An Experimental and Theoretical Multi-Mbar Study of Ti-6Al-4V
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An Experimental and Theoretical Multi-Mbar Study of Ti-6Al-4V Bengt E. Tegner1, Simon G. MacLeod2, Hyunchae Cynn3, John Proctor1, William J. Evans3, Malcolm I. McMahon1 and Graeme J. Ackland1 1
SUPA, School of Physics and Astronomy, and Centre for Science at Extreme Conditions. The University of Edinburgh, EH9 3JZ, U.K. 2 Institute of Shock Physics, Imperial College London, SW7 2AZ, U.K. 3 Lawrence Livermore National Laboratories, Livermore, CA 94550, U.S.A. ABSTRACT We report results from an experimental and theoretical study of the room temperature (RT) compression of the ternary alloy Ti-6Al-4V. In this work, we have extended knowledge of the equation of state (EOS) from 40 GPa to 221 GPa, and observed a different sequence of phase transitions to that reported previously for pure Ti. INTRODUCTION The commercial and industrial importance of the two-phase ternary alloy Ti-6Al-4V (wt %) is well documented and known to be heavily dependent on its mechanical properties (see for example [1]). Ti-6Al-4V (hereafter referred to as Ti64) crystallizes predominantly in the twoatom α -phase (hexagonal-close-packed or hcp) at ambient conditions, with a much smaller fraction by volume crystallizing in the β -phase (body-centered-cubic or bcc) around the grain boundaries. The alloying of substitutional and interstitial impurities increases the strength of Ti64 compared with pure Ti. Al is the α -phase stabilizer and dominant substitutional strengthener. At room temperature (RT), using diamond anvil cells (DACs) and angle-dispersive X-ray diffraction (ADXRD) [2], the α -phase of Ti64 has been observed to transform into the threeatom hexagonal ω -phase at 27 GPa. A more recent RT DAC study, using energy-dispersive Xray diffraction (EDXRD), did not observe any phase transformation up to 32.4 GPa [3]. Shock studies of Ti64 are fairly extensive, but no evidence of a phase transformation has been observed (see for example [4] and references contained therein). Pure Ti has been compressed at RT using DACs to 220 GPa, and the transformation sequence α → ω → γ → δ was reported [5, 6], where the γ and δ phases are orthorhombic distortions of the hcp and bcc structures, respectively. The effects of uniaxial stress on the α → ω transition in Ti were studied by Errandonea et al. [7] using DACs and ADXRD. They found that the transition pressure was dependent on the pressure transmitting medium (PTM) used, and ranged from 4.9 GPa (no PTM) up to 10.5 GPa (argon PTM). In addition, they observed the coexistence of the α and ω phases over a large pressure range. Our motivation for conducting the present study was to determine whether or not Ti64 exhibited similar behavior to that reported for pure Ti at multi-megabar pressures.
EXPERIMENT AND THEORY Experimental details We performed a number of static high-pressure compression experiments using gasmembrane driven DACs at the High Pressure Collaborative Access Team (HPCAT) beamline 16IDB, at the Advanced Photon Source (APS), in Chicago. We collected ADXRD patterns from commercially-sourced Ti64
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