Ion Species/Energy Dependence of Irradiation-Induced Lattice Structure Transformation and Surface Hardness of Ni₃Nb and Ni₃Ta Intermetallic Compounds

Abstract

Bulk samples of Ni₃Nb and Ni₃Ta intermetallic compounds were irradiated with 16 MeV Au, 4.5 MeV Ni, 4.5 MeV Al, 200 MeV Xe and 1.0 MeV He ions, and the change in near-surface lattice structure was investigated by means of the grazing incidence x-ray diffraction (GIXD) and the extended x-ray absorption fine structure (EXAFS). The Ni₃Nb and Ni₃Ta lattice structures transform from the ordered structures (orthorhombic and monoclinic structures for Ni₃Nb and Ni₃Ta, respectively) to the amorphous state by the Au, Ni, Al and Xe ion irradiations. Irrespective of such heavy ion species or energies, the lattice structure transformation to the amorphous state almost correlate with the density of energy deposited through elastic collisions. In the case of the samples irradiated with 1.0 MeV He ions, however, no amorphization was observed even when the density of elastically deposited energy is the same as that for Au irradiated sample which showed the amorphous phase. The change in Vickers hardness induced by the amorphization was also measured and was discussed in terms of ion fluence and the density of deposited energy.

1. Introduction

Conventionally, the processes of quenching, forging or rolling have been used so far as a method for modifying metal materials. These processes utilize the thermal energy and/or strain energy which are deposited into materials. While, energetic ion beam irradiation is also a potential method to deposit the energy into materials and to modify various material properties. Some readers may think of the ion implantation as a method of materials modification by using ion beam irradiation. The ion implantation process has often been used so far to inject various atoms into matrix materials¹⁾. While on the other hand, the energy deposited by ions has almost exclusively been used to simulate degradation or damage on the structure and several properties of materials related to nuclear power plants and space crafts (so called “Radiation Damage”)²⁾. However, energetic ion irradiation locally gives high density energy deposition into a target, and it can induce non-thermal equilibrium phases. Therefore, the ion irradiation can add various new properties to materials, which cannot be realized by conventional materials processing methods. Also, the effect of energetic ion irradiation on materials can be controlled by changing ion species, energies and/or ion fluence.

In the present study, we have used bulk Ni-based intermetallic compounds as target materials. Intermetallic compounds show various characteristic properties such as high temperature strength³⁾, oxidation resistance⁴⁾, hydrogen storage⁵⁾, superconductivity⁶⁾ or shape memory⁷⁾, depending on their chemical compositions. The varieties of these properties derive from the difference in their ordered lattice structures.

