Effect of femtosecond laser processing on mechanical properties of plates made of submicrocrystalline VT1-0 titanium alloy is studied using active deformation and fatigue testing involving cantilever bendin

Kolobov, Yu. R.

Korneev, E. A.

Kudryashov, S. I.

Kuzmenko, I. N.

Skomorokho, A. N.

English

DSpace at Belgorod State University

ISSN 1063-7842, Technical Physics, 2018, Vol. 63, No. 3, pp. 385–390. © Pleiades Publishing, Ltd., 2018.Original Russian Text © Yu.R. Kolobov, E.A. Korneeva, I.N. Kuz’menko, A.N. Skomorokhov, S.I. Kudryashov, A.A. Ionin, S.V. Makarov, A.Yu. Kolobova, S.S. Manokhin,V.I. Betekhtin, A.G. Kadomtsev, 2018, published in Zhurnal Tekhnicheskoi Fiziki, 2018, Vol. 88, No. 3, pp. 396–401.PHYSICAL SCIENCE OF MATERIALSEffect of Processing by Femtosecond Pulsed Laseron Mechanical Properties of Submicrocrystalline TitaniumYu. R. Kolobova, b,*, E. A. Korneevac, I. N. Kuz’menkoa, A. N. Skomorokhovd,S. I. Kudryashove, f, A. A. Ionine, S. V. Makarove, f, A. Yu. Kolobovab, g,S. S. Manokhina, b, V. I. Betekhtinh, and A. G. Kadomtsevha Belgorod State National Research University,Belgorod, 308015 Russiab Institute of Problems of Chemical Physics, Russian Academy of Sciences,Chernogolovka, Moscow oblast, 142432 Russiac Joint Institute for Nuclear Research,Dubna, Moscow oblast, 141980 Russiad Institute of Physics and Power Engineering, State Research Center of Russian Federation,Obninsk, 249033 Russiae Lebedev Physical Institute, Russian Academy of Sciences,Moscow, 119991 Russiaf National Research Nuclear University MEPhI, Moscow, 115409 Russiag National Research Technological University MISiS, Moscow, 119991 Russiah Ioffe Institute, Russian Academy of Sciences,St. Petersburg, 194021 Russia*e-mail: kolobov@bsu.edu.ruReceived May 31, 2017Abstract—Effect of femtosecond laser processing on mechanical properties of plates made of submicrocrystal-line VT1-0 titanium alloy is studied using active deformation and fatigue testing involving cantilever bending.DOI: 10.1134/S106378421803009XINTRODUCTIONSubmicrocrystalline (SMC) and nanostructured(NS) alloy-free titanium is a promising material formodern metal biomaterials and surgical implants [1].Formation of the SMC and NS states in titaniumalloys leads to substantial improvement of mechanicaland physicochemical properties, in particular, proper-ties needed for applications as materials for medicalimplants in traumatology, orthopedics, stomatology,and several branches of medicine and technology.Surface modification by femtosecond laser pulsesis a promising method for an increase in biocompati-bility of the SMC titanium and biological tissues.Femtosecond laser radiation (FLR) provides lowercontamination in comparison with alternative meth-ods for surface processing, is highly technological, andcan be used for processing of items with complicatedsurfaces. A relatively small depth of the zone of ther-mal effect of less than one micrometer is an importantadvantage of ultrashort (femtosecond) laser pulses [2].Such an advantage makes it possible to modify thinsurface layers in the absence of heating of bulk mate-rial, which, normally, leads to a decrease in mechani-cal strength due to degradation of the SMC and NSstates or complete elimination of such states owing tothe reverse processes and recrystallization. Thus, theapplication of femtosecond lasers is important formodification of the surface of titanium alloys in theSMC and NS states, since the development of theabove processes at relatively high temperatures leads toa decrease in the strength of the materials typical ofcourse-grain structures [3, 4].Controlled modification of surface profiles withthe aid of FLR is a promising method for an increasein osteointegration, since the surface nanorelief spe-cifically affects the behavior of various cells and, inparticular, provides an increase in the bioactivity ofosteoblasts [5, 6]. Controlled modifications of surfacerelief of metal implants using FLR and the corre-sponding effect on biocompatibility have been studiedin several works (see, for example, [5–7]). Note insuf-ficient data on the effect of FLR on the mechanicalproperties of titanium alloys. In this work, we study theeffect of FLR on