Elsevier

Journal of Alloys and Compounds

Volume 696, 5 March 2017, Pages 130-135
Journal of Alloys and Compounds

Effect of B2-ordered phase on the deformation behavior of Ti-Mo-Al alloys at elevated temperature

https://doi.org/10.1016/j.jallcom.2016.11.211Get rights and content

Highlights

  • Ti-Mo-Al alloys dominated by B2-ordered phase were prepared by arc-melting.

  • As-cast Ti-Mo-Al alloys exhibited impressive compressive strength at 1073 K.

  • Strong strain hardening was observed in Ti50Mo25Al25 alloy.

Abstract

The microstructures and mechanical properties of as-cast Ti-Mo-Al alloys with various compositions were systematically investigated in this study, aiming to explore the potentials of B2-ordered phase (β2) in high temperature applications. According to the experimental results, these as-cast Ti-Mo-Al alloys were mainly composed of β2 phase and contained a large number of dislocation walls and sub-grain boundaries. Overall they exhibited a high level of compressive strength at 1073 K, with the maximum value approaching ∼879 MPa. The impressive mechanical properties of β2-dominating Ti-Mo-Al alloys are related to the excellent stability of B2 structure and extensive dislocation entanglements at elevated temperature.

Introduction

The alloys developed on the basis of Ti-Al system have great potentials in aerospace applications because of their low density, high strength and good corrosion resistance [1], [2], [3]. Compared with conventional Ti alloys, the phase transitions induced by alloying into Ti-Al base alloys are more complicated since the established disorder/order transformation along solidification might be altered and new ordered phases could be formed. Among the novel ordered phases reported in Ti-Al-X ternary systems, the β2 phase (indicated as β0 in some literature) with B2 structure is a very common one and has been widely observed, especially when X represents refractory elements such as Nb, Mo, Cr, W, Ta and V [4], [5], [6].

The significance of β2 phase for the development of Ti-Al base alloys mainly lies in the improvement of mechanical properties. It is well known that the attractive high temperature strength of these alloys generally arises from the stiff intermetallic compounds (e.g. γ-TiAl (L10) and α2-Ti3Al (D019)), which, on the other hand, bring about the limited room temperature ductility and inadequate workability. The β2 phase, being able to deform with multiple slip systems, has been recognized as a critical toughening phase to α2 phase. For example, Gogia et al. demonstrated that the room temperature ductility of Ti-11Nb-24Al alloy characterized with α22 dual phases was up to 6–10% while there was almost no ductility of single α2 alloys [7]. Recent studies further pointed out that an even enhanced deformability could be achieved in the alloys composed of β2+O (Ti2AlNb, orthorhombic) or α22+O phases, e.g. Ti-27Nb -22Al [8] and Ti-23Nb-22Al [9], [10], [11]. Moreover, β2 phase has been employed to strengthen ductile β phase (A2). Naka et al. made numerous efforts in constructing β+β2 microstructure for Ti-Al base ternary and quaternary alloys used at elevated temperatures, as inspired by the classic γ-Ni matrix+γ′-Ni3Al structure in nickel-based superalloys [12].