Mainly in 1980s and 1990s, a lot of researchers reported lattice disordering and amorphization phenomena induced by charged particle irradiations for various intermetallic compounds such as NiTi^8–11), NiAl^9,12), NiAl₃¹³⁾, Ni₂Al₃¹⁴⁾, FeAl¹²⁾, Zr₃Fe^15,16), ZrFe₂¹⁶⁾, ZrCr₂¹⁶⁾, Zr₃Al^17,18), NiZr₂¹⁹⁾, NiZr¹⁹⁾, Ni₃Zr¹⁹⁾. AuIn₂²⁰⁾, In₂Pd²⁰⁾ and CuTi²¹⁾. Especially, in-situ observations under MeV electron irradiation by using high voltage transmission microscopes (HVEM) has often been used for the investigation of the dependence of irradiation-induced amorphization and lattice disordering on irradiation temperature, dose rate and total dose of irradiating electrons^8,15,19). Even reecently, by using HVEM, new results for the electron beam induced amorphization for CoTi, Co₃Ti, Cr₂Ti, Cr₂Al, Ti₂Pd, and TiNi_1−xFe_x have been reported^22–25). To clarify the amorphization kinetics, some theoretical approaches and computer simulations have also been performed^11,26–29). The tendency to be disordered or amorphized by the irradiation, however, strongly depend on the kinds of target intermetallic compounds. Therefore, to understand fully the mechanism of irradiation-induced amorphization and lattice disordering, we have to perform irradiation experiments also for intermetallic compounds which have never been investigated yet. Moreover, in most of previous studies, not bulk but thin film samples have been used to perform the lattice structure observation by means of transmission electron microscopes (TEM). As target surfaces act as strong sinks against irradiation-produced lattice defects, effects of the irradiation on lattice structures of thin films are expected to be different from those of bulk samples. In addition, if we chose thin film samples as ion irradiation targets, it would be difficult to investigate the irradiation effects on macroscopic properties of them, such as mechanical, magnetic and electrical properties. Recently, our research group has used energetic ion irradiations to modify the lattice structure and the mechanical property of bulk intermetallic compounds. In particular, we have chosen Ni-based intermetallic compounds as ion irradiation targets because they are applied for structural materials^30,31). They are promising as a next generation type of high temperature materials and it is needed to investigate not only structure transformation, but also mechanical property. In our previous results, we showed the lattice structure transformation, which was induced by energetic ion irradiation, and the change in Vickers hardness for Ni₃Al³²⁾, Ni₃V^33,34), Ni₃Nb and Ni₃Ta³⁵⁾ intermetallic compounds. At room temperature, Ni₃V intermetallic compound shows the tetragonal ordered structure (D0₂₂ structure), and Ni₃Al intermetallic compound shows the cubic ordered structure (L1₂ structure). However, both ordered structures change into the disordered fcc structure (A1 structure) by the energetic ion irradiation^32–34). While for Ni₃Nb and Ni₃Ta intermetallic compounds, the lattice structures transformed from the ordered orthorhombic or monoclinic structure to the amorphous state by the ion irradiation³⁵⁾. These results were obtained under the same irradiation condition that all intermetallic compound samples were irradiated with 16 MeV Au ions at room temperature. Also, we have performed Al and I ion irradiation for Ni₃Al intermetallic compound and confirmed the transition to the disordered fcc structure³²⁾.

To use the energetic ion beam irradiation as a tool for material modifications, we need to find irradiation parameters which dominate the irradiation effect. In the present study, we have paid attention to ion species and energy dependence of the structural change from the ordered structure to the amorphous state in Ni₃Nb and Ni₃Ta interatomic compounds from the point of view of energy deposition through the electronic excitation and the elastic collisions.

In addition to the use of bulk samples instead of thin films, another advantage of our study is to utilize the extended X-ray absorption fine structure (EXAFS) spectroscopy at a synchrotron facility as a means to estimate the change in lattice structure. As EXAFS is so called “non-destructive” method, we do not need to make thin films but can use bulk samples also for the EXAFS measurements, which are used for the irradiation and hardness measurements. Moreover, through the EXAFS measurement, we can observe local atomic arrangements around selected atomic elements, i.e, Nb or Ta in the present study. Such information will be quite useful for the investigation of irradiation effects on the lattice structure of intermetallic compounds. In this report, we will also show the ion species and energy dependence on hardness for Ni₃Nb and Ni₃Ta intermetallic compounds.

We have chosen 16 MeV Au ions, 4.5 MeV Ni ions, 4.5 MeV Al ions, as irradiating ions to Ni₃Nb and Ni₃Ta targets. In addition to these ions irradiation, we have performed 1.0 MeV He ions irradiation to the two kinds of targets. For Ni₃Nb, 200 MeV Xe ion irradiation was also performed. Although some results for the Au ion irradiation have already been reported³⁵⁾, we will discuss here the ion species/energy dependence on the irradiation effect including the previous results of Au ion irradiation. As can be seen in Fig. 1, the shapes of the depth profiles of energy deposition by 16 MeV Au, 4.5 MeV Ni, 4.5 MeV Al and 1.0 MeV He ions for Ni₃Nb and Ni₃Ta are approximately the same. This point is very important when effects of ion irradiation are compared among several ion irradiations with different species and/or energies. Moreover, the Ni ion irradiation, what is called, “the self-ion irradiation” will reveal whether the lattice structure transformation is affected by the accumulation of irradiating ions or not. The value of electronic stopping power, Se, for 200 MeV Xe ion irradiation is much larger than that for 1.0 MeV He, 4.5 MeV Al, 4.5 MeV Ni or 16 MeV Au ions. Through such high-Se ion irradiation experiment, therefore, we can discuss the contribution of the high density electronic excitation to the irradiation-induced lattice structure transformation of Ni₃Nb.

