Irradiation induced non-equilibrium phase formation and structural transformation in the Fe–Ti–Nb multilayered films

Abstract

Six sets of the Fe–Ti–Nb multilayered films were first prepared and then subjected to ion beam irradiation with 180 keV Xe ions. The unique amorphous phases, phase separation induced dual amorphous phase and orientation-preferred BCC–Fe crystalline phase were observed in these Fe–Ti–Nb multilayered films at different irradiation doses. In addition, it is also revealed that the Ti-rich HCP crystalline phase could transform into a metastable FCC phase in the Fe₁₅Ti₇₅Nb₁₀ film. The possible mechanisms of these non-equilibrium phase formation and structural transformations are discussed based on atomic collision theory, geometric crystallography and ab initio calculations.

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Introduction

Fe-based amorphous alloys have attracted more and more attention in recent years due to their ultra-high strength, excellent soft magnetic properties, good corrosion resistance, and their rather low cost.^1–3 For example, a variety of Fe-based amorphous alloys have been developed by Inoue et al. and other researchers.⁴ Among these amorphous alloys, Ti-containing Fe-based amorphous alloys are known as one of the potential lightweight engineering materials for industrial applications with high strength and ductility, especially for Fe–Ti–Nb ternary metal system. In Park's research, Fe–Ti–Nb amorphous alloys were obtained to own good compressive mechanical properties, i.e., high yield (1.9–2.2 GPa) and fracture (2.2–2.6 GPa) strength together with large plasticity (6–13%).⁵ Meanwhile, it has also been found by extensive researches that microstructure and phase transformations have a significant influence on the mechanical properties of amorphous alloys.^6,7 Therefore, the present study focuses on the non-equilibrium phase formation and structural transformations of the Fe–Ti–Nb multilayered films upon ion beam irradiation (IBI).

Comparing with other non-equilibrium producing techniques such as the liquid melt quenching developed in 1960,⁸ solid-state reaction and mechanical alloying introduced in 1980s,^9–11 the ion beam irradiation has been proven to be very effective in producing metastable amorphous alloys.¹² There are three essential features involved in the IBI. First, the individual metal layers elevate the interfacial free energies of the initial multilayered films up to a highly energetic state desired. Second, ion irradiation could be conducted to trigger the interfacial mixing at a low temperature. Third, the irradiation dose could be controlled by adjusting the ion current, thus enable researchers to trace the details of the structural phase transformations taking place in the films. Consequently, IBI has been widely employed to study the non-equilibrium formation and transformation of metal systems.^13–15

In order to investigate the related microstructures and phase transformations of the Fe–Ti–Nb ternary metal system, six sets of multilayered films with overall compositions of Fe₈₀Ti₁₂Nb₈, Fe₆₅Ti₂₁Nb₁₄, Fe₅₀Ti₃₀Nb₂₀, Fe₃₀Ti₄₂Nb₂₈, Fe₁₅Ti₅₁Nb₃₄ and Fe₁₅Ti₇₅Nb₁₀ were first prepared by electron depositing and then subjected to ion beam irradiation by 180 keV Xe ions upon different doses in the present study.

Experimental procedure

In the experiments, 180 keV Xe ions were used as irradiating ions in IBI. Based on the TRIM calculation,¹⁶ the thickness of the Fe–Ti–Nb system should be controlled about 40 nm. Thus, all the Fe–Ti–Nb multilayered films were designed to be around 35 nm. Multilayered films were first prepared by alternatively depositing pure Fe (99.99%), Ti (99.99%) and Nb (99.99%) at a rate of 0.1–0.5 Å s⁻¹ onto newly cleaved NaCl single crystal substrates in an e-gun evaporation system with a vacuum level of 10⁻⁶ Pa. The as-deposited films were then subjected to irradiation in an implanter with a vacuum level better than 5 × 10⁻⁴ Pa. The doses were in a range from 4 × 10¹⁴ Xe⁺ per cm² to 7 × 10¹⁵ Xe⁺ per cm². During irradiation, liquid nitrogen was injected to cool the sample holder and the current density was confined to be 15 μA cm⁻². The structures of all the Fe–Ti–Nb multilayered films were examined by bright field image and selected area diffraction (SAD) of transmission electron microscopy (JEM 200 CX) and high-resolution transmission electron microscopy (HRTEM) (TECNAI G² 20). And the X-ray fluorescence (XRF) (XRF-1800) and energy dispersive spectroscopy (EDS) examination were used to ascertain the real compositions of the deposited films, as well as the resultant alloy phases formed.

