Radiation damage in helium ion irradiated nanocrystalline Fe

https://doi.org/10.1016/j.jnucmat.2011.10.052Get rights and content

Abstract

Fe films with an average columnar grain size varying from 49 to 96 nm are deposited by magnetron sputtering technique. Sputtered films have predominant body centered cubic structure together with a small fraction of face centered cubic phase. Bulk Fe with an average grain size of 700 nm is also irradiated at the same condition for comparison. Helium bubbles are observed in Fe films and bulk Fe irradiated by 100 keV helium ions to a fluence of 6 × 1020 ions/m2 at room temperature. Smaller grains lead to lower density of He bubbles. Radiation hardening in Fe films is much less than that of bulk Fe, and is a combined consequence of He bubble induced hardening and radiation induced compressive stress in Fe films.

Introduction

Energetic particles during radiation interact with atoms of the target material and induce a large number of point defects, such as vacancies and interstitials. Point defects aggregate to form vacancy clusters, voids, dislocation loops and stacking fault tetrahedron [1], [2], [3], [4]. Void swelling and He embrittlement may lead to severe degradation of mechanical properties [5], [6]. It is of great interest to develop radiation tolerant materials that can promote the annihilation of radiation induced defects. Studies have shown that certain grain and phase boundaries may act as preferential trapping sites for irradiation induced point defects and their clusters, promote recombination of interstitials and vacancies, and thus alleviate the degradation of mechanical properties [7], [8]. It has been established that He plays an important role on the evolution of microstructures and mechanical properties of irradiated materials [9], [10], [11]. Helium, in existence of high concentration of vacancy clusters, can quickly combine with vacancy clusters to form He bubbles [6]. Bubble density in some of He ion irradiated metals can approach 1023/m3 or greater [12], [31], [33]. Understanding these phenomena is important to develop radiation tolerant materials.

Since stainless steels are commonly used as nuclear reactor structural components, its matrix element, Fe, has been investigated extensively for radiation response [13], [14], [15], [16], [17]. He implantation induced radiation damage in Fe has been widely studied by modeling and simulation, including the formation and diffusion of He clusters and bubbles [18], [20], He-grain boundary interactions [21], [22], [36], He-dislocation interactions [23], He cluster thermal stability [24], [27] and bubble pressure and size [28]. These studies have demonstrated that He atoms can be absorbed by both dislocations and grain boundaries. He has a higher binding energy to grain boundaries due to their greater excess volume [11], [36]. Bubble density and size are related to grain boundary structures [19], [21]. Experimentally, He desorption has been investigated in both single crystal and polycrystalline Fe [26], [29], where polycrystals are found to be more favorable for He absorption.

Nanostructured materials have high volume fraction of interfaces, including high angle grain boundaries, phase boundaries and layer interfaces. These high energy defects may act as point defect sinks and thus lead to enhancement of radiation tolerance [3], [25], [30]. Recent studies show that Cu/Nb, Cu/V and Fe/W multilayer films can significantly reduce radiation induced defects due to their abundant interfacial area [30], [31], [32], [33], [34]. Size dependent reductions of He bubble density, lattice distortion, swelling as well as radiation hardening have been observed in He ion irradiated Cu/V multilayers [31], [32]. Furthermore the study of He ion irradiated Cu/Mo multilayers shows that segregation of He bubbles to interfaces is directly related to the vacancy-to-He concentration ratio [33]. A relatively low concentration of vacancies allows He to diffuse to the interface and form He bubbles at interfaces. Another type of boundary, metal/oxide interface, can also dramatically enhance radiation tolerance. Uniformly distributed nanoscale oxide precipitates in oxide-dispersion-strengthened (ODS) alloys, have shown superior void swelling resistance and high temperature thermal stability [14]. Nanocrystalline Fe and its alloys subjected to different heavy ion (Kr) radiations have been investigated by Karpe et al., who showed that Kr ions could trigger faster grain growth because of denser collision cascade [35]. Size dependent radiation response has also been found in He irradiated stainless steel, in terms of delayed void nucleation, lower void densities and reduced void swelling by decreasing grain size [8]. It has also been shown that the binding energy of He with grain boundaries increases linearly with grain boundary excess volume, which means high volume fraction of grain boundary may trap more He atoms [36]. Despite the aforementioned studies, He ion induced damage in nanocrystalline Fe films has rarely been studied experimentally. In this paper, we present the study of radiation induced defects (mainly He bubbles) and the evolution of mechanical properties in nanocrystalline Fe film. Bulk Fe with large grain size is also irradiated for comparison. This study helps to understand the role of grain boundaries on reducing He induced radiation damage in metals.

