In situ Evidence of Defect Cluster Absorption by Grain Boundaries in Kr Ion Irradiated Nanocrystalline Ni
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TRODUCTION
RADIATION of metals by high energy particles generates vacancy-interstitial pairs (Frenkel pairs). These point defects can be removed through mutual recombination or a reaction with defect sinks, such as neutral sinks (grain boundaries (GBs), voids), biased sinks (dislocations), and variable sinks (impurity atoms and coherent precipitates).[1] Meanwhile, these point defects can aggregate to form defect clusters, which can be dislocation loops, voids, or stacking fault tetrahedron.[2] High densities of radiation-induced defects can result in significant degradation of mechanical stability of structural metallic materials. The enhancement of radiation tolerance of metallic materials via mitigation of the formation of defect clusters is a challenge. The chemistry and microstructures of the materials play an important role on the nucleation and migration of defect clusters. For instance, austenitic stainless steels subjected to cold working or modified with the addition of certain elements showed reduced swelling under neutron radiation at 673 K (400 C).[3] Oxide dispersionstrengthened ferritic alloys showed enhanced resistance to void swelling and fracture embrittlement under Helium radiation and electron irradiation.[4–6] Layer interfaces in immiscible multilayers such as Cu/Nb,[7,8]
C. SUN, M. SONG, K.Y. YU, Y. CHEN, Ph.D. Students, and X. ZHANG, Associate Professor, are with Department of Mechanical Engineering, Materials Science and Engineering Program, Texas A&M University, College Station, TX 77843-3123. Contact e-mail: [email protected] M. KIRK, Materials Scientist, is with the Materials Science Division, Argonne National Laboratory, Argonne, IL 60439. M. LI, Materials Researcher, is with the Nuclear Engineering Division, Argonne National Laboratory, Argonne, IL 60439. H. WANG, Associate Professor, is with Department of Electrical and Computer Engineering, Texas A&M University. Manuscript submitted April 9, 2012. Article published online February 12, 2013 1966—VOLUME 44A, APRIL 2013
Cu/Mo,[9] Cu/V,[10] and V/Ag[11] can effectively mitigate swelling, lattice distortion, and radiation hardening. GBs are considered as sinks for irradiation-induced defects.[12–16] A substantial reduction of radiation damage is anticipated in nanocrystalline (NC) materials, given their larger volume fraction of GBs, compared to the conventional coarse-grained (CG) materials. Several molecular dynamic simulations have been reported on the impact of GBs on the radiation behavior of NC metals. Samaras et al. observed significant atomic migration toward the surrounding GB during thermal spikes in self-ion irradiated NC Ni.[17] Bai et al. showed that GBs act as defect sinks as well as sources, and they emit interstitials to annihilate the nearby vacancies within the grains.[18] Experimental studies also showed that irradiation-induced damage was significantly reduced in NC metals. Singh et al. found that grain refinement can delay the nucleation of voids and mitigate void swelling in stainless steel with a grain size from 0.45 to 50
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