Evolution of synthesis of FCC nanocrystalline solid solution and amorphous phase in the Ti-Ta based alloy by high milling energy

https://doi.org/10.1016/j.jallcom.2020.155980Get rights and content

Highlights

  • A nanocrystalline fcc phase of Ti-Ta based alloy was synthetized by high milling energy.

  • The Sn content and milling time influence on the formation of fcc Ti-based alloy.

  • Thermodynamic calculations with Miedema’s models are in agreement with XRD and HRTEM results.

  • Nanocrystalline grain size and high microstrain value promote formation of fcc phase in Ti-Ta-based alloy.

Abstract

Pure Ti and Ti-based alloys exhibit two equilibrium phases: at lower temperatures they have a hexagonal close packed (α-Ti) crystalline structure while at higher temperatures they possess a body-centered cubic (β-Ti) structure. However, in special conditions, they could exhibit a metastable face-centered cubic (γ-Ti) crystalline structure. This paper aims to investigate the influence of Sn amount and milling time on the formation of γ-Ti phase on the Ti-Ta based alloy. Four alloys with compositions Ti-13Ta-xSn (x: 3, 6, 9 and 12 at. %) were obtained by high energy milling. The alloys were analyzed by X-ray diffaction patterns, transmission and scanning electron microscopy. For the Ti-13Ta-3Sn alloy, the β-Ti phase is formed in greater quantity, ∼70% at 100 h. For the Ti-13Ta-6Sn and Ti-13Ta-9Sn alloys, the phase that is in greater quantity is γ-Ti, around 100% at 100h. The Ti-13Ta-6Sn alloy shows a greater tendency to form the γ-Ti phase, since the quantity is close to 100% at 50 h milling time. Thermodynamic calculations are comparable with experimental data. The calculations were made using the MAAT software based on Miedema’s model. The MAAT is a free software that can be download from www.rpm.usm.cl.

Introduction

Titanium and Ti-based alloys are widely used in engineering applications such as aerospace, biomedical, chemical and nuclear industries due to their high strength to weight ratio [1], excellent corrosion resistance [2] and negligible biological impact on the human body [3]. In this context, Ti-Ta binary alloys have attracted a great interest due to their excellent combination of high strength, a relative low modulus, and a corrosion resistance that is superior to that of pure Ti [4]. The addition of 13 at.%Ta gives a good combination of high strength and low elastic modulus [5], but increasing the addition of Ta beyond at.%13 significantly increases the density of the alloy, which is unfavorable for many applications [5,6], especially as biomaterial. Ta and Sn are interesting elements when used in biomedical applications since both are classed as highly biocompatible due to their negligible effects on the human body [7], and also because Sn is an inexpensive alloying element.

Pure Ti and Ti-based alloys exhibit an allotropy change at the β-transus temperature. At lower temperatures they exhibit a hexagonal close packed (hcp) crystalline structure (called α-phase) and above this temperature they possess a body centered cubic (bcc) crystalline structure (called β-phase).The β-transus temperature for pure Ti is 1155 ± 2 K and for Ti-based alloys it depends on the type and amount of alloying elements [6]. A great variety of properties of Ti-based alloys are related to this allotropic phase transformation, which is very important for engineering applications. Ti-based alloys are classified according to their microstructure as α-phase, α+β-phase and β-phase [6]. The main characteristics of each group are:

  • i)

    The α-phase alloys exhibit high elastic modulus, good creep resistance, good weldability and excellent corrosion resistance,

  • ii)

    The α+β-phase alloys have high strength, good corrosion resistance and moderate fracture toughness and weldability, their properties depending on the α/β relative proportion, and

  • iii)

    The β-phase alloys exhibit high strength and fatigue resistance, lower elastic modulus, good formability, high hardenability and moderate corrosion resistance [6,[8], [9], [10]].

