Eur. Phys. J. D (2013) 67: 159DOI: 10.1140/epjd/e2013-30767-4

Regular Article

THE EUROPEANPHYSICAL JOURNAL D

Effect of laser fluence on surface, structural and mechanicalproperties of Zr after irradiation in the ambient environmentof oxygen

Mohsan Jelani, Shazia Bashira, Muhammad Khaleeq-ur Rehman, Riaz Ahamad, Faizan-ul-Haq, Daniel Yousaf,Mahreen Akram, Naveed Afzal, Muhammad Umer Chaudhry, Kahliq Mahmood, Asma Hayat, and Sajjad Ahmad

Centre for Advanced Studies in Physics, GC University Lahore, Pakistan

Received 24 December 2012 / Received in final form 28 February 2013Published online 1 August 2013 – c© EDP Sciences, Societa Italiana di Fisica, Springer-Verlag 2013

Abstract. The laser irradiation effects on surface, structural and mechanical properties of zirconium (Zr)have been investigated. For this purpose, Zr samples were irradiated with Excimer (KrF) laser (λ ≈ 248 nm,τ ≈ 18 ns, repetition rate ≈30 Hz). The irradiation was performed under the ambient environment ofoxygen gas at filling pressure of 20 torr by varying laser fluences ranging from 3.8 to 5.1 cm−2. The surfaceand structural modification of irradiated targets was investigated by scanning electron microscope (SEM)and X-ray diffractometer (XRD). In order to explore the mechanical properties of irradiated Zr, the tensiletesting and Vickers micro hardness testing techniques were employed. SEM analysis reveals the graingrowth on the irradiated Zr surfaces for all fluences. However, the largest sized grains are grown for thelowest fluence of 3.8 J cm−2. With increasing fluence from 4.3 to 5.1 J cm−2, the compactness and densityof grains increase whereas their size decreases. XRD analysis reveals the appearance of new phases of ZrO2

and Zr3O. The variation in the peak intensity is observed to be anomalous whereas decreasing trend inthe crystallite size and residual stresses has been observed with increasing fluence. Micro hardness analysisreveals the increasing trend in surface hardness with increasing fluence. The tensile testing exhibits theincreasing trend of yield stress (YS), decreasing trend of percentage elongation and anomalous behaviourof ultimate tensile strength with increasing fluence.

1 Introduction

Over the past two decades, lasers have been developed asan efficient and reliable tool for material processing ap-plications. It has several advantages over other processingtechniques, e.g., reduced heat-affected zone, control overthe laser fluence, speed, versatility, non-contact nature,contamination free and environmental friendly ablationprocesses [1]. Lasers are widely used in industries for weld-ing, cutting, heat treatment and surface modification [2].

Laser ablation of solid targets in the presence of am-bient gas has attained a significant importance due toits vast ranging applications e.g. pulsed laser deposition,nano particle synthesis and laser-induced breakdown spec-troscopy [3,4]. During laser ablation the presence of thegas as an ambient environment significantly enhancesthe ablation performance due to the confinement effect,the energy deposition by the plasma ions of gaseous media,the chemical reactivity and diffusion of ambient gases [5,6].

Zirconium (Zr) is a one of the most important engi-neering and biocompatible material. It is used for claddingof fuel rods in nuclear reactors because of its low neutronabsorption cross section. It exhibits comparatively good

a e-mail: shaziabashir@gcu.edu.pk

mechanical properties, corrosion resistance, thermal sta-bility and high reactivity towards oxygen at high temper-ature [7]. It is therefore of basic interest to investigatethe laser irradiation effects on the surface and mechanicalproperties of Zr. In this regard various research studieshave been conducted on Zr and its alloys by using laserand other radiation sources.

Ursu et al. [8] studied the nitriding and oxidizing oftitanium and Zr samples by the action of microsecondpulsed TEA CO2 laser irradiation in the presence of ni-trogen and oxygen gas environment. Due to the significantformation of nitrides and oxides layer, an increase in hard-ness has been observed. Nishitani et al. [9] had used thepulsed Nd+ doped glass laser to irradiate the pure tran-sition metal targets (Al, Fe, Ni, Nb, Ti, W and Zr) inthe ambient environment of air and nitrogen gas to formtheir oxides and nitrides. It has been observed that ox-ide and nitride formation mainly depends on the boilingpoints of the metals. Montross et al. [10] studied the lasershock processing and its effects on microstructure and me-chanical properties of metal alloys. It has been observedthat the laser shock processing can significantly improvefatigue performance, hardness and mechanical propertiesof some metals and alloys.