Fig. 1

Depth profiles of energies deposited through (a) electronic excitation and through (b) elastic collision for one ion per unit path length for 16 MeV Au⁵⁺, 4.5 MeV Ni²⁺, 4.5 MeV Al²⁺, 200 MeV Xe⁺¹⁴ and 1.0 MeV He⁺ ion irradiation in Ni₃Nb alloys, and depth profiles of energies deposited through (c) electronic excitation and through (d) elastic collision and for one ion per unit path length for 16 MeV Au⁵⁺, 4.5 MeV Ni²⁺, 4.5 MeV Al²⁺ and 1.0 MeV He⁺ ion irradiation in Ni₃Ta. (a) Ni₃Nb, (b) Ni₃Nb, (c) Ni₃Ta, (d) Ni₃Ta.

2. Experimental Procedure

Ingots of Ni₃Nb and Ni₃Ta were manufactured by arc melting and subsequently homogenized at 1273 K for 96 and 144 hours, respectively. The two kinds of ingots were cut into sheet-type samples and mechanically polished. Ni₃Nb samples were irradiated at room temperature with 16 MeV Au⁵⁺, 4.5 MeV Ni²⁺, 4.5 MeV Al²⁺ and 1.0 MeV He⁺ by using 3-MV tandem accelerator and a single-ended accelerator at Takasaki Advanced Radiation Research Institute, National Institutes for Quantum and Radiological Science and Technology. Some Ni₃Nb samples were also irradiated with 200 MeV Xe⁺¹⁴ ions by using 20-MV tandem accelerator at Nuclear Science Research Institute, Japan Atomic Energy Agency. Ni₃Ta samples were irradiated with 16 MeV Au⁵⁺, 4.5 MeV Ni²⁺, 4.5 MeV Al²⁺ and 1.0 MeV He⁺ ions at room temperature. The irradiation parameters are listed in Table 1. Figures 1(a) and 1(b) show the depth profiles of the energy deposited through the electronic excitation and the elastic collisions by one ion per unit path length for 16 MeV Au⁵⁺, 4.5 MeV Ni²⁺, 4.5 MeV Al²⁺, 200 MeV Xe⁺¹⁴ and 1.0 MeV He⁺ ion irradiations in Ni₃Nb intermetallic compound. Figures 1(c) and 1(d) show the depth profiles for 16 MeV Au⁵⁺, 4.5 MeV Ni²⁺, 4.5 MeV Al²⁺ and 1.0 MeV He⁺ ions irradiation in Ni₃Ta intermetallic compound. These profiles were calculated by using the SRIM code³⁶⁾. For the calculations, we used the value of 9.09 g/cm³ and 12.04 g/cm³ as the density of Ni₃Nb and Ni₃Ta samples, respectively. As can be seen in the figure, the effect of the ion irradiation is restricted only at the region from the sample surface to the depth of about 2 μm except for Xe ion irradiation, the depth affected by which is about 10 μm. Therefore, we have chosen the grazing incidence X-ray diffraction (GIXD) to observe the surface lattice structures (from the surface to the depth of about 200 nm).

Table 1 Irradiation parameters.

Target	Ion species	Au5+	Ni2+	Al2+	Xe14+	He+
	Energy	16 MeV	4.5 MeV	4.5 MeV	200 MeV	1.0 MeV
Ni3Nb	Projected Range[µm]	1.54	1.48	1.51	8.57	1.59
	Se [keV/nm]	6.32×100	2.85×100	4.92×100	3.95×101	7.21×10−1
	Sn [keV/nm]	2.87×100	4.72×10−1	6.52×10−2	1.67×10−1	1.23×10−3
	Ion Fluence [m−2]	1.0×1017	5.0×1017	5.0×1018	1.0×1018	4.0×1020
		5.0×1017	2.5×1018	1.0×1019	5.0×1018	1.2×1021
		5.0×1018	2.5×1019	1.0×1020	1.6×1019	1.7×1021
Ni3Ta	Projected Range[µm]	1.42	1.35	1.44		1.59
	Se [keV/nm]	6.16×100	2.77×100	4.81×100		7.08×10−1
	Sn [keV/nm]	3.31×100	5.31×10−1	7.36×10−2		1.38×10−3
	Ion Fluence [m−2]	1.0×1017	5.0×1017	5.0×1018		4.0×1020
		5.0×1017	2.5×1018	1.0×1019		1.2×1021
		5.0×1018	2.5×1019	1.0×1020		1.7×1021

