martensite. The intersections of α″ martensite and the contribution of dynamic Hall-Petch effect could result in spectacular work hardening behavior in comparison to other β-type titanium alloys. The presence of sub-sized grains in the microstructure was related to the martensite reversion; this could further increase the strain hardening rate in the experimental alloy. 1. Introduction Metastable β titanium bio-alloys show a variety of mechanical properties depending on their chemical composition. The design strategy for these alloys was originally developed by Morinaga et al. [1] and was based on the semi-empirical approach of “d-electron design”. It was shown that the β phase mechanical stability could be related to several electronic parameters, including Bo (the Bond Order) as a measure for the average covalent bond strength between Ti and a particular alloying element and Md as criteria for the average d-orbital energy level [2]. Accordingly, chemical composition defines the stabi- lity of the β phase which is the main factor dictating the corresponding elastic/plastic deformation mechanisms [3]. In alloys holding a rela- tively low β phase stability, twinning or martensitic transformation may occur which results in super-elasticity and improved mechanical properties [4–6]; while the dislocation slip would be the main deformation mechanism where β phase possesses a high stability [7].In a more detailed view, Molybdenum equivalence (Mo equ ) can be em- ployed to express the stability of the β phase in titanium alloys con- taining a variety of β stabilizing elements. As is evident from Fig. 1 [8], by decreasing Mo equ the deformation mechanism in β titanium alloys varies from dislocation slip to twinning and deformation-induced transformation. Interestingly, a region can be observed in which both martensite transformation and mechanical twinning act together. It is expected that the alloys belonging to this region present desirable mechanical properties. Consequently, a new class of metastable β tita- nium alloys containing non-toxic and allergy-free elements (i.e. Ti-Nb- Ta-Zr (TNTZ) series) has been developed based on the d-electron design which would be susceptible for several reversible or irreversible de- formation mechanisms. There are lots of reports on the elastic properties of the TNTZ al- loying system. Niinomi et al. [9] investigated the nonlinear elastic https://doi.org/10.1016/j.msea.2018.09.038 Received 10 July 2018; Received in revised form 11 September 2018; Accepted 12 September 2018 ⁎ Corresponding author. E-mail address: zareih@ut.ac.ir (A. Zarei-Hanzaki). Materials Science & Engineering A 738 (2018) 15–23 Available online 13 September 2018 0921-5093/ © 2018 Published by Elsevier B.V. T