As a “β stabilizer” approximately four times stronger than niobium, molybdenum exhibits impressive capability in stabilizing the cubic structure for Ti-Al base alloys. The β2 (B2) phase in Ti-Mo-Al system is usually denoted as Ti2MoAl, but it has been observed in a wide composition range including stoichiometric one (Ti50Mo25Al25) and non-stoichiometric ones (Ti45Mo5Al50, Ti54Mo20Al26 and Ti50Mo12.5Al37.5, etc.) [13], [14], [15]. Previous works on the Ti-Mo-Al alloys dominated by B2 structure focused on their ordering and site occupancy behavior [15], [16], [17], [18], [19], [20]. For example, Chen and Jones reported the flexible occupancy of Mo atoms in either sublattice by examining characteristic X-ray intensities ratios of three Ti-Mo-Al B2 alloys [19]. Based on the Rietveld refinement of obtained X-ray and neutron diffraction data, Singh et al. further divided the site occupancies in B2 phases of non-stoichiometric Ti-Mo-Al alloys into two groups. Assuming Ti atoms occupied A sites while Al and Mo occupied B sites, they suggested that excess Ti atoms occupied B sites in the alloys with Ti≧50 at.% and excess Mo atoms occupied A sites in the alloys with Ti≦50 at.% [18]. Although the degree of ordering and site occupancy in B2 phases were declaimed to affect the mechanical properties of Ti-Mo-Al alloys, there have been limited publications investigating the composition-dependent mechanical behavior of B2 dominating Ti-Mo-Al alloys. The effects of β2 phase were mostly studied in the context of γ+β2, α22 and γ+α22 alloys [4], [21]. Thomas et al. once examined the flow stress of Ti2MoAl as a function of temperature, which showed more impressive thermal stability of β2 phase and higher strength than that of Ti2NbAl and Ti5VAl2 at T≧1050 K. Nevertheless, their work only covered the near stoichiometric composition, Ti50.6Mo25.3Al24.1 [22]. Considering the wide existence of β2 phase in Ti-Mo-Al alloys and possible optimization of mechanical performance by monitoring the volume fraction of β2 phase, a thorough study about the effect of β2 on the deformation behavior of Ti-Mo-Al alloys is essentially necessary. In this work, we investigate the microstructure and mechanical properties of Ti-Mo-Al alloys with various chemical compositions, aiming to provide insights into the alloy design strategy for Ti-Al base alloys.

Pure Ti (99.999%), Mo (99.99%) and Al (99.95%) were selected as starting materials to fabricate Ti-Mo-Al alloys with various nominal compositions (as listed in Table 1). Unless mentioned otherwise, the compositions appeared in this work are in atomic percent. The samples were prepared by conventional arc melting technique in Argon atmosphere. All the ingots were flipped over and re-melt for at least five times to ensure chemical homogeneity.

The fine-polished bulk samples were examined using a Bruker D8 Advance X-ray diffractometer (XRD) with Cu Kα radiation to identify constituent phases in the as-cast Ti-Mo-Al alloys. Their microstructures were characterized on a JEOL JSM-7800F scanning electron microscope (SEM) under back-scattered electron (BSE) imaging mode. The detailed chemical composition of constituent phases was determined by energy-dispersive spectroscopy (EDS) on a Zeiss Ultra 55 SEM. We further examined their microstructure and lattice structure by transmission electron microscopy (TEM) on a JEOL JEM-ARM 200F microscope operated at 200 kV.

The hardness of investigated alloys was measured by Vickers hardness indenter at a load of 0.5 kg. Compression tests were conducted on an Instron 5982 machine at 1073 K. The compression specimens with the dimension of about 2 × 2x4 mm were electron discharge machined from the ingots. For each alloy, at least three compression specimens were tested to ensure the repeatability. All the compression tests were performed at a constant strain rate ε˙ of 2.1 × 10−4 s−1.

Section snippets

Results and discussion

Fig. 1 shows the XRD spectra of as-cast Ti-Mo-Al alloys investigated in this study. It is clearly seen that these Ti-Mo-Al alloys exhibited characteristic reflections of body-centered cubic (BCC) structure. With increasing concentration of Mo, the reflections (100) and (210), which are associated with B2-ordered structure (β2 phase), become visible. However, in Fig. 1 the superlattice reflections (e.g. (210)) are not so conspicuous for the strongly ordered composition, Ti50Mo25Al25. These weak

Conclusions

The as-cast Ti-Mo-Al alloys investigated in this study is dominated by B2-ordered phase (β2). These alloys exhibit a high level of compressive strength, which is up to ∼879 MPa at 1073 K. The impressive mechanical properties of β2-dominating Ti-Mo-Al alloys at elevated temperature are related to the excellent stability of B2 structure and extensive dislocation entanglements occurred during deforming process.

Acknowledgements

The authors are very grateful for the financial support from the New Energy and Industrial Technology Department Organization (NEDO) in Japan through the Energy and Environment New Technology Leading Program No. P14004.

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