Study of precipitation and dislocations in nitrogen implanted Zircaloy-4

Surface and Coatings Technology 83 (1996) 115-l 19

Study of precipitation and dislocations in nitrogen implanted Zircaloy-4 Guoyi Tang a3b,B.H. Choi a, W. Kim a, K-S. Jung a, J.H. Lee a, T.Y. Song a, D.S. Shon a, J.G. Han’ b Department

a Korea Atomic Energy Research Institute, Taejon 305-600, South Korea of Mechanical Engineering, Tsinghua University, Beijing, People’s Republic of China ’ Sung Kyun Kwan University, Suwon 440-420, South Korea

Abstract The purposeof this investigation is to develop the relationshipbetweenimplantation conditions and material properties for nitrogen implantedZircaloy-4. The distribution of precipitation in annealedZircaloy-4 is homogeneous. The size of most precipitate particlesis in the range lo-200 nm and the dislocation density is very low. Microanalysis of the precipitation of the substrate showsthat theseparticlesare Zr(Fe,Cr)z ternary intermetallics, which have both hexagonal Laves-phase Zr(Fe,Cr), structure and cubic Zr(Fe,Cr), structure with a higher Fe:Cr ratio (from 1.53 to 2.47). The average value of microhardness of the substrate was measured to be Hv 221. After implanting nitrogen in Zircaloy-4, the dislocationdensitywassignificantly increasedand a dislocation substructurewasformed in the implantedlayer. The original precipitate did not changeobviously. The ZrN phasewasdiscovered

in the implanted layer. The dislocation substructure and the amount of ZrN were varied with increasing implantation temperature and dose. As the implantation temperature varied from 310 “C to 640 “C, the average value of microhardness was increased to Hv 580 and Hv 740 respectively. The effect of implantation temperature and dose on the microstructure, phase structure and composition,and surfacehardnessis presentedin this paper. Keywords:

Zircaloy-4; Ion implantation; Cladding tube; Precipitation

1. Introduction Zircaloy-4 has been the most commonly used cladding material in boiling-water reactors (BWR) and pressurized-water reactors (PWR) owing to its low thermal neutron cross-section, excellent corrosion resistance, adequate strength and good formability. Nodular corrosion and fretting wear between fuel cladding tubes and spacer grids have been found to be the major mechanism for cladding tube failure during service of nuclear reactors [l-3]. The corrosion resistance and wear resistance of Zircaloy-4 are sensitive to microstructure characteristics. The morphology, crystal structure and chemical composition of the intermetallic particles, and the dislocation substructure of Zircaloy-4 can affect the lifetime of cladding tubes [ 4-71. Recently, surface modification using ion beams has proved to be an attractive technique. Properties of the metal surface such as the hardness, wear and corrosion resistance can be significantly improved by ion implantation [8,9]. In the present work, Zircaloy-4 specimens exhibiting different properties after various implantation processes were selected for analysis of intermetallic particles and dislocations. 0257-8972/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved

The main aim is to investigate the effect of implantation conditions such as the temperature and dose on the microstructure and properties.

2. Materials

and experimental

procedure

The composition of Zircaloy-4 used in this study is 1.24 Sn, 0.21 Fe, 0.11 Cr, 0.13 0 and 0.008 Si in wt.%. Specimens were taken from a 9 mm thick Zircaloy-4 sheet. The specimens for transmission electron microscope (TEM) analyses were taken from the sheet by slicing the sample. They were mechanically thinned and chemically thinned in a solution of 45% HN03, 45% Hz0 and 10% HF to a wall thickness of 0.04 mm, and then preliminary electropolishing from one side was performed on the punched 3 mm disks, in an electrolyte consisting of 7% perchloric acid in ethanol at 25 V and -40 “C. The polished specimens were implanted by a 120 keV nitrogen beam with a dose of 1 x 1017-1 x 1O1’ ions cmd2. The corresponding current densities were in the range 18-92.6 PA cmw2 and the substrate temperature varied from 100 “C to 724 “C during the implant-

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ation,, The ion beam was scanned in order to obtain uniform implantation over the specimens.The pressure in the experimental chamber during the implantation 3 x 10W5-3x low4 Torr was kept at about (4 x 10m3-3x 10e2 Pa). After implantation, the procedure was repeated on the other side that had not been implanted. A JEOL 2000FX analysiselectron microscope(AEM) equipped with an Oxford Link systemenergy-dispersive X-ray spectrometer (EDS) was used to determine the crystal structure and carry out elemental microanalysis of the particles. About 40 different sizesof particle in the range lo-100 nm in every specimenwere evaluated by EDS. The crystal structure of the particles was determined by selectedarea diffraction (SAD). 3. Results 3.1. Substrate miuostructwe

of Zidoy-4

The original unimplanted sheet of Zircaloy-4 had a fully recrystallized structure with mean grain diameter of 11.1f 1.6pm for the near equiaxial grains. The elements Fe and Cr in the specimenswere mainly distributed in the intermetallic particles. The concentrations of Fe and Cr in the matrix were too low to be detectedby standard EDS quantitative microanalysis. TEM observation shows that the sizedistribution of the second-phaseparticles is in the range lo-200 nm, and most of the particles are less than 100nm in size, They were characterizedby severalmorphologies including near spherical, polygonal, rectangular and square. Most particles were spherical and polygonal. They are located both at and within the grain boundaries (Fig. 1). The microchemical analysis(EDS) and structural analy-