Section snippets

Experimental

Nanocrystalline Fe films with thickness of ∼1 μm were synthesized on oxidized silicon (SiO2) substrates by using DC magnetron sputtering technique at room temperature. The deposition rate was varied from 0.36 to 0.71 nm/s in order to tailor the average grain size. The purity of the Fe target is 99.95%. A base pressure of 6.7 × 10−6 Pa was reached before depositions and the Ar partial pressure during sputtering was ∼0.5 Pa. Before and after deposition, the curvature of substrate was measured by using a

Results

X-ray diffraction (XRD) spectra of the as-deposited Fe films at different deposition rate are shown in Fig. 1 by thicker solid lines. Strong bcc (1 1 0) texture is observed in Fe films, and bcc (1 1 0) peaks in all specimens deviate slightly from the standard position, as indicated by the vertical dash line. Such deviation implies the existence of residual stress in the as-deposited film, which will be discussed later. Furthermore these as-deposited films also show a small shoulder peak, identified

Microstructure and irradiation induced He bubbles

Microstructure characterizations show that higher deposition rate leads to smaller grain sizes. In sputtered Fe films, grain boundaries formed by island coalescence during deposition have very low mobility even at a substrate temperature as high as 1/2 Tm (Tm is the melting temperature of Fe) [38]. As such the growth of Fe films at room temperature in the current study is mainly determined by island nucleation mechanism [38]. The grain size of sputtered Fe should be directly correlated to the

Conclusions

We examined the radiation response of the sputtered nanocrystalline Fe films with various grain sizes, ∼49–96 nm, and the bulk Fe with an average grain size of 700 nm. The density of He bubbles in nanocrystalline Fe film is significantly lower in comparison to that in bulk Fe. There is an insignificant variation of average bubble diameter between the irradiated film and bulk Fe. Grain and phase boundaries are preferential sites for accumulation of He bubbles. Radiation hardening is less

Acknowledgements

X.Z. acknowledges the financial support by the US Army Research Office – Materials Science Division, under Contract No. W911NF-09-1-0223. L.S. acknowledges the support by NSF under Grant No. CMMI-0846835. Support by the Center for Integrated Nanotechnologies (CINT) under user agreement at Los Alamos National Laboratory is also acknowledged. We also acknowledge the usage of microscopes at the Microscopy and Imaging Center at Texas A&M University.

References (61)

  • P.J. Maziasz

    Journal of Nuclear Materials

    (1993)
  • S.J. Zinkle et al.

    Journal of Nuclear Materials

    (1995)
  • Y. Chimi et al.

    Journal of Nuclear Materials

    (2001)
  • B.N. Singh et al.

    Journal of Nuclear Materials

    (1995)
  • N. Hashimoto et al.

    Journal of Nuclear Materials

    (2004)
  • R.L. Klueh et al.

    Journal of Nuclear Materials

    (2006)
  • D. Stewart et al.

    Journal of Nuclear Materials

    (2011)
  • P.B. Johnson et al.

    Journal of Nuclear Materials

    (1980)
  • R.E. Stoller et al.

    Journal of Nuclear Materials

    (2011)
  • C.S. Deo et al.

    Journal of Nuclear Materials

    (2007)
  • W.J. Phythian et al.

    Journal of Nuclear Materials

    (1995)
  • S.J. Zinkle et al.

    Journal of Nuclear Materials

    (1993)
  • V.A. Borodin et al.

    Journal of Nuclear Materials

    (2007)
  • P.A. Thorsen et al.

    Scripta Materialia

    (2004)
  • F. Gao et al.

    Journal of Nuclear Materials

    (2007)
  • F. Gao et al.

    Journal of Nuclear Materials

    (2009)
  • L. Yang et al.

    Nuclear Instruments & Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms

    (2007)
  • M. Rose et al.

    Nuclear Instruments & Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms

    (1997)
  • D. Xu et al.

    Journal of Nuclear Materials

    (2007)
  • K. Morishita et al.

    Nuclear Instruments & Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms

    (2003)
  • D. Xu et al.

    Journal of Nuclear Materials

    (2009)
  • X. Zhang et al.

    Nuclear Instruments & Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms

    (2007)
  • E.G. Fu et al.

    Journal of Nuclear Materials

    (2009)
  • E.G. Fu et al.

    Journal of Nuclear Materials

    (2010)
  • N. Li et al.

    Journal of Nuclear Materials

    (2009)
  • N. Karpe et al.

    Materials Science & Engineering, A: Structural Materials: Properties, Microstructure and Processing

    (1994)
  • R.J. Kurtz et al.

    Journal of Nuclear Materials

    (2004)
  • R. Bullough et al.

    Journal of Nuclear Materials

    (1980)
  • J.H. Evans

    Journal of Nuclear Materials

    (1977)
  • T.R. Malow et al.

    Acta Materialia

    (1997)
  • Cited by (0)

    View full text