Communication In Situ Synchrotron Radiation Study of TiH 2 -6Al-4V and Ti-6Al-4V: Accelerated Alloying and Phase Transformation, and Formation of an Oxygen-Enriched Ti 4 Fe 2 O Phase in TiH 2 -6Al-4V MING YAN, M.S. DARGUSCH, C. KONG, J.A. KIMPTON, S. KOHARA, M. BRANDT, and M. QIAN In situ heating, synchrotron radiation X-ray diffraction has been used to study the alloying and phase trans- formation behavior of TiH 2 -6Al-4V and Ti-6Al-4V al- loys. Accelerated alloying and phase transformation were observed in the powder compact of the TiH 2 -6Al- 4V alloy subjected to a high heating rate. In addition, an oxygen-stabilized Ti 4 Fe 2 O phase, which is present as sub-micron or nanoscaled particles, has been identified in the TiH 2 -6Al-4V alloy. The implications of these experimental findings have been discussed in terms of alloying, improved densification and oxygen scavenging in titanium and titanium alloys. DOI: 10.1007/s11661-014-2631-4 Ó The Minerals, Metals & Materials Society and ASM International 2014 Titanium hydride, normally referred to as the d-TiH 2 phase [face-centred cubic (fcc) with a = 4.36 A ˚ ], is a compound stable at room temperature (RT) but will decompose at elevated temperatures. Its transformation is dependent on atmosphere, heating rate, and powder size (if TiH 2 in present as powder), as well as tempera- ture. [1–3] TiH 2 has attracted extensive research interest for applications in hydrogen storage, [4] metal foaming, [5] and hydroprocessing. [6] In the context of powder metallurgy (PM), research on TiH 2 was initiated in the 1960s [7] but recent years have seen a significant renewed interest in PM TiH 2 for a few good reasons, including [8–13] : (a) the use of TiH 2 powder can result in higher sintered densities and often better mechanical properties; (b) TiH 2 powder can be more affordable than the hydride-dehydride (HDH) Ti powder; and (c) the use of TiH 2 powder can lead to lower oxygen content in as-sintered Ti materials. This is due to the release of atomic hydrogen after heating of TiH 2 , which can reduce the surface oxide film on the TiH 2 powder particles. As a result of these benefits, PM TiH 2 shows promise as means to develop more affordable, high-performance PM titanium alloys. The dehydrogenation of TiH 2 involves a number of phase transformations according to the reassessed phase diagram of Ti-H. [14] A shell of a phase was found to envelop each remaining core of titanium hydride parti- cle. [1] This envelope of a phase not only controls the kinetics of dehydrogenation but is also expected to affect the alloying process of b-stabilizers. [1] Ivasishin and Savvakin [15] first proposed that the use of a fast heating rate could avoid the formation of this a shell during the decomposition of TiH 2 ; doing so is expected to favor the alloying process with b-stabilizers as the a shell acts as a barrier to the diffusion of b-stabilizers. This represents another potential unique feature of the PM TiH 2 but no direct evidence has been reported yet. One reason is that most relevant studies including the sintering of titanium alloys using TiH 2 powder were carried out at heating rates of less than 30 °C/min, [3,8,10–13] which has been shown to be inadequate to avoid the formation of the a phase. [3] The other reason is that it requires the use of in situ, instantaneous and high-temperature phase identification to monitor phase transformation during the dehydroge- nation of TiH 2 . In situ synchrotron radiation X-ray diffraction offers one such ideal method for this purpose. This study undertakes a systematic investigation of the alloying and phase transformation behaviors of TiH 2 - 6Al-4V via in situ heating, synchrotron radiation X-ray diffraction. For comparison, a sample of HDH Ti-6Al-4V has also been studied. We will show that there are distinct differences in oxide phase development between HDH-Ti based and TiH 2 -based Ti-6Al-4V, and more importantly, TiH 2 -6Al-4V responds uniquely to different heating rates, including the accelerated a fi b transformation under high heating rates. We will further demonstrate that there is formation of sub-micron or even nanosized Ti-Fe-O particles in the TiH 2 -6Al-4V subjected to a high heating rate. The existence of such oxygen-enriched particles may offer a new oxygen scavenging pathway for developing cost-affordable, high-performance Ti materials. A powder mixture of TiH 2 -based Ti-6Al-4V, consisting of TiH 2 powder (~44 lm; 99.4 pct purity), elemental Al powder (~1.5 lm; 99.7 pct purity), and 45Al-55V master alloy powder ( < 45 lm; 99.5 pct purity), was prepared using a Turbula mixer with a mixing time of 60 minutes. Cylinder samples (F6 mm 9 0.5 mm) were compacted using the powder mixture under a uniaxial pressure of MING YAN, Research Fellow, is with the Centre for Additive Manufacturing, School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, VIC 3001, Australia, and also with the Queensland Centre for Advanced Materials Processing and Manufacturing, The University of Queensland, Brisbane, QLD 4072, Australia. Contact e-mails: ming.yan@rmit.edu.au, uqmyan@ gmail.com M.S. DARGUSCH, Associate Professor, is with Queens- land Centre for Advanced Materials Processing and Manufacturing, The University of Queensland. C. KONG, Senior Research Officer, is with the Electron Microscopy Unit, University of New South Wales, Sydney, NSW 2052, Australia. J.A. KIMPTON, Principal Scientist, is with the Australian Synchrotron, 800 Blackburn Road, Clayton, VIC 3168, Australia. S. KOHARA, Senior Chef Scientist, is with the SPring-8/JASRI, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan. M. BRANDT and M. QIAN, Professors, are with the Centre for Additive Manufacturing, School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University. The first author of this paper had extensive discussions with Dr. Kohara during his visit to Dr. Kohara’s lab in the SPring-8 Japan Synchrotron. Dr. Kohara’s experience in synchrotron radiation has contributed a lot to the present study. Manuscript submitted July 13, 2014. Article published online October 30, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 46A, JANUARY 2015—41