To investigate the change in local atomic arrangements around Ta atoms in the unirradiated Ni₃Ta sample and those irradiated with Ni or Au ions, we performed the EXAFS measurement around the Ta-L3 absorption edge (9.88 keV) at the 27B beamline of the synchrotron radiation facility of High Energy Accelerator Research Organization (KEK-PF). We used the computer software, WinXas³⁷⁾, to analyze the obtained EXAFS spectra. In the analyses, all EXAFS spectra were Fourier transformed using k³ weighting with the k range from 20–30 to 100–150 nm⁻¹.

We performed the micro Vickers hardness measurements at room temperature for unirradiated and irradiated Ni₃Nb and Ni₃Ta samples. To detect the hardness near the sample surface, the applied load was 10 gf (98.07 mN). The time interval of indentation was kept at 10 seconds. Indent depth is about 1 μm which is enough to cover the irradiated region of sample surface.

3. Results and Discussion

First of all, we discuss the change in surface lattice structures by the ion irradiations. Figure 2 shows the wide-scanned GIXD spectra for Ni₃Nb samples irradiated with Au, Ni, Al, Xe and He ions. For comparison, the result for the unirradiated sample is also shown. For the spectrum of the unirradiated Ni₃Nb, several diffraction peaks are clearly observed. These peaks correspond the orthorhombic ordered structure which is the thermal equilibrium phase of Ni₃Nb at room temperature³⁸⁾. To see the change in spectra in more detail, Fig. 3 shows the narrow-scanned spectra around 43° for the Ni₃Nb samples irradiated with Au, Ni, Al, Xe and He ions. The intensity of each peak decreases with increasing the ion fluence. To emphasize the change in shape of the GIXD peak by each irradiation, all the peaks are normalized as the intensity of the highest peak for each spectrum becomes unity and are plotted in the figure. For the Ni₃Nb samples irradiated with 4.5 MeV Ni ions (Fig. 3(b)), the peaks corresponding to the orthorhombic structure completely disappear for fluences of 2.5 × 10¹⁸/m² and 2.5 × 10¹⁹/m², and a broad peak appears around 43°. The broad XRD spectra for the Ni ion irradiation are similar to the spectra for Au ion irradiation³⁵⁾ (Fig. 3(a)). A Ni₃Nb intermetallic compound matrix consists of Ni and Nb atoms. Even when Ni ions are implanted into Ni₃Nb matrix with the fluence of 2.5 × 10¹⁹/m², the total number of Ni atoms near the surface of Ni₃Nb does scarcely change, and accumulated Ni ions does not have any effect on the GIXD spectrum. Therefore, the spectrum change by the Ni ion irradiation is caused by the energy transferred from the energetic Ni ions to the sample and not by the accumulation of Ni ions in the sample. For the change in the spectra by the Al ion irradiation (Fig. 3(c)), the same tendency has been observed as that for the Ni ion irradiation. The peaks corresponding to the orthorhombic structure completely disappear and a broad peak appears around 43° by the Al ion irradiation with fluence of 1.0 × 10²⁰/m². However, for the Au ion irradiated samples, a broad peak is observed with the fluence of 5.0 × 10¹⁷/m² and 5.0 × 10¹⁸/m². Also, a similar peak is confirmed in Ni ion irradiated samples with fluence of 2.5 × 10¹⁸/m² and 2.5 × 10¹⁹/m². This result shows that for the irradiation with smaller mass ions, a larger fluence is needed to change the lattice structure of the Ni₃Nb samples. Such a trend becomes more remarkable in the case of 1.0 MeV He ion irradiation (Fig. 3(e)). Even for the fluence of 1.7 × 10²¹/m², the lattice structure of the Ni₃Nb sample is never changed, but the peaks for the orthorhombic structure are still clearly observed. The spectrum changes have also been observed for the 200 MeV Xe ion irradiation (Fig. 3(d)). In the spectrum with the fluence of 1.6 × 10¹⁹/m², all the peaks for the orthorhombic lattice structure disappear, and a broad peak appears around 43°.

