Strain and structure order variation of pure aluminum due to helium irradiation

Abstract

Damage induced by helium ion irradiation in 1060 pure aluminum was investigated with a home-made MT3-R ion implanter. In this process, 10¹⁵, 10¹⁶ and 10¹⁷ ions per cm² irradiation were used with a voltage of 50 kV. In order to evaluate the irradiation damage in samples, surface morphology changes, strain variation and microstructure transformation have been examined using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Besides, detailed collision events of energetic ions with target atoms were simulated using SRIM-2008. The result shows that the irradiation dose plays the most important role in changing the target surface layer. Increasing the irradiation dose further damaged the surface layer morphology and increased surface roughness. In this process, strain changes and phase transformation caused by the ordered crystal structure being broken were discussed and a mechanism was proposed to explain the phenomenon that appeared at different irradiation stages. In addition, the variation of dislocations observed by TEM confirmed this conclusion.

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1 Introduction

In recent years, the ion irradiation technique has become a novel method in the field of material processing, as well as being relevant to radiation damage occurring in nuclear protection materials and aeronautical materials. When energetic ions impinge on a sample surface, ions with several tens of keV or higher energies can be implanted.¹ As a result of momentum transfer between incident ions and target atoms, some damage would be induced in the sample surface. Earlier research revealed that various energetic ions caused defects on different materials, including micro-cracks, bubbles, blisters, pinholes, crystal damage, etc.^2–6 In order to examine the characteristic changes of irradiated samples, some experiments have been conducted.^7–10 Li et al.⁷ used a micro-compression test to study the effect of a nanometer-scale helium bubble on the strength and deformability of sputter-deposited Cu and Cu/Nb multi-layers. Cui et al.⁸ observed interaction between lattice dislocations and grain boundaries in austenitic stainless by in situ transmission electron microscopy (TEM). Aabdin et al.⁹ performed argon ion irradiation to switch the natural nanostructure ON and OFF in Bi₂Ti₃ materials. Xiao et al.¹⁰ transformed amorphous silicon into polycrystalline silicon after gallium implantation by annealing treatment, which provided sufficient energy for recrystallization.

Moreover, with energetic ion bombardment of a target surface, secondary electrons and secondary ions generated at the surface are utilized to form high resolution images.¹¹ In addition, surface or near-surface atoms of the target can get rid of surface binding energy when they receive sufficient momentum from incident ions, which was employed to modify or mill the micro and nanometer-scale structural shape of a target precisely. For instance, by setting the ion-projected range and angle of incidence of the ion beam, Romero-Gomez et al.¹² found that well-ordered nanorods appear in the TiO₂ film surface layer. A nanovoid structure was observed in an Ar⁺ implanted GaSb wafer,¹³ and a similar phenomenon was found in a N⁺ irradiated TiO₂ film.¹ Besides, nanocavities were found in He⁺ irradiated Si and Ge.¹⁴ Nano-strings, nano-bead chains and nano-pore membranes were fabricated with smooth surfaces using the focused ion beam technique.¹⁵

As an alloy with low density and high strength, pure aluminum was commonly employed as anti-radiation material of aircraft, which is always exposed to the radiation of energetic charged particle currents in space, such as solar wind¹⁶ (during a period of solar minimum, the flux of He⁺ is 10³ per cm² per day, with most particles at energies of tens to hundreds of keV (ref. 17)). Current research focuses on the modification of aluminum alloys with heavy ion irradiation,^18–20 and there is rather little research of irradiation damage. Under current conditions, it is convenient to produce particles with a tens of keV range perpendicular to the target surface. Moreover, light particles with several tens of keV constitute a major part of light particles in the space radiation environment. Hence, it is reasonable to choose 50 kV as an acceleration voltage that combines with the device and application. The main purpose of this work was to generate accelerated and energetic ions to simulate energetic charged particles (tens of keV range) via a home-made ion irradiation system. By varying irradiation time, helium ions with different doses were implanted to the surface layer of 1060 pure aluminum. The relevant strain variation and order degree of the sample structure were demonstrated, and these findings provide keen insights into the durability and safety use of anti-radiation materials in space radiation.