The α-to-β-phase transformation can be promoted by controlling temperature, pressure, type and amount of alloying elements [6]. In general terms, when the temperature is increased the formation of the β-phase is favoured, and the pressure has a small effect on the transformation in pure Ti. Smith et al. [11] studied the effect of pressure on Ti-Mo, Ti-24Nb-4Zr-8Sn (Ti2448) and Ti-36Nb-2Ta-0.3O alloys and observed that the β-phase is stable up to 40 GPa. Observing the p-T diagram of pure titanium [12], the β-phase is stable at high temperatures and the α-phase transforms to the ω phase at temperatures lower than 1155 K when the pressure increases. Alloying elements influence in different ways the β-transus temperature: α-stabilizers increase the β-transus temperature (typical elements in this group are Al, O, N, C [6]), whereas β stabilizers move the β-transus towards lower temperatures (in this group are Ta, Nb, Z, W, V, Mo, Fe, Mn, Cr, Co, Ni, Cu, Si, H [6,13]). There are also alloying elements such as Sn and Zr that show no influence on the β-transus [6]. The α and β phases are equilibrium phases, but a metastable phase with face cubic centered (fcc) crystalline structure (called γ-phase) has been reported in pure Ti and Ti-based alloys [[14], [15], [16], [17], [18], [19], [20]]. The γ-phase has been obtained using different methods such as: deformation of square cross-section nanopillars of Ti-0.1 wt% O [14], deformation of Ti–5Ta–2Nb wt.% by explosive cladding [16], rolling at room temperature of pure Ti [21], water quenching of Ti compact [22], and high-energy milling of Ti-25Al at.% [17]. A common characteristic of all processes where γ-phase has been reported is that it requires the presence of high deformation and nanocrystalline grain size. Theoretical studies are in agreement with the experimental conditions mentioned: Xiong et al. [23] studied the effects of size and temperature on the phase transformation of pure Ti from hcp to fcc and found that the γ-phase is stable in nanoparticles, nanowires and nanofilms when the sizes are smaller than 27, 19 and 9 nm, respectively, at around 777 K. Using ab-initio calculations the structural stability and electronic structure of γ-phase was studied [24]. The results showed that this phase is locally stable because the elastic stability criterion for a cubic crystal is fulfilled by the calculated elastic constants. On the other hand, the stability of γ-phase bands embedded in hcp-Ti matrix using density functional theory calculations was studied [21]. Three computational experiments were carried out, i) pure hcp structure containing 32 atomic layers, ii) 29 atomic layers hcp structure and 3 atomic layers fcc structure (referred to as the 3L-FCC structure), and iii) 25 atomic layers hcp structure and 7 atomic layers fcc structure (referred to as the 7L-FCC structure). Under uniaxial tensile loading, the 3L-FCC and 7L-FCC structures are stable and the pure hcp structure deformed to 4% has higher potential energy than the 3L-FCC structure at zero strain, and at a strain of 5.5% has higher potential energy than the 7L-FCC structure at zero strain [21].

Yu et al. [14] studied the in-situ formation of the γ-phase at elevated temperatures using transmission electron microscopy. They observed that hcp→fcc transformation occurs at temperatures around 873 K when Ti-0.1 wt%O foils were deformed. The γ-phase is stable at ambient temperature and displays considerable dislocation-based plasticity under nano compression tests. Bolokang et al. [22] reported that milling of pure Ti (30 h under Ar atmosphere) and water quenching of the compact after sintering (from 1473 K) gives rise to the formation of γ-phase. Zhang and Ying [25] obtained γ-phase in Ti–25 at.% Al powders applying high-energy milling (8 h) and heat treatment up to 923 K. Manna et al. [26] concluded that the formation of γ-phase of pure Ti during milling is due to the increment of plastic strain out of nanograin size and that the presence of impurities have no influence in the transformation. Jing et al. [27] studied the effect of heat treatments on γ-phase formation in Ti–20Zr–6.5Al–4V (wt. %). Samples were heated at 1223 K for 1 h under vacuum atmosphere, water quenched, and subsequent aging treatments were performed between 673 and 973 K for 2 h. The microstructure obtained showed the presence of hcp-phase, bcc-phase (residual), γ-phase and martensite-phase (α’’). Prasanthi et al. [16] studied the formation of γ-Ti phase in the α+β Ti–5Ta–2Nb alloy obtained by shock loading in an explosive clad. By means of high resolution transmission electron microscopy (HRTEM) they found that the γ-phase had a needle shaped morphology and presented an orientation relationship (OR) given by (1¯1¯0)γ//(011¯0)α and (1¯11¯)γ//(112¯0)α between γ and α phases. Van Heerden et al. [28] showed that the transformation from α to γ occurred in Ti layers in Al/Ti multilayers by vapour deposition. Chakraborty et al. [29] reported that polycrystalline Ti thin films, gradually transform from γ to α when increasing film thickness. The γ-phase is stable when films are much thinner than 144 nm and the α-phase when Ti films are thicker than 720 nm. Also, the γ Ti phase has been obtained in pure Ti and Ti alloys powders fabricated via high energy milling [17,22,26,30].