Fig. 1. Micrograph Zircaloy-4.

of second-phase

particles

in

unimplanted

Cr

Fig. 2. The diffraction pattern and EDS of n hexagonal Zr(Cr,Fe), particle; the zone axis direction is [23i],

sis (SAD) identified the particles to be the hexagonal Laves-phaseZr(Cr,Fe), type with lattice parameters a0 = 0.51 nm, co = 0.83 nm. Some rectangular or square particles were identified as cubic type Zr(Cr,Fe), with lattice parameter a0 = 0.71 nm (Fig. 2). The Fe:Cr ratio for most Zr-Fe-0 ternary particles in the Zircaloy-4 varied from 1.53 to 2.47. The larger the size of the particle was, the higher was the Fe:Cr ratio in the particles. The Fe:Cr ratio for the particles of larger individual size (greater than or equal to 0.8 urn) can be over 7.5.The typical microstructure of Zircaloy-4 in the annealed state is g-phaset intermetallic particles. The dislocation density in the substrate is lower. The distribution of dislocations was arranged randomly and some straight dislocations were also seen. The interactions between moving dislocations and second-phase particles revealed by TEM show that some interactions between dislocations and particles exist as a dislocation

G. Tang et al./Swface and Coatings Technology 83 (1996) 115-119

(4

Fig. 4. Emissive dislocation from a grain boundary.

‘ 1/, 200 0

100

200

Fig. 5. The relationship temperature.

(‘4 Fig. 3. Dislocations (a) cutting through second-phase particles.

I

I

I

300 400 Temperature

I 500 (“Cl

I 600

,L 700

, 800

between microhardness and implantation

the particles, (b) around

line cuts through particles, but most interactions exist as a dislocation line around particles. The dislocation is pinned by fine particles (Fig. 3). We also found that some dislocations were emitting from the grain boundary. As they encountered a larger particle, dislocation pile-ups were formed (Fig. 4). 3.2. Nitsogen ion implantation Microhardness test results show that the hardness increased with increasing implantation temperature and dose; when the implantation temperature was over 640 “C, the microhardness value was significantly decreased (Fig. 5). TEM analysis shows that no significant change in the composition and crystal structure of the original intermetallic particles occurred after ion implantation processing. The ZrO,, ZrC and ZrN particles were identified by EDS and dark-field technique

after the various implantation processes. These ZrO,, ZrN and ZrC particles were very small, in the range 5-20 nm (Fig. 6). The high dislocation density and mosaic structure were discovered in the sub-surface of the implanted specimens. The characteristics of the dislocations changed with increasing implantation temperature and dose. Nanoscale dislocation cells were formed in the specimen implanted at 170 “C. The boundary between cells was clear and their mean size was about 15.61+ 1.96 nm (Fig. 7). when the implantation temperature was raised to 310 “C and 450 “C. The size of dislocation cells in the sub-surface was also increased. Their mean size was about 25.93 I4.7 nm for 310 “C and 288.7 + 38.8 run for 450 “C. After the implantation temperature was raised to 640 “C, the dislocation cells disappeared, but the dislocation density in the implanted layer was still higher. When the implantation temperature reached 724 “C, the dislocation density was significantly decreased. We discovered that the dislocation cells became larger and clearer, as the implantation dose