Fig. 2

Wide-range GIXD spectra for (a) Au, (b) Ni, (c) Al, (d) Xe and (e) He ion irradiated Ni₃Nb samples. For comparison, the spectrum for unirradiated specimen is shown in each figure.

Fig. 3

Narrow-range GIXD spectra for (a) Au, (b) Ni, (c) Al, (d) Xe and (e) He ion irradiated Ni₃Nb samples. For comparison, the spectrum for unirradiated specimen is shown in each figure. Indices on spectrum for unirradiated sample correspond to the orthorhombic structure.

It is well known that a broad peak in XRD spectrum indicates a state which loses the long-range ordering of atomic arrangements³⁹⁾. This is like liquid, what is called an amorphous state. Therefore, irrespective of heavy ion species, the lattice structure of Ni₃Nb tends to transform by the irradiation from the orthorhombic ordered structure to the amorphous state even at room temperature. The critical ion fluence which causes the amorphization, however, depends on the ion species and/or ion energy. In the case of 1.0 MeV He ion irradiation, within the ion-fluece range of the present experiment, the amorphization was not observed. The formation of the amorphous phase for Ni-Nb alloy by liquid quenching has been reported⁴⁰⁾. The change in XRD spectra by the liquid quenching is similar to that observed in the present irradiation experiment.

We have found a similar irradiation effect for the Ni₃Ta intermetallic compound. Figure 4 shows the results of narrow-scanned GIXD spectra for the unirradiated Ni₃Ta sample and those irradiated with 16 MeV Au, 4.5 MeV Ni, 4.5 MeV Al and 1.0 MeV He ions. The spectrum for the unirradiated sample corresponds to the monoclinic ordered lattice structure, which is a thermal equilibrium structure of Ni₃Ta at room temperature⁴¹⁾. After the irradiation, the peaks corresponding to the monoclinic structure tend to disappear with increasing in ion fluence. For Au, Ni or Al ion irradiation they nearly completely disappear and the broad peak appears around 43 degree for the irradiation with high fluences. Therefore, this result implies that Ni₃Ta tends to transform from the monoclinic ordered structure to the amorphous state by each ion irradiation at room temperature. However, for He ion irradiation (Fig. 4(d)), ordered peaks remain to the high fluence of 1.7 × 10²¹/m².

Fig. 4

Narrow-range GIXD spectra for (a) Au, (b) Ni, (c) Al and (d) He ion irradiated Ni₃Ta samples. For comparison, the spectrum for unirradiated specimen is shown in each figure. Indices on spectrum for unirradiated sample correspond to the monoclinic structure.

Figure 5 shows the k³-weighted EXAFS spectra around Ta L3 absorption edge for unirradiated Ni₃Ta and Ni₃Ta samples irradiated with the Au and Ni ions, and their EXAFS – FT spectra. When paying attention to the first peak which corresponds to nearest neighbor atoms for the Ta atom, even after the irradiation, it is still more outstanding than any other peaks which are strongly suppressed or nearly disappear by the irradiation. The same effect of the ion irradiation on the EXAFS spectrum has already been observed for the Au ion-irradiated Ni₃Nb³⁵⁾, which is shown in the Fig. 6. The present EXAFS results clearly confirm that the amorphization is induced by the ion irradiation in the both materials, i.e., the long range ordering tends to disappear and only the short range ordering still survives the irradiation.

Fig. 5

(a) k³-weighted EXAFS spectra around Ta L3 absorption edge for unirradiated Ni₃Ta specimen and those irradiated with Au or Ni ions, and (b) their FT EXAFS spectra. (a) Ni₃Ta, (b) Ni₃Ta.

Fig. 6

FT EXAFS spectra around Nb K absorption edge for the unirradiated Ni₃Nb specimen and that irradiated with Au ions¹³⁾.