The α→β, α→γ and β→γ transformations could be induced by deformation during milling [26,[31], [32], [33]]. When external stress is applied, the strains could promote a crystal structure transformation, which it is called transformation induced by deformation [[34], [35], [36], [37]]. The strain promotes the atom movements in specific crystal planes to reduce the free energy, this displacement encouraging a phase transformation, this displacement encouraging a phase transformation. Some examples of transformation induced by deformation are; i) transformation from austenite (γ) to martensite (ε or α’) in steels (fcc to hcp or bcc) [34,[38], [39], [40], [41]], ii) transformations from β to α” phase (bcc to orthorhombic) [37,42] or from α to ω phase (bcc to hcp) [[42], [43], [44]] in Ti-based alloys. These kinds of transformations exhibit a specific orientation relationship (OR), in which two crystals (parent phase and product phase) are related in specific planes and directions. The α-Ti to β-Ti transformation exhibits a OR of (0001)α//(110)β and [112¯0]α//[111]β [[45], [46], [47]] and Wang et al. [48], report a OR of (011¯0)α//(110)β and [0001]α//[111]β, in a process of high speed machining. Zhu et al. [49] reported the OR of the α-Ti to γ-Ti transformation as: (112¯0)α//(2¯20)γ or (101¯0)α//(220)γ with [0001]α//[111]β, (0101¯)α//(11¯0)γ and Yang et al. [50] as [0001]α//[111]β. Wu et al. [21] and Hong et al. [51] reported this transformation in rolling and cryogenic channel die compression processes with a OR {101¯0}α//{110}γ and 0001α//001γ, respectively. For the β-Ti to γ-Ti transformation, Zhang and M. Aindow [52] reported two possible types of OR in a Ti-Al-Nb-Zr alloy, one is the Kurdjumov-Sachs; {111}γ//{101¯}β and 11¯0γ//111β and the second is the Pitsch; {001¯}γ//{01¯1}β and 11¯0γ//111β. Also Li et al. [53] proposed two OR; the firsts is (11¯1)γ//(110)β and [110]γ//[11¯1]β, in this case a shear is produced along the (110)β in the direction [1¯10]β with a Shockley partial dislocations of 2c6[1¯10]β and a second shear is produced in the plane (110)β, which promotes this transformation. The second is (001)γ//(110)β and [110]γ//[11¯1]β with a glide in the (1¯12)β and Shockley partial dislocations of 3c6[11¯1]β are produced.

In the current literature there are very few works on Ti-based alloys using Sn as alloying elements, and even fewer about Ti-based alloys with fcc structure with Sn for possible biomedical applications. Therefore, it seems interesting to explore the synthesis of Ti-13Ta-xSn alloys with a fcc crystal structure for possible new applications, such a biomedical use. Based on the above, the main goal in this work is to study the effect of amount of Sn and milling time on the formation of γ-Ti phase on the Ti-Ta based alloy. The alloys were obtained by mechanical alloying in a planetary mill. In addition, theoretical thermodynamic models for describing and explaining the formation of solid solution and amorphous phases were used.

Section snippets

Material and methods

The elemental powders utilized were Titanium grade IV with particle sizes of P80 < 149 μm NOAH Technologies, Tantalum with particle of P80 < 45 μm and 99.9% purity Sigma-Aldrich and Tin with particle sizes of P80 < 150 μm and 99.8% purity Sigma-Aldrich. The nominal compositions of the alloys studied were Ti-13Ta-xSn (x = 3, 6, 9 and 12 at.%). Mechanical alloying was carried out in a Retsch PM400 planetary mill with a speed of 250 r.p.m. The milling was carried out at room temperature; in order

Characterization of unmilled powder

Fig. 1 shows the SEM images of raw powders of Ti, Ta and Sn. In Fig. 1a, pure Ti has a spongy and irregular morphology, with a size of approximately 150 μm. This morphology presents good compactability. The morphology is characteristic of the Kroll process for Ti primary metal production [62,63]. Fig. 1b shows that Ta particles exhibit an irregular morphology with sizes smaller than 45 μm approximately. Fig. 1c shows that pure Sn powders have a spheroidal morphology characteristic of the

Conclusions

The amount of Sn and the milling times have a strong influence on the presence of three Ti-based phases, which are α-Ti, β-Ti, and γ-Ti. The α-Ti and β-Ti phases are equilibrium phases whereas γ-Ti is metastable phase. For the Ti-13Ta-3Sn alloy the β-Ti phase is formed in great quantity, ∼72% at 100 h. For the Ti-13Ta-6Sn, Ti-13Ta-9Sn and Ti-13Ta-12Sn alloys, the phase that is in greater quantity is γ-Ti, around 100% for the Ti-13Ta-6Sn, Ti-13Ta-9Sn milled at 100h. No evidence of Ti hydrides

CRediT authorship contribution statement

C. Aguilar: Conceptualization, Data curation, Formal analysis, Funding acquisition, Methodology, Project administration, Resources, Software, Supervision, Writing - original draft, Writing - review & editing. E. Pio: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Writing - original draft. A. Medina: Data curation, Formal analysis, Resources, Supervision, Writing - original draft. C. Martínez: Data curation, Formal analysis, Investigation, Project

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to acknowledge financial support from FONDECYT project no 1190797 and FONDEQUIP/EQM project no 140095. We also want to thank professor Dr. Juan Donoso for the critical revision of the manuscript.

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