Results and discussion

Formation and phase separation of amorphous phase

In order to present the detailed phase formation and structural phase transformations emerged in the six sets of Fe–Ti–Nb multilayered films, which were irradiated to the dose range from 4 × 10¹⁴ to 7 × 10¹⁵ Xe⁺ per cm², all the experimental results of the samples were summarized in the Table 1. One can see clearly that unique amorphous phases can be formed in the Fe₆₅Ti₂₁Nb₁₄, Fe₅₀Ti₃₀Nb₂₀, Fe₃₀Ti₄₂Nb₂₈ and Fe₁₅Ti₅₁Nb₃₄ multilayered films, yet at different irradiation doses. Meanwhile, in the multilayered films with composition of Fe₆₅Ti₂₁Nb₁₄ and Fe₃₀Ti₄₂Nb₂₈, crystalline phases were formed again with increasing the irradiation dose. However, in the other two films with too much of Fe or Ti, unique amorphous phase was difficult to be obtained.

Table 1 The phase transformations observed in the multilayered films irradiated to the dose range from 6 × 10¹⁴ to 7 × 10¹⁵ Xe⁺ per cm^2a

Ion dose (Xe^+ per cm^2)	Fe80Ti12Nb8	Fe65Ti21Nb14	Fe50Ti30Nb20	Fe30Ti42Nb28	Fe15Ti51Nb34	Fe15Ti75Nb10
a Fe: Fe-rich BCC phase; Ti: Ti-rich HCP phase; Nb: Nb-rich BCC phase; F: Ti-rich FCC phase; A: amorphous phase.
0	Fe + Ti + Nb	Fe + Ti	Fe + Ti + Nb	Fe + Ti + Nb	Fe + Ti + Nb	Fe + Ti + Nb
6 × 10^14	Fe + Ti + Nb + A	Fe + A	Fe + A	Fe + Ti + Nb	Fe + Ti + Nb	Fe + Ti + Nb
1 × 10^15	Fe + A	A	Fe + A	Fe + Ti + Nb	Fe + Ti + Nb	Ti + Nb + F
5 × 10^15	Fe + A	Fe + A	Fe + A	A	Fe + Ti + Nb + A	F
7 × 10^15	Fe + A	Fe + A	A	Fe + A	A	F

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Further, all the formed amorphous phases were examined by HRTEM, and an interesting phenomenon of phase separation was observed in the formed amorphous phase in Fe₃₀Ti₄₂Nb₂₈ multilayered film after irradiated to the dose of 5 × 10¹⁵ Xe⁺ per cm². Fig. 1 shows the SAD patterns and corresponding bright field image of the amorphous phase by TEM and HRTEM. One can see clearly from the Fig. 1(a) that the SAD pattern is consisted of two halos, which demonstrated the formation of amorphous phase. The bright field image of local region by HRTEM was divided by a groove in the middle into two parts, brighter area in the left and darker area in the right, and the corresponding SAD patterns were shown in the Fig. 1(c) and (d), respectively. From the comparison of the two figures, it can be seen that only one halo was obtained in the two figures and the radiuses in the two figures were also different, suggesting that phase separation may occur in the film. The compositions of the two regions were measured by EDS to be Fe₃₄Ti₄₄Nb₂₂ and Fe₁₈Ti₃₈Nb₄₄, respectively. As the lattice parameter of BCC–Fe is smaller than that of BCC–Nb, it is reasonable that the radius of the halo reflected from Fe-rich amorphous phase is bigger than that of the halo reflected from Nb-rich amorphous phase.

Fig. 1 (a) The SAD pattern of the resultant phases of Fe₃₀Ti₄₂Nb₂₈ multilayered films after irradiated to the dose of 5 × 10¹⁵ Xe⁺ per cm² by TEM, the corresponding (b) bright field image and (c) (d) the SAD pattern of local region by HRTEM.

According to the atomic collision theory, a highly energetic state of Fe–Ti–Nb multilayered films was leaded by a sequence of ballistic collisions triggered by IBI. When the collision cascade is terminated, the highly energetic state has to relax to equilibrium. However, as the relaxation period is extremely short, i.e., 10⁻¹⁰ s, atomic rearrangement is hard to be taken place. Consequently, no complicated structured crystalline phase was retained, and only crystalline phase with simple structure, such as BCC, FCC and HCP, or the disordered structure, i.e., amorphous phase, could be preserved. Therefore, the phase formation and transformations are the results of the competitions between simple structural crystalline phases and amorphous phases. Meanwhile, structure and composition fluctuation caused by the ballistic collision may be kept after the extremely short relaxation period, thus phase separation induced dual amorphous phase formed in local regions.