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The purpose of the present work is to investigate theeffect of laser fluence on surface, structural and mechanicalproperties of Zr. A pulsed excimer (KrF) laser (248 nm,18 ns, 30 Hz) was employed to irradiate the Zr targetsin the presence of oxygen environment. SEM analysis wascarried out to explore surface modifications. These surfacestructures were correlated with structural modification ofZr explored by XRD analysis. In order to establish the re-lation of these surface and compositional changes with me-chanical properties of the material, the tensile testing andVickers micro hardness testing techniques were employed.

2 Experimental work

In the present research work, the Zr metal with purityof 99.999% is used as a target material. The rectangularshaped samples with an average length of 43 mm, widthof 5 mm and thickness of 3 mm were selected. The targetswere grinded and mechanically polished with silicon car-bide papers. The polished specimens were sealed into silicatubes, evacuated up to a base pressure of 10−6 torr by us-ing rotary pump followed by diffusion pump. The anneal-ing of samples is performed in a muffle furnace at 800 ◦Cfor 1 h to relieve internal stresses and defects. After an-nealing, the specimens were ultrasonically cleaned andwere mounted on a sample holder attached to two dimen-sional motor controlled stage. The mounted samples wereplaced in UHV chamber made up of stainless steel. Thechamber was evacuated to a base pressure of 10−6 torr andthen pure oxygen gas (99.999%) at a pressure of 20 torrwas filled in the chamber.

An excimer laser system (GAM LASER Inc. USAmodel EX200) with wavelength of 248 nm, pulse dura-tion of 18 ns, repetition rate of 30 Hz was employed assource of irradiation. After passing through a plano con-vex lens of focal length 50 cm and a quartz transmissionwindow attached with one port of the chamber, the beamhits the target at an angle of 90◦ with respect to thetarget surface. The samples were exposed and scannedalong the length once in one direction at very low scan-ning speed of 0.6 mm s−1. All targets were irradiatedby 2100 number of laser pulses at the repetition rateof 30 Hz. In order to explore the effect of laser fluenceon the surface, structural and mechanical modification oftarget material all other parameters were kept constant,and the targets were exposed by four various laser flu-ences of 3.8, 4.3, 4.7 and 5.1 J cm−2. Laser fluence waschanged by changing the output pulse energy. The valuesof energies corresponding to these fluences are 90, 100,110 and 120 mJ. Laser fluence was evaluated by using thefollowing relation [11]:

Fluence = Energy/Area. (1)

The laser focused spot size is evaluated by considering thearea of the rectangle. The area (A) of rectangular beamafter focusing through a lens of focal length 50 cm wasmeasured by using the following relation:

A = a × b, (2)

Where “a” is length of the rectangular beam and is equalto 2.35 mm and “b” is width of the rectangular beam andis equal to 1 mm.

The length and width of rectangular beam are mea-sured by irradiating the Zr target with laser radiation andthen exploring the surface by using SEM analysis. In thisway the laser spot size comes out to be 0.0235 cm2.

The surface morphology of unirradiated and laserirradiated samples was investigated by scanning elec-tron microscope (SEM) (JEOL-JSM-6480). The structuralanalysis of the pre and post irradiated specimens is con-ducted by using X’Pert PRO (MPD) X-ray diffractome-ter. The Vickers hardness tester (Zwick/Roell ZHU-5030),equipped with a pyramid shaped diamond indenter wasemployed for the measurement of surface micro hardness.The test was conducted by using 200 g load for den-tations time of 15 s at room temperature. The tensiletesting of unirradaited and irradiated samples was car-ried out by using universal tensile testing machine (AG-1Shimadzu). All samples were deformed under the gaugelength of 40 mm with cross head speed of 1 mm/min atroom temperature.

3 Results and discussion

3.1 Surface morphology

The SEM images of Figure 1 reveals the surface morphol-ogy of (a) unirradiated and laser irradiated Zr for variousfluences of (b) 3.8, (c) 4.3, (d) 4.7 and (e) 5.1 J cm−2.