the mechanical properties of theSMC VT1-0 titanium.385386 KOLOBOV et al.Fig. 1. Original SMC VT1-0 titanium sample: (a) TEM image of the microstructure and (b) histogram of the grain distributionwith respect to size.100 200 300 400 8000Math. expectation = 220 nm102430(a) (b)N/N0500 600 70068201214161822242628Grain size, nmMode = 140 nmMedian = 190 nmStandard deviation = 11 nm500 nmMATERIALS AND METHODSWe study mechanical properties of the originalSMC VT1-0 titanium alloy and the FLR-modifiedsample. The SMC state with homogeneous globularstructure with a mean grain size of about 220 nm (Fig. 1)is obtained for rods with a diameter of 8 mm usingmechanical and thermal processing that involvesradial-displacement, helical, and section rolling [8].Residual microstress (first-order stress) is eliminateddue to 3-h-long annealing in a Nabertherm LT5/13/P330 muffle furnace in air at a temperature of350°C. Then, test samples (plates) are cut from theannealed rods with the aid of spark cutting using aSODICK AQ 300L device. The plates with workingsizes of 2 × 0.5 × 27 and 2 × 0.2 × 27 mm are used fortension tests, and the plates with working sizes of 3 ×0.6 × 16 mm are used for fatigue tests using symmetriccantilever bending. The samples are mechanicallygrinded and polished using a Struers LaboPol-5device prior to the laser processing and testing.The mean sizes of the grains are determined usingthe random linear intercept method [9, 10]. The Ima-geScope software is employed for construction andmathematical processing of histograms of grain-sizedistributions. Figure 1b presents the histogram of thesize distribution of grains for the original VT1-0 tita-nium alloy and the corresponding statistical parame-ters. The distribution density function is approximatedusing the logarithmic law.Laser ProcessingIn the experiments on the surface modification ofthe SMC titanium using FLR, we employ linearlypolarized radiation of an Avesta Project Ti:Sa femto-second laser setup with a central wavelength of λ ≈744 nm, a FWHM of the radiation band of 12 nm, apulse duration of about 100 fs in the interaction region,a pulse energy of up to 8 mJ, and a pulse repetition rateof 10 Hz. The laser energy is chosen to avoid notice-able degradation of the energy density distribution onthe surface of the target due to self-focusing in air andthe corresponding chromatic emission, filamentation,and scattering by air plasma.The processing is performed in air at a f luence ofF = 0.3 J/cm2 and a scanning speed of  = 18 μm/s.The scanning direction and polarization vector areoriented in such a way that the periodic parallelgrooves that result from the laser modification aredirected along the plate (i.e., in parallel to the axis ofquasi-static tension and tension–compression axis incyclic tests).To choose the modification regimes, we use theresults of the previous work, in which we have struc-turally modified SMC titanium using FLR. Theresults of [11] show that different types of relieves canbe obtained at different laser f luences and scanningrates. The modification regime with the parametersF = 0.3 J/cm2 and  = 18 μm/s is optimal with respectto biocompatibility. Note the absence of deep poresand cracks that may serve as concentrators of stressand cause a decrease in the strength parameters of thematerial (especially, under cyclic loads).Methods for Mechanical TestsThe fatigue tests are performed using an InstronElectropuls 3000 device and supplemented with thevvTECHNICAL PHYSICS  Vol. 63  No. 3  2018EFFECT OF PROCESSING BY FEMTOSECOND PULSED LASER 387Fig. 2. Loss angle ϕ between load ϕσ and shift ϕε.ForceShiftLoss angleForωtceShiftstudy of viscoelastic properties using the dynamicmechanical analysis (DMA) [12]. In the fatigue tests,we employ the cantilever bending at an oscillation fre-quency of 10 Hz at room temperature. The tests areperformed in the symmetric mode with a cycle asym-metry coefficient of R = –1 in the presence of loadingat constant amplitude of the deflection. The size of theworking region of the sample in the tests is 0.60 ×3.0 × 16.0 mm. The amplitudes of force are used tocalculate stress with the aid of the formulawhere F is the amplitude of the force exerted on thesample, l is the length of the working part of the sam-ple, b is the width of the sample, and h is the thicknessof the sample.In the fatigue tests, we measure the instantaneousforce (load) and shift (deformation) (Fig. 2) and per-form the DMA using the correlation method. In thecorresponding procedure, we determine the phase dif-ference (loss angle ϕ) of the periodic variation in theload and deformation.