Preferred orientation of BCC–Fe phase

In the Fe₅₀Ti₃₀Nb₂₀ multilayered films, preferred orientations of BCC–Fe phase were observed at as-deposited state and after irradiated to the doses less than 8 × 10¹⁴ Xe⁺ per cm². Fig. 2 shows the SAD patterns of the Fe₅₀Ti₃₀Nb₂₀ multilayered films at different states. The diffraction rings and symmetrical texture spots in the Fig. 2(a) demonstrated that the resultant phase at as-deposited state was a mixture of Fe-rich BCC phase, Ti-rich HCP phase and Nb-rich BCC phase, with an orientation-preferred phase. The orientation-preferred phase was indexed to be a BCC–Fe phase with lattice parameter of 2.91 Å, which is a little bigger than that of pure BCC–Fe phase. The preferred orientations of BCC–Fe phase were maintained when the irradiation dose was increased to 6 × 10¹⁴ Xe⁺ per cm², as the Fig. 2(b) shown. However, Fig. 2(c) shows that preferred orientation of BCC–Fe phase disappeared with further increasing the irradiation dose.

Fig. 2 The SAD patterns of the resultant phases of Fe₅₀Ti₃₀Nb₂₀ multilayered films (a) at as-deposited state, (b) after irradiated to the dose of 6 × 10¹⁴ Xe⁺ per cm² and (c) after irradiated to the dose of 8 × 10¹⁴ Xe⁺ per cm².

The formation of preferred orientation of BCC–Fe phase was assumed to be attributable to the direction of atomic vapor during deposition. The preferred orientated BCC–Fe phase maybe resulted from the epitaxial-like growth of the Fe layer on the Ti layer. According to the researches of Bauer and Ramirez,^17,18 the epitaxial monolayer or multilayer formed usually with the most densely packed plane paralleled. Thus, mutual epitaxial of the HCP and BCC metals is consisting of stacking sequences of densely packed planes, i.e., (110)_BCC and (001)_HCP. From purely geometrical considerations, Fig. 3(a) shows the schematic diagram of the orientations (110)_Fe deposited on (001)_Ti. and b = a_HCP are the nearest-neighbor distance of BCC–Fe phase and HCP–Ti phase. Orientations preferred are those in which the most densely packed rows ([010]_HCP) in (001)_HCP are paralleled to those ([1̄11], [11̄1], [001]) in (110)_BCC. The first two are Kurdjumov-Sachs (KS) orientation, and occurs when or . The last one is Nishiyama–Wassermann (NW) orientation occurs when or . Another NW relationship is also preferred when the [112]_HCP and the [11̄0]_BCC have the same distance, which is called NW-y configuration occurred when or a_HCP/a_BCC = 1. In the experiment, the lattice parameter of BCC–Fe phase was expanded from 2.82 Å (pure BCC–Fe) to 2.91 Å, which is almost equal to the a_Ti = 2.92 Å. Therefore, the formation of orientation-preferred BCC–Fe phase may be attributable to the NW-y configuration, and the Fig. 3(b) shows the possible orientation. In order to have an intuitional analysis, an atomic projection of Fe (110) deposited on Ti (001) by NW-y configuration along [001]_Ti was shown in the Fig. 4. It can be seen that good coherent relationship can be achieved with stretching Fe along [1̄10]_Fe or compressing Ti along [112]_Ti.

Fig. 3 (a) Schematic diagram of the orientations Fe (110) deposited on Ti (001) preferred and (b) the possible orientation happened in Fe₅₀Ti₃₀Nb₂₀ multilayered films.

Fig. 4 The atomic projection of Fe (110) deposited on Ti (001) by NW-y configuration along [001]_Ti.

Moreover, the mechanisms of thin crystalline films grow near equilibrium maybe the Vomer–Weber (VW), the Stranski–Krastanov (SK) or the Frank–van der Merwe (FM) mode, depending on the relative magnitudes of the surface energies and the interfacial energy.¹⁹ In the SK or VW mode, three-dimensional crystals form on the substrate. If possible, nucleation and growth will along unidirectional steps, thus preferred orientation of BCC–Fe crystal phase could be observed. From the geometric crystallography analysis and the film growth mechanism discussion above, the experiment results are reasonable.