In our previous study³⁵⁾, we performed only the Au ion irradiation for Ni₃Nb and Ni₃Ta intermetallic compounds. In the present study, we have investigated the lattice structure transformation by using five kinds of ion species with different energies. Therefore, we can discuss irradiation parameters for describing the irradiation-induced amorphization. First of all, the GIXD results for Ni₃Nb are summarized in terms of ion fluence in Fig. 7. Open circles show the amorphous state and X-marks show the ordered crystal structure. A triangle-mark shows the case where it is difficult to decide whether the specimen is completely amorphized or not. The figure indicates that there does not exist any clear correlation between the transformation to the amorphous state and the ion fluence.

Fig. 7

Lattice structure transformation for Ni₃Nb in terms of ion influence.

Next, we consider the processes of energy deposition by the energetic ions in materials. When an energetic ion penetrates a target, it loses its energy via two nearly independent processes: the energy loss through the elastic collisions, which dominate the energy loss of ions in the low energy range, and the energy loss through the electronic excitation and ionization, which overwhelm the energy loss through the elastic collisions in the high energy range. To discuss the ion-irradiation induced amorphization in Ni₃Nb in terms of the two interaction processes, the density of energy deposited through the electronic excitation, E_elec, and that through the elastic collisions, E_elas, in the region observed by the GIXD measurement have been calculated by using the following equation;   

\[E_{elec} = {<}S_e{>} \phi\](1)

  

\[E_{elas} = {<}S_n{>} \phi,\](2)

where, ${<}S_e{>}$ and ${<}S_n{>}$ are the average values of electronic stopping power, $S_e$, and the nuclear stopping power, $S_n$, which were calculated from the sample surface to the depth of about 200 nm, which is a region observed by the GIXD method, and ϕ is the ion-fluence Since the changes of Se and Sn in relation to the depth are quite small near the surface, the values of Se and Sn for each ion with the incident energy can used instead of the averaged values of <Se> or <Sn>. The values of Se and Sn, for each ion species and the ion fluence are listed in Table 1. Figure 8(a) shows the transformation of the lattice structure for Ni₃Nb samples in terms of the total deposited energy density, (E_elec + E_elas). Figure 8(b) shows the lattice structure transformation in terms of E_elec. According to these figures, the transformation to the amorphous state does not well correlate with the density of the total deposited energy or the density of energy deposited by the electronic excitation process, E_elec. Finally, we have paid attention to the density of energy deposited through the elastic collisions, E_elas. As can be seen in Fig. 8(c), the transformation to the amorphous state correlate with the energy deposited through elastic collisions, E_elas, much better than total deposited energy density or that deposited by the electronic excitation, E_elec. Strictly speaking, however, in the case of Al and Xe ion irradiations, it seems that a larger value of E_elas is necessary for the amorphization of Ni₃Nb than for the case of Au and Ni ion irradiation. In comparison to the result for 1.0 MeV He ion irradiation, this tendency is more remarkable. In Fig. 9 the GIXD spectrum for Au ion irradiation with the fluence of 5.0 × 10¹⁷/m² is compared with that for He ion irradiation with the fluence of 1.2 × 10²¹/m². Although the value of E_elas is about the same for the two kinds of irradiation, (E_elas is about 1.6 × 10¹⁷ eV/μg), the Au ion irradiated sample shows a broad peak corresponding to the amorphous phase, but the He ion irradiated sample is not amorphized yet and some peaks which correspond to the orthorhombic structure can still be observed.

Fig. 8

Lattice structure transformation for Ni₃Nb in terms of (a) density of total deposited energy (elastic collisions and electronic excitation), (b) density of energy deposited through electronic excitation and (c) that through elastic collisions.

Fig. 9

Comparison of GIXD spectra for Au ion irradiated and He ion irradiated Ni₃Nb specimens. Density of energy deposited through elastic collisions is the same (1.6 × 10¹⁷ eV/μg) for the both irradiations.