2 Experimental

A 1 mm thickness rolling commercial 1060 pure aluminum sheet was selected as the target material and the composition is shown in Table 1. Specimens were obtained by a wire-electrode cutting to a size of 10 × 10 × 1 mm. Scanning electron microscopy (SEM) samples were polished with diamond spray polishing compounds until ground up to a mesh number of 1000 by SiC sandpapers. TEM thin foils were prepared from the sample surface layer. Firstly, the sample was ground from the undersurface to a plate with a thickness of less than 30 μm. After that, the plate was thinned in a Gatan-691 precision ion polishing system until electron transparency occurred, and the depth of the sample surface layer analyzed was between 0.2 and 0.5 μm.

Table 1 Chemical composition of 1060 aluminum alloy (wt%)

Fe	Si	Cu	Zn	V	Mn	Ti	Mg	Al
0.35	0.25	0.05	0.05	0.05	0.03	0.03	0.03	Balance

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The irradiation experiment was carried out in a home-made MT3-R ion implanter, using helium ions implanted on and oriented perpendicular to the surface of the samples. The schematic diagram of the MT3-R ion implanter is shown in Fig. 1. During irradiation, a 10 μA cm⁻² beam current was produced using a 50 kV acceleration voltage in a vacuum, which maintained a base pressure of 10⁻⁴ Pa. In order to reach an implantation dose of 10¹⁵, 10¹⁶ and 10¹⁷ ions per cm², dwell times of 22 s, 136 s and 1620 s were proceeded in the 10 × 10 mm² specimens, respectively.

Fig. 1 Schematic diagram of the home-made MT3-R ion implanter.

After that, the morphologies of the samples before and after irradiation were observed using Hitachi S3400 SEM. Roughness of the sample surfaces was measured using an MFP-3D atomic force microscope. Strain and phase analyses were carried out employing an X-ray diffraction (XRD) system, which performed between 15° and 90° (2θ) at room temperature with a step size of 0.02°. And the evaluation of dislocations in the samples before and after irradiation was studied with JOEL JEM-2100F TEM. Dislocation density was estimated by the line intercept method,²¹ and the value was calculated using the following equation:

where ρ is the dislocation density, L is the length of randomly drawn lines on the TEM image, N is the number of intersections (drawn lines with dislocations), and t is the foil thickness.

3 Results and discussion

Before the experiment, a simulation was performed on SRIM-2008 software. SRIM is a software using the Monte Carlo method to calculate ion distribution and defects generated by incident ions in a target surface layer.²² A stopping and range calculation predicts that, with 50 kV accelerating voltage, the peak concentration of energetic ions appears at 3405 angstroms beneath the target surface, and the distribution is in agreement with a Gaussian distribution. Namely, incident ions tend to aggregate at a special depth R₀ (here, 3405 angstroms), and this tendency can be described as a Gaussian form:where R is the depth that one incident ion stays, and ΔR_p is the longitudinal straggling (815 angstroms, the value is given by SRIM, which is equivalent to the standard deviation of the Gaussian curve²³), in which, when incident ions travel in solid matter, the energetic ions lose their energy through electronic and nuclear collision processes. In SRIM, the energy loss was divided into ionization, vacancies and phonons by ions or recoils, as shown in Table 2. In addition, crystal lattice defects were created in this process and the calculation results are illustrated in Fig. 2. The plot shows a number of 98 target atom displacements made by per incident ions, and these displacements give rise to either a target vacancy or a replacement collision. In this case, a number of 93 target vacancies and 4 replacement collisions were created.

Table 2 Energy loss of incident ions in the target calculated by SRIM-2008 for a helium ion beam of 50 keV energy (%)

Energy loss	Ionization	Vacancies	Phonons
Ions	44.31	0.13	0.44
Recoils	23.77	2.51	28.85

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Fig. 2 Collision events calculated by SRIM-2008 of helium irradiation (50 keV).