G. Tanget al.lS’urface and Coatings Technology 83 (1996) 115-119

reported in Ref. [ 111, Fe in Zr-Cr intermetallic particles can lead to an increasein the interfacial energy,thereby increasing the tendency for coarsening. Therefore, the Fe content of the larger particles is higher than that of the smaller particles. After implantation processing, a lot of fine particles in the surface and sub-surface of the specimenswere formed. The ZrO,, ZrC, ZrN particles are located in the sub-surface, and most particles were in the range 3-10 nm. When a moving dislocation encounters second-phaseparticles it will not, in general, ~ be able to cut through them becausethe second-phase particles are generally stronger than the matrix. Consequently,the dislocation will have to bow between the second-phaseparticles and around them, leaving a dislocation loop around the particle [ 121. The stresscP required for this processis approximately -~~-~-where L is the distance of closestapproach between the particles, G is the shear elastic shear modulus, and b is the Burgers vector. When fine dispersion particles are present,very large stressesmust be applied before dislocation motion can occur, and the material has a high strength. Becausethe dislocation lines in the implanted layer would be strongly pinned by these fine particles (ZrO,, ZrN and ZrC), the surface hardness and wear resistanceof implanted specimensrapidly increased.The susceptibility to nodular corrosion of Zircaloy decreases with increasing number density of precipitates. Thus uniform and fine second-phase particles cause higher corrosion resistanceand wear resistance[7,13-151. The characteristic of the dislocations was altered during the implantation and the properties of the implanted layer were influenced. Dislocation networks were formed in the nitrogen ion implanted layer and the Fig. 7. The nanoscale dislocation cells in the implanted specimen zone affected by the implantation, and the size of the at 170 “C. dislocation cells increased with increasing implantation temperature and dose. It is reported in Refs. [16-181 that this dislocation network has beenfound to contribincreased with a constant implantation temperature of ute to increasethe hardness,wear resistanceand corro300 “C, and the mean sizeof dislocation cells was about 181.3i- 0.4 nm. sion resistance. 4. Discussion

5. Conclusions

The characteristics of precipitates observed in this investigation show that Zircaloy-4 sheetin the annealed statehas two different crystal structuresof the Zr(Cr,Fe), phase (h.c.p. and f.c.c.).Most of the second-phaseparticles are hexagram Laves Zr(Cr,Fe),. The distribution of particles is uniform and the Fe content in the particles is higher than the Cr content. It is well known that the size and coarsenessof the particles depend mainly on the degree of matrix supersaturation and the interfacial energy. The higher the interfacial energy is, the stronger the driving force for particle coarsening [lo]. As

(1) There are two different crystal structures in the intermetallic particles of Zircaloy-4 sheet:hexagram Laves Zr(Cr,Fe), phase and f.c.c.Zr(Cr,Fe),. Most of the second-phaseparticles are hexagonal Laves Zr(Cr,Fe), and their morphology is near spherical and polygonal. The morphology of f,c,c,Zr(Cr,Fe), is near rectangular and square. The Fe content in all particles is higher than the Cr content. (2) The microstructure of Zircaloy-4 sheet is CIphaset intermetallic particles. The dislocation density in the substrate is low. The distribution of

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dislocations is arrayed randomly. The interaction between moving dislocations and second-phase particles revealed by TEM shows that some interactions between dislocations and particles exist as a dislocation line cuts through particles, but most interactions exist as a dislocation line around particles. The dislocation is pinned by fine particles. (3) The composition and crystal structure of original particles in the substrate are not altered by ion implantation. Many fine ZrO,, ZrC and ZrN particles were discovered in the ion implanted layer. The size of these particles is in range 3-20 nm, and they strongly pin the dislocations. (4) The high dislocation density and mosaic structure were discovered in the sub-surface of the implanted specimens. The characteristic of the dislocations varied with increasing implantation temperature and dose. Nanoscale dislocation cells were formed in the specimen implanted at temperatures between 170 and 450 “C. The boundary between cells was clear. The size of dislocation cells in the sub-surface also increased with increasing implantation temperature. When the implantation temperature was above 64.0 “C, the dislocation cells disappeared, and when the implantation temperature reached 724 “C, the dislocation density was significantly decreased. The dislocation cells become clearer with increasing implanted dose at 300 “C.

Warner, The Science and Design of Engineering Materials, Irwin, Chicago, IL, 1995, p. 312. 1111 N.V. Bangaru, R.A. Busch and J.H. Schemel, in Zirconium in the Nuclear Industry, ASTM STP 939, ASTM, Philadephia, PA, 1987, p. 341. [=I F.J. Humphreys, in M. Hlewis and D.M.R. Taplin (eds.), Micromechanisms of Plasticity and Fracture, University of Waterloo Press, Waterloo/Parsons Press, Dublin, 1983, p. 1. in the Nuclear Cl31 B. Cheng and R.B. Adamson, in Zirconium Industry, ASTM STP 939, ASTM, Philadephia, PA, 1987, p. 387. r141 Y.H. Jeong, KS. Rheem, C.S. Choi and Y.S. Kim, J. Nucl. Sci.

Acknowledgment

Cl51 J.S. Bryner, J. Nucl. Mater., 82 (1979) 84. Et61 J.R. Treglio, A.J. Perry and R.J. Stinner, Adu. Mater. Processes, 5 (1995) 29. r-171 Yu.P. Sharkeev, A.N. Didenko and E.V. Kozlov, Surfi Coat.

The research is supported by the Long-Term Nuclear Research and Development Program in Korea.

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