In Situ Synchrotron Radiation Study of TiH2-6Al-4V and Ti-6Al-4V: Accelerated Alloying and Phase Transformation, and Formation of an Oxygen-Enriched Ti4Fe2O Phase in TiH2-6Al-4V

Abstract

In situ heating, synchrotron radiation X-ray diffraction has been used to study the alloying and phase transformation behavior of TiH 2 -6Al-4V and Ti-6Al-4V alloys. 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. Titanium hydride, normally referred to as the d-TiH 2 phase [face-centred cubic (fcc) with a = 4.36 Å ], 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 temperature. [1][2][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][9][10][11][12][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 particle.[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][11][12][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 du...

An Overview of Densification, Microstructure and Mechanical Property of Additively Manufactured Ti-6Al-4V — Comparison among Selective Laser Melting, Electron Beam Melting, Laser Metal Deposition and Selective Laser Sintering, and with Conventional Powder

“…The figure shows that the as-built density of AM Ti-6Al-4V is mostly higher than 99% of theoretical density and the material can be fully dense if processing parameters are appropriately selected. SLS Ti-6Al-4V is exceptional to this conclusion, which as-built densities are around 95% and similar to the as-sintered density of the conventional PM Ti-6Al-4V [4][5][6][7][8][9][10][11][12][13]. …”

“…The high cost of the ELI raw powders, however, can be an issue to AM Ti-6Al-4V from cost perspective. Figure 22 compares the price of a variety of powders, showing that the cost on the AM powders is extraordinary [4][5][6][7][8][9][10][11][12][13]. They are mostly higher than $400 per kg while hydrogenation-dehydrogenation (HDH) Ti powders that are widely used in conventional PM Ti are merely around $30 per kg.…”

“…and Ti powders produced via the Kroll, Hunter and HDH approaches (price of Ti-6Al-4V is similar to these Ti powders). Price of fine powders of Ti-6Al-4V for the 3D printing is extraordinary [4][5][6][7][8][9][10][11][12][13].…”

Powder metallurgy of titanium – past, present, and future

“…It has also been reported that, under certain circumstances, less expensive alloying elements may have beneficial effects on PM Ti alloys [354]. For example, Yan et al [355] postulated that the reaction kinetics between Ti and Fe are accelerated when TiH 2 powder is used as the feedstock in a PM process. Therefore, it was proposed that this phenomenon could enable the use of Fe to scavenge oxygen in PM Ti alloys via the formation of submicron Ti 4 Fe 2 O particles.…”