The same data analysis has been performed for the Ni₃Ta samples. Figure 10 and Figs. 11(a), (b) and (c) show the lattice structure transformation from the monoclinic phase to the amorphous phase in terms of ion-fluence, density of total deposited energy, density of energy deposited by the electronic excitation and density of energy deposited by elastic collisions, respectively. The figures indicate that the amorphization of Ni₃Ta samples correlates with the density of energy deposited by elastic collisions, E_elas, much better than the density of total deposited energy or that by electronic excitation. The amorphization of Ni₃Ta is, however, not described only by E_elas, which is shown clearly in Fig. 12. Even by the irradiation with the same value of E_elas, the amorphization occurs by the Au ion irradiation, but the monoclinic structure still exists after the He ion irradiation.

Fig. 10

Lattice structure transformation for Ni₃Ta in terms of ion influence.

Fig. 11

Lattice structure transformation for Ni₃Ta in terms of (a) density of total deposited energy (elastic collisions and electronic excitation), (b) density of energy deposited through electronic excitation and (c) that through elastic collision.

Fig. 12

Comparison of GIXD spectra for Au ion irradiated and He ion irradiated Ni₃Ta specimens. Density of energy deposited through elastic collisions is the same (1.4 × 10¹⁷ eV/μg) for the both irradiations.

Here, we discuss the meaning of E_elas as a parameter for describing irradiation effects. The value of E_elas is the density of energy which is directly transferred from an incident ion to the target atoms. The transferred energy causes the displacement of target atoms from their regular lattice sites if the transferred energy is larger than some value (the displacement threshold energy, E_d). The atoms displaced from the lattice sites are called “Primary Knock-on Atom (PKA)”. The total number of displacements per one atom is called dpa (displacements per atom) and has often been used as a unit of irradiation dose especially in the research field of radiation damage for nuclear materials. The value of dpa is nearly proportional to the value of E_elas for energetic (>1 MeV) ions. Therefore, if an irradiation effect is well correlated with the value of E_elas, the effect is attributed to the total number of atomic displacements or the accumulation of individual atomic displacement. In many cases, however, irradiation effects are not well described only by dpa. Another important parameter for the description of irradiation effects is the spectrum or the energy of primary knock-on atom (PKA). If the PKA energy is large, its energy is transferred in the narrow region of the target, causing the collective displacements or displacement cascade. For the ions used in the present irradiation, except for 200 MeV Xe ion irradiation, the average PKA energy is larger for the irradiation with larger mass ions. The present result clearly shows that the irradiation with the largest mass ions (Au ions) amorphizes the Ni₃Nb and Ni₃Ta samples much more effectively than the irradiation with the smallest mass ions (He ions). Therefore, to understand the mechanism of the ion irradiation-induced amorphization of Ni₃Nb and Ni₃Ta, not only the accumulation of individual atomic displacement but also the collective displacements or displacement cascade by high energy PKAs have to be considered. The particle species dependence of the amorphization of Ni₃Nb and Ni₃Ta is similar to that for the irradiation-induced amorphization of AuIn₂²⁰⁾, Zr₃Fe¹⁶⁾ and NiAl₃¹³⁾. The amorphization of AuIn₂ by He ions requires much higher dpa-values than those necessary for the amorphization by heavy ions²⁰⁾. The critical temperature for the amorphization of Zr₃Fe by electron irradiation is about 220 K, which is much lower than 570–600 K for Ar ion irradiation¹⁶⁾. At 100 K, the NiAl₃ was completely transformed into an amorphous state for Xe, Ne, and electrons. At 300 K only Xe irradiation produced a complete transformation to the amorphous state. The critical temperature for the amorphization depends on irradiating ion species¹³⁾. The difference in amorphization between by light ions or electrons and by heavy ions, which have commonly been observed in many kinds of intermetallic compounds, can be explained as follows; for heavy ion irradiation, the direct amorphization inside the displacement cascade occurs, and for light ion or electron irradiation, the amorphization is caused as a result of point defect accumulation.