Figure 1b reveals the grain growth with distinctboundaries for the lowest fluence of 3.8 J cm−2. Figures 1cand 1d exhibits that the density and compactness of grainsincreases and their size decreases when the laser fluence isenhanced from 3.8 to 4.7 J cm−2. The appearance of grainboundaries becomes more distinct and wider for these flu-ences. With the further increase in fluence up to a valueof 5.1 J cm−2, the size of the grains decreases whereasthe density and compactness increases. The appearanceof grain boundaries becomes less distinct and diffusive atthis fluence. These micro structural changes induced bylaser irradiation can be briefly explained as follows.

Laser induced rapid heating and cooling, the tremen-dous temperature gradient and the gas dynamic flows areresponsible for inducing thermal stresses, enhanced diffu-sion and grain growth [12,13].

The theoretical values of thermal diffusion depth andoptical penetration depths are 0.48 μm and 10 nm, respec-tively. Thermal penetration depth (Dth) is evaluated byusing the formula [11]:

Dth = 2√

κt, (3)

where, κ = thermal diffusivity and τ = pulse duration ofthe laser beam and

κ = K/ρCp. (4)

Here K, ρ and Cp are thermal conductivity, den-sity and specific heat capacity, respectively, with cor-responding values of 22.6 W m−1 K−1, 6.52 g cm−3

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Fig. 1. SEM images revealing the surface morphology of (a) unirradiated and excimer laser irradiated Zr for various fluencesof (b) 3.8, (c) 4.3, (d) 4.7 and (e) 5.1 J cm−2.

and 0.27 J g−1 K−1, respectively, for Zr. By substitut-ing the values of K, ρ and Cp in equation (4), the valueof thermal diffusivity comes out to be 12.838 mm2/s. Bysubstituting the value of thermal diffusivity and pulse du-ration of 18 ns in equation (3), thermal diffusion depthcomes out to be 0.48 μm.

Grain growth occurs as a result of migration of grainboundaries. Grain boundary movement is a recrystalliza-tion process which arises during or after deformation inorder to minimize the stored strain and surface energy.Atoms from strained boundary can jump to adjacent ones,releasing the strain energy and reorienting the lattice lead-ing to large and smooth grain boundaries [14]. The en-ergy associated with grain boundaries depends on thebond angles and the degree of misorientation. Therefore,

the driving force of grain growth is the reduction of thisenergy along with the decrease in total boundary area.The strained grains will continue to grow if the metalremained at elevated temperature even after the recrys-tallization [15]. By increasing the laser fluence from thevalue of 3.8 to 4.7 J cm−2, the processes like melting andrecrystallization takes place. Due to hydro dynamical ef-fects the cooling and crystallization of metals resided inthe irradiated zone generates intense boiling. After melt-ing and boiling, the material resolidifies and solute oxy-gen atoms diffuse and segregate to grain boundaries alongwith the formation of oxides. This diffusion and segre-gation of oxygen atoms along grain boundaries makes thegrain boundaries to be distinct and wider [13]. As the laserfluence is increased the specimen melting and generated

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Fig. 2. XRD patterns of unirradiated and excimer laser irra-diated Zr for various fluences of 3.8, 4.3, 4.7 and 5.1 J cm−2.

thermal stress also increases. These stresses compress theliquid and also cause the grain boundaries to become com-pact, thick and deep. During cooling and resolidificationprocess, the large sized grains break into smaller grains,thereby increasing their density with decreasing size. Infact, the segregation of oxygen atoms along the grainboundaries reduces the disorder and internal energy andhence the mobility of the grain boundaries [16]. At a max-imum fluence of 5.1 J cm−2, due to enhanced diffusion,the appearance of grain boundaries becomes diffusive andcomparatively less distinct [13].

3.2 XRD analysis

Figure 2 represents the XRD patterns of unirradiated andlaser irradiated Zr for various fluences of 3.8, 4.3, 4.7and 5.1 J cm−2. For unirradiated target, various diffrac-tion peaks have been observed at different angles [17]. Forirradiated specimens the new peaks are identified. Thesepeaks correspond to phases of ZrO2 and Zr3O after expos-ing Zr in the ambient environment of oxygen [18–20]. Forunexposed target the peak identified at 47.7◦ correspondsto Zr. For irradiated targets this phase has been trans-formed into Zr3O and a new peak is identified at 48.2◦.