Structural transformation from HCP–Ti to FCC–Ti phase

As Table 1 shown, an interesting phase transformation from HCP–Ti to FCC–Ti phase was observed in the Fe₁₅Ti₇₅Nb₁₀ multilayered films with increasing the irradiation dose. Only one FCC–Ti phase was retained in the films after irradiated to the doses more than 5 × 10¹⁵ Xe⁺ per cm². For comparison, Fig. 5 shows the SAD patterns of the Fe₁₅Ti₇₅Nb₁₀ multilayered films at as-deposited state and after irradiated to the dose of 7 × 10¹⁵ Xe⁺ per cm². One can see from the diffraction rings in Fig. 5(a) that a mixture of Fe-rich BCC phase, Ti-rich HCP phase and Nb-rich BCC phase was obtained. While, only one Ti-rich FCC phase was observed after the films was irradiated to high dose, as shown in the Fig. 5(b), and the lattice parameter of the FCC–Ti phase was indexed to be 4.11 Å.

Fig. 5 The SAD patterns of (a) the resultant phases of Fe₁₅Ti₇₅Nb₁₀ multilayered films at as-deposited state and (b) the newly formed FCC–Ti after irradiated to the dose of 7 × 10¹⁵ Xe⁺ per cm².

The formation of the metastable FCC–Ti phase and such a phase transformation from BCC–Ti to FCC–Ti may be the results of the atomic collision physical process of IBI discussed above. In the thermodynamic aspect, the energy difference between the HCP–Ti and FCC–Ti phases calculated by Miedema et al. is relatively small to be 0.5 kJ mol⁻¹.²⁰ Thus, the transformation from HCP–Ti phase to a metastable FCC–Ti phase is possible by techniques far from equilibrium, e.g. IBI. In the kinetics aspects, the transformation is readily, as both HCP and FCC structures are close-packed with the same atomic packing density and the only difference between them is the stacking order. The shearing mechanism (SM) proposed by Liu et al. is probably one of the possible explain for the transformation.²¹ The phase is transformed with the sliding of the (0002)_HCP plane atoms along the vector direction of 1/3〈11̄00〉. The lattice constant relationship between the two structures can be readily determined to be . With the relationship, the lattice constants of the FCC–Ti phase can be calculated to be 4.13 Å, which is consistent with 4.11 Å obtained in experiment.

To further confirm the process of phase transformation in the Fe₁₅Ti₇₅Nb₁₀ multilayered films, ab initio calculation based on Cambridge Sequential Total Energy Package (CASTEP) was carried out. In the present calculations, the exchange and correlation effects are described by the function of Perdew and Wang,²² which employs generalized gradient approximation (GGA).²³ The interaction between the valance electrons and ionic cores is represented by ultrasoft pseudopotentials (US-PP). Brillouin zone integrations were performed using and 16 × 16 × 16 Monkhorst–Pack grid,²⁴ which led to 120 irreducible κ points. The lattice parameter of FCC–Ti determined by CASTEP is to be 4.09 Å, which is in good accordance with the experimental observation (a_FCC–Ti = 4.11 Å). The results are also consistent with that calculated by Wang et al. using VASP with PAW-GGA potential to be 4.10 Å.²⁵

Conclusions

Upon the appropriate ion beam irradiation, the unique amorphous phases could be obtained in the films with overall compositions of Fe₆₅Ti₂₁Nb₁₄, Fe₅₀Ti₃₀Nb₂₀, Fe₃₀Ti₄₂Nb₂₈ and Fe₁₅Ti₅₁Nb₃₄. Meanwhile, phase separation caused by composition fluctuation was also observed in the Fe₃₀Ti₄₂Nb₂₈ multilayered film and the phase separation further results in the formation of dual amorphous phase. In addition, BCC–Fe phases in the Fe₅₀Ti₃₀Nb₂₀ multilayered films exhibit the preferred orientations at as-deposited state and after irradiated to doses less than 8 × 10¹⁴ Xe⁺ per cm². It was assumed to be attributable to the unidirectional epitaxial-like growth of atomic vapor during deposition. Besides, for the FCC–Ti phase formed in the Fe₁₅Ti₇₅Nb₁₀ multilayered films at high irradiation doses, the lattice constant is calculated to be compatible with both the SM prediction and ab initio calculation.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (50971072, 51131003), the Ministry of Science and Technology of China (973 Program 2011CB606301, 2012CB825700), and the Administration of Tsinghua University.

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