It has been reported so far that some intermetallic compounds such as NiZr₂⁴²⁾, RCo₂ and RCo₃ (R = rare earth elements)⁴³⁾, and NiTi⁴⁴⁾ show lattice structure transformation through the high density electronic excitation. On the other hand, in the case of Ni₃Nb and Ni₃Ta, the present study shows that the energy deposited through the electronic excitation does little affect the lattice structure transformation. The value of electronic stopping power, Se, for 200 MeV Xe ion irradiation is much larger than for other four ions. As can be seen in Fig. 8(c), however, the energy deposited by the elastic collisions dominates the irradiation-induced amorphization even for the 200 MeV Xe ion irradiation.

Figure 13(a) (b) show the Vickers hardness for Ni₃Nb and Ni₃Ta as a function of ion fluence. The result shows that the hardness increases with increasing the ion-fluence. At a given ion-fluence, the increase in hardness is larger for the irradiation with larger mass ions. This ion species dependence is similar to that for the GIXD result. Then, in Fig. 14, the Vickers hardness change is plotted as a function of the density of energy deposited through elastic collision. It is worth noting here that the density of elastically deposited energy is different from the value of E_elas we have used for the analysis of GIXD results. As the GIXD measures only the lattice structure near the sample surface, the value of Sn for the incident energy can be used. By contrast, the Vickers hardness tester measures the hardness for the whole of region affected by the irradiation. Therefore, for the plot in Fig. 14, we used the averaged density of elastically deposited energy, <E_elas>, which was calculated by the following equation,   

\[{<}E_{elas}{>} = \left( \int_0^R S_n (x)dx/R \right) \times \phi\](3)

where, Sn(x) is the nuclear stopping power as a function of depth, x, R is the projected range of each ion, and ϕ is the ion fluence. As is similar to the case of GIXD result, for the description of the irradiation-induced increase in hardness, the density of energy deposited through elastic s, < E_elas > is better than ion-fluence, and the < E_elas > dependence of hardness can correspond to the conventional knowledge that the hardness for the amorphous state is larger than that for the crystalline structure. Figure 14, however, shows that the experimental data for Al and He ion irradiations largely deviate from the curve for Au and Ni ion irradiations. This result implies that other factors should also be considered in addition to the elastically deposited energy. To clarify the ion species/energy dependence of hardness, the nano-indentation measurement, which detect the hardness only near the sample surface, where the values of Se and Sn little change, is now in progress.

Fig. 13

Vickers hardness as a function of ion fluence for (a) Ni₃Nb and (b) Ni₃Ta.

Fig. 14

Vickers hardness as a function of density of energy deposited through elastic collision for (a) Ni₃Nb and (b) Ni₃Ta.

4. Summary

Bulk samples of Ni₃Nb and Ni₃Ta intermetallic compounds were irradiated at room temperature with 16 MeV Au, 4.5 MeV Ni, 4.5 MeV Al and 1.0 MeV He ions. Some Ni₃Nb samples were also irradiated with 200 MeV Xe ions. The effects of the ion irradiation on the lattice structure were investigated by means of GIXD and EXAFS. The effect of the ion irradiation on the surface hardness was examined by using a Vickers hardness tester. The amophization was observed in Au, Ni, Al and Xe irradiated samples. This phenomenon is almost correlated with the density of energy deposited through the elastic collisions. In the case of the samples irradiated with 1.0 MeV He ions, no amorphization was observed even when the density of elastically deposited energy is the same as that for Au irradiated sample which showed the amorphous state. The Vickers hardness increased with increasing ion fluence, but the hardness does little correlate with the ion fluence or the density of elastically deposited energy. The present result implies that to understand the effect of energetic ion irradiation on the lattice structure and the surface hardness of Ni₃Nb and Ni₃Ta, not only the energy deposited through elastic collisions (total number of atomic displacements) but also other irradiation factors such as the energy spectrum of primary knock on atoms have to be considered.

Acknowledgments

The authors would like to thank the staff of Takasaki Advanced Radiation Research Institute, National Institutes for Quantum and Radiological Science and Technology for operating the 3-MV tandem and the single-ended accelerators. They also thank the staff of Nuclear Science Research Institute, Japan Atomic Energy Agency for operating 20-MV tandem accelerator. The present study has been carried out under the collaboration program between Osaka Prefecture University and National Institutes for Quantum and Radiological Science and Technology and that between Osaka Prefecture University and Japan Atomic Energy Agency.

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