Fig. 3 shows the surface morphologies of the reference sample and the samples with 10¹⁵ ions per cm², 10¹⁶ ions per cm² and 10¹⁷ ions per cm² of irradiation, respectively. According to a ternary Al–Fe–Si system, 1060 pure aluminum is made up of an aluminum matrix, Al–Fe phases and Al–Fe–Si phases.²⁴ Fig. 3(a) illustrates that the reference sample is composed of an aluminum matrix, and precipitate phases. Besides, due to the peeling of particles, trails formed in the polishing process were also observed. When energetic helium ions bombarded the sample surface perpendicularly, pinholes and micro-cracks (the density is about 10⁵ per cm²) began to appear on the sample surfaces, whereas the precipitate phases disappeared partially, as depicted in Fig. 3(b). In Fig. 3(c), the pinholes (the density is about 10⁵ per cm²) became deeper and bigger, while the precipitate phases basically disappeared. This phenomenon was aggravated with 10¹⁷ ions per cm² irradiation, as shown in Fig. 3(d). Therefore, rules can be found that deeper and bigger pinholes (the density is about 5 × 10⁵ per cm²) appeared and micro-cracks extended, but precipitate phases disappeared gradually with the addition of irradiation dose. According to the above results, the ion dose was confirmed as a factor with a major effect, having a great impact on defect formation and phase transformation²⁵ in the target surface layer.

Fig. 3 SEM images of specimens: (a) reference, (b) 10¹⁵ ions per cm², (c) 10¹⁶ ions per cm², and (d) 10¹⁷ ions per cm².

In order to study more details of the surface morphology before and after helium irradiation, the average roughness, R_a, of the samples was measured by AFM, as illustrated in Fig. 4. The average roughness of the reference sample is 3.235 nm, and a rising tendency was found after irradiation. With 10¹⁵ ions per cm², 10¹⁶ ions per cm² and 10¹⁷ ions per cm² of irradiation, the average roughness became 3.633 nm, 5.516 nm and 6.178 nm, respectively. In addition, the absolute values of the maximum and minimum roughness of the samples presented a similar tendency. The increased roughness of the sample surface may be due to the sputtering effect in the crystal of ion irradiation.²⁶

Fig. 4 Average, maximum and minimum roughness of the sample surfaces.

X-ray diffraction patterns of the reference and the irradiation samples are shown in Fig. 5. The diffraction peaks correspond to the aluminum matrix with a face-centered cubic crystal structure (PDF#04-0787). Sharp peaks at 2θ equal to 38.472°, 44.738°, 65.133° and 78.227° that appear in the spectrum of the reference sample are recognized as the (111), (200), (220) and (311) planes, and these peaks can also be observed in the samples with different irradiation doses respectively. However, in comparison with the reference sample, with 10¹⁵ ions per cm² helium ion irradiation, four peaks of the α-Al phase appear at higher Bragg angles. But the deviation angle became lower with one order of magnitude of irradiation ions added (10¹⁶ ions per cm²). When the irradiation dose was increased to 10¹⁷ ions per cm², these peaks get shifted toward higher Bragg angles again. Based on the XRD results and the Bragg equation, the interlayer spacing of the (220) planes was calculated. The interlayer spacings of the reference sample and the samples with 10¹⁵ ions per cm², 10¹⁶ ions per cm² and 10¹⁷ ions per cm² of irradiation are 286.1 Å, 285.6 Å, 286.9 Å, and 285.4 Å respectively.

Fig. 5 X-ray diffraction patterns of the reference and irradiation samples.