The peak intensity corresponding to diffraction planeZr3O (012) initially increases with increasing fluencefrom 3.8 to 4.7 J cm−2. However, for the highest fluenceof 5.1 J cm−2 a decrease in peak intensity is observed.As the laser fluence increases from 3.8 to 4.7 J cm−2, thesolute oxygen atoms segregates on the target surface anddiffuse across the grain boundaries due to enhanced heatgeneration and energy deposition. Thus, increases its (oxy-gen) concentration into the material, which is indicated bythe increase in intensity of the peak Zr3O (012) [13]. Atthe maximum fluence of 5.1 J cm−2, due to fast meltingand cooling, the resolidification and recrystallization pro-cesses starts to take place. The large sized grains breaks

Fig. 3. The variation of crystallite size of laser irradiated Zrfor various fluences.

up into smaller ones which causes the reduction in peakintensity [21,22].

Average crystallite size of unirradiated and irradi-ated Zr targets is calculated by using Debye-Scherrerformula [23]:

Crystallite size (D) =kλ

FWHM cosθ(5)

where K is Scherrer constant with value 0.94, λ is X-ray’swavelength of Cu-Kα radiation and its value is 1.54 A.FWHM is the full width half maxima of the respectivediffraction peak in radian and θ is the Bragg’s angle ofdiffraction.

Figure 3 shows a plot of variation of crystallite sizecorresponding to the plane Zr3O (012) as a function offluence. This figure illustrates the decrease in crystallitesize by increasing laser fluence. This decrease in crystallitesize is due to enhanced diffusion of oxygen atoms intothe interstitial sites and also along the grain boundaries ofthe specimen. This diffusion enhances the peak broadeningand therefore reduces the crystallite size [22].

XRD provides one of the non-destructive methodsfor measuring residual stresses. The residual strain andstresses corresponding to the plane Zr3O (012) are evalu-ated by using following formulas [24]:

Strain(σ) =d − d0

d0, (6)

Stress = σ × E, (7)

where d0 is the original or unstrained plane spacing, E isYoung’s modulus having value of 88 GPa for Zr metal [25]and σ is induced strain. The variation of induced stressesfor various fluences is displayed in Figure 4. The exis-tence of both tensile and compressive stresses is observed.This figure illustrates that by increasing fluence tensilestresses are transformed into compressive stresses. These

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Fig. 4. The variation of residual stress of laser irradiated Zrfor various fluences.

stresses are induced due to lattice distortion and by ther-mal shocks induced by laser energy deposition [24]. Ini-tially the laser induced shock waves generate the tensilestresses and cause temperature gradient. By increasing thefluence, the enhanced diffusion of oxygen relaxes the ten-sile stresses and transforms them into compressive thatcauses the reduction in crystallite size [26]. These resultsare well correlated with change in surface morphologicalimages of Figures 1b–1e. For lower fluences large sizedgrains are grown and for increasing fluence, their size de-creases and compactness increases gradually.

The laser induced pressure (GPa) and energy de-posited per atom (eV/atom) are also evaluated for var-ious laser fluences and are represented in Figure 5. Itwas observed that both pressure and deposited energy peratom increases linearly by increasing the laser fluence. Thethreshold displacement energy for Zr is 28 eV [27] whereasenergy deposited per atom varies from 12 to 16 eV/atom.This implies that deposited laser energy is significantlyhigher than melting energy per atom and is capable to pro-duce defects in the material. Similary the Young’s mod-ulus of Zr is 88 GPa [25], whereas pressure exerted dur-ing laser irradiation (energy absorbed per unit volume)varies from 3.5 × 103 to 5 × 103 GPa, which is significantlyhigher than the Young’s modulus and can generate per-manently enhanced defects. These defects are thereforeobservable after irradiation.

3.3 Micro hardness

Figure 6 represents the variation of micro hardness of ex-cimer laser irradiated Zr as a function of laser fluence.An increase in value of hardness is observed with increas-ing laser fluence. This variation in micro hardness is at-tributed to micro structural defects. The increase in themicro hardness may be credited to increasing diffusion of

Fig. 5. The variation of energy deposited per atom (eV/atom)and of laser induced pressure during irradiation of Zr for vari-ous fluences.