Indeed, when energetic helium ions were introduced into aluminum, energetic incident ions underwent a series of energy loss collisions with both electronic collisions and nuclear collisions, which generated target surface layer defects, such as interstitial atoms, replacement atoms, vacancies and lattice distortion. With an increasing irradiation dose, a mechanism was proposed at different irradiation stages. In the case of 10¹⁵ ions per cm² irradiation, the main type of irradiation damage was considered as interstitial which exerted compression stress on the surrounded crystals. Consequently, the distance of the diffraction planes was decreased by compression stress in the irradiation surface layer, leading to a slight right shift (0.14°) of the diffraction peaks. On the other hand, the target surface was cleaned with energetic ion bombardment which brought about a high intensity of α-Al peaks. When the irradiation dwell time was added to 136 s, the number of vacancies increased with increasing irradiation dose.²⁷ Herein, helium performed as cavity nucleation sites and then more vacancies, vacancy clusters and interstitial atoms were generated. In order to reduce lattice strain, vacancy clusters grow with absorbing additional vacancies.²⁸ Therefore, distortion generated by vacancy clusters is greater than interstitial, which produces a large tensile stress on the surrounding crystal,²⁹ and leads to an intensity decrease and a left shift (0.2°) of the peaks in the diffractogram. However, these vacancy clusters have a tendency to collapse and form a mass of line defects.³⁰ Fig. 6 shows TEM images of 1060 pure aluminum without irradiation and with 10¹⁶ ions per cm² irradiation. In the virgin sample, only a few dislocations (the density is about 1.8 × 10¹¹ per cm²) were observed, as shown in Fig. 6(a). However, a high density of dislocations (the density is about 3.3 × 10¹¹ per cm²) is presented in the sample with 10¹⁶ ions per cm² irradiation, as illustrated in Fig. 6(b).

Fig. 6 TEM images of 1060 aluminum: (a) virgin sample; (b) 10¹⁶ ions per cm² irradiation.

During irradiation processing, most of the energy loss during collisions is transformed into thermal energy. In comparison with ambient temperature, this results in a significantly high temperature (about 630 K, 10¹⁷ ions per cm² irradiation) of the target surface layer, which promotes the diffusion of vacancies and atoms, crystal lattice recovery,³¹ and helium-vacancy cluster growth,³² and releases stress concentration in the irradiation sample surface layer. This suggests that the diffraction pattern shifts toward higher Bragg angles with 10¹⁷ ions per cm² ion irradiation. In addition, the regularity of the variation of the two peaks corresponding to the (041) and (624) planes of the Al₉Fe₂Si₂ phase is consistent with the peak variation of α-Al. Whereas, significant peak changes of the (111) plane are also observed in Fig. 5, indicating a decrease of organizational order of the Al₉Fe₂Si₂ phase with increasing irradiation dose, and finally inducing amorphization with 10¹⁷ ions per cm² irradiation.

4 Conclusions

Crystal defects, strain changes, phase transformation and morphology alterations in a 1060 pure aluminum surface layer introduced by helium irradiation are explored in this work. Sample surface morphologies are varied with helium irradiation and the changes mainly appear in the form of pinholes and micro-cracks. These damages grow with an increase of the irradiation dose. Mean roughness of the sample surface increases with the addition of irradiation dose and a peak value of 6.178 nm is obtained at 10¹⁷ ions per cm² irradiation. Meanwhile, strain variation is present in helium irradiation samples, and the degree of irradiation damage depends on the dosage of irradiation. With 10¹⁵ ions per cm² irradiation, predominant distortion was caused interstitially, which presents compression stress. However, in samples with 10¹⁶ ions per cm² irradiation, this was taken over by tensile stress which was mainly induced by vacancy clusters. In addition, the structure disorder of the Al–Fe–Si phase is caused by helium irradiation, which is increased with the addition of irradiation dose, and eventually leads to amorphization of the Al–Fe–Si phase with 10¹⁷ ions per cm² irradiation.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 51175238), the Industrial Supporting Plan of Zhenjiang City (no. GY2013011), "Six talent peaks" of high level talent selection and training project of Jiangsu Province (no. 2013-ZBZZ-031), the research fund of Jiangsu Province Cultivation base for State Key Laboratory of Photovoltaic Science and Technology (No. SKLPSTKF201504) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors would like to thank Jian Chuan Ming, from Chang Zhou Bo Rui Heng Electronic Technology Co., Ltd for his technical direction. Rui Zhang, Zheng Dong Wu and Yu Shan Li from Jiangsu University are appreciated for sample preparation.

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