Fig. 6. The variation of micro hardness of excimer laser irra-diated Zr for various fluences.

oxygen atoms across the grain boundaries with increas-ing fluence [28]. As a result of this diffusion, the tensilestresses are transformed into compressive (Fig. 4) and areresponsible for the reduction in crystallite size (Fig. 3) andincreasing trend of micro hardness [29,30]. This increase inhardness depends upon many factors including the densityof oxide contents, lattice defects, size and distribution ofgrain, phase composition and crystal structure [31]. Theseresults are well correlated with SEM results (Fig. 1), andcan be verified by using Hall-Petch relationship [32]:

H = H0 + KH(d)−1/2, (8)

where H is the hardness, d is the average grain size,whereas H0 and KH are experimental constants.

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Fig. 7. The comparison of stress-strain curves measured bytensile testing machine of unirradiated and excimer laser irra-diated Zr for various fluences.

Fig. 8. The variation of yield stress (YS) of laser irradiatedZr for various fluences.

3.4 Tensile tests

The tensile stress-strain curves of unirradiated andlaser irradiated Zr for various fluences of 3.8, 4.3, 4.7and 5.1 J cm−2 are represented in Figure 7. The graphof Figure 8 represents the variation of yield stress (YS)for various laser fluences. This figure exhibits that YS in-creases with increasing laser fluence. The graph of Fig-ure 9 illustrates the variation in the ultimate tensilestrength (UTS) as a function of laser fluence. This fig-ure reveals that UTS initially increases with increasingthe fluence from 3.8 to 4.7 J cm−2 and then shows a de-crease for the maximum fluence of 5.1 J cm−2. The vari-ation in the tensile stress, ultimate tensile strength and

Fig. 9. The variation of ultimate tensile strength (UTS)of laser irradiated Zr for various fluences of 3.8, 4.3, 4.7and 5.1 J cm−2.

percentage elongation are associated with the micro struc-tural changes produced due to laser irradiation. For mostof the materials, yield stress and grain size are correlatedwith Hall-Petch equation [33]:

σy = σ0 + ky(d)−1/2. (9)

In this expression, σy is yield stress, d the average graindiameter, σ0 and ky are constant for the specific material.According to this relation grain size reduction improvesthe material strength and simultaneous loss of ductility.The grain boundaries act as a barrier for dislocation mo-tion. The large scale atomic disorderness and differentorientations between neighbouring grains creates repul-sive stress field which opposes the continued dislocationmotion [34]. With the propagation of more dislocations,there is a dislocation pileup. It causes dislocation diffu-sion across the grain boundary due to enhanced repul-sive forces and therefore generates further deformation inthe material. For the small sized grain boundaries, (SEMresults of Fig. 1), dislocation pileup will be smaller andtherefore large amount of stress is required to move dis-locations across a boundary. Thus, higher applied stresswill be required to move the dislocation, which will beresponsible for the higher yield strength (Fig. 8) [34]. Atmaximum fluence of 5.1 J cm−2, these piled-up disloca-tions cause the sliding of grain boundaries and rotation ofgrains resulting a slight decrease in ultimate tensile stress(Fig. 9) [35]. This rotation provides larger path for disloca-tion movement by eliminating the barrier presented by thegrain boundaries between them. This mechanism can leadto the softening and localization of the material. These re-sults are compatible with SEM results, according to whichinitially increasing laser fluence up to range of 4.7 J cm−2

causes the growth of grains with the appearance of widerand distinct boundaries (Fig. 1) representing more oxygen

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diffusion. At the maximum fluence of 5.1 J cm−2 theseboundaries become diffusive due to refilling by shock lique-fied material. This is responsible for softening of materialand correspondingly a reduction in the UTS is observed.

4 Conclusions

The effect of laser fluence on surface, structural and me-chanical properties of Zr has been investigated. The graingrowth is observed for all fluences, however, a decrease ingrain size and increase in the width and depth of grainboundaries is observed for increasing laser fluences. Atthe maximum fluence the appearance of grain boundariesbecomes again diffusive and comparatively less distinct.XRD analysis reveals the appearance of new phases ofZrO2 and Zr3O. The variation in the peak intensity is ob-served to be anomalous whereas decreasing trend in thecrystallite size and residual stresses has been observedwith increasing fluence. Micro hardness test showed thecontinuous increase in hardness by increasing laser fluencedue to the enhanced diffusion and compressive nature ofresidual stresses. In the tensile results it was found that YSincreases, whereas percentage elongation decreases withincreasing fluence regularly. The UTS shows anomalousbehavior with the increase of laser fluence. These tensilechanges are well correlated with SEM and XRD analysisand are associated with Hall-Petch effect.

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