σ = 26 ,FlbhTECHNICAL PHYSICS  Vol. 63  No. 3  2018Fig. 3. Surface morphology of the FLR-processed VT1-0 titaniu20 μm(a) (b)The measured tangent of the loss angle tanϕ char-acterizes scattering of the oscillation energy ΔW due toinelastic internal microscopic processes in solid:Here, ΔW/W is the part of energy that must be trans-ferred to the oscillation system over a single period toprovide constancy of total oscillation energy W. Anincrease in quantity tanϕ indicates an increase in theintensity or activation of additional inelastic processesthat accompany the cyclic deformation of solid (e.g.,reversal and microplastic deformation, growth ofcracks, etc.) [13].The tensile tests are performed with the aid of anInstron 5882 electromechanical device at a deforma-tion rate of 1.5 mm/min using an extensometer with aworking length of 10 mm.A Quanta 600 FEG scanning electron microscopewith field emission is used to study surface morphol-ogy and elemental composition of the processed sam-ples.EXPERIMENTAL RESULTSAND DISCUSSIONFigure 3 shows the morphology and cross sectionof the FLR-processed titanium surface.The regimes of laser processing and the structurethat is formed due to such processing have been ana-lyzed in [11]. The structure exhibits ordered micro-and nanorelief, in particular, grooves that are parallelto the scanning direction.Figure 4 presents the deformation curves obtainedin the tensile tests for the original SMC titanium plateswith thicknesses of 0.2 and 0.5 mm and the plates withlaser-modified surfaces. It is seen that the laser-induced modifications of the surface and near-surfacelayers weakly affect characteristics of the materialΔϕ =π1tan .2WWm samples: (a) ×5000, (b) ×20 000, and (c) cross section ×5000.5 μm 20 μm(c)388 KOLOBOV et al.Fig. 4. Deformation curves for the SMC VT1-0 titaniumsamples with thicknesses of 0.2 and 0.5 mm with laser-modified surfaces in comparison with the deformationcurves of the original SMC titanium samples.0800σ, MPa20060012ε, %1086424003 4 1 21 − ns 0.5 mm2 surface laser modification− ns 0.5 mm +3 − ns 0.2 mm4 surface laser modification− ns 0.2 mm +Fig. 5. Synchronously recorded experimental results on (1)maximum stress per cycle σ and (2) tangent of loss angle ϕversus number of deformation cycles.300500σ max, MPa20(σ0.2 − σmax)/σ0.20450400123502500.020.0500.040.030.01tanϕFig. 6. Plot of tangent of loss angle tanϕ vs. relative devia-tion of the maximum stress per cycle from the yield point.0.0060.016tanϕ0.0020.0120.0100.0080.0040.5(σ0.2 − σ)/σ0.20.40.30.200.014Fig. 7. Plots of maximum stress per cycle vs. number ofcycles preceding destruction (fatigue curve) for the nano-structured VT1-0 titanium plates with a thickness of0.6 mm (open squares) in the absence of coating and(closed squares) after femtosecond laser processing.300700σ, MPa1006005004002003 5 6 704Laser coatingWithout coatinglogNunder study (mechanical strength, yield point, andYoung modulus). The Young modulus can be esti-mated in the first approximation using the slope of thelinear fragment of deformation curve σ–ε. However,the laser-modified samples exhibit higher plasticity incomparison with original samples.Figure 5 shows the synchronously recorded resultson the maximum stress in the cycle σ (curve 1) andtangent of the loss angle (curve 2) versus number ofcycles. The curve of cyclic strengthening (curve 1 inFig. 5) shows that the maximum stress in the cycle (atthe fixed deflection) remains almost unchanged anddecreases only before destruction. Quantity tanϕ(curve 2) also remains unchanged and sharplydecreases only prior to destruction.Thus, the beginning of the destruction of the sam-ple under cyclic load can be determined using bothcurves of cyclic strengthening and a decrease in theamplitude of stress and an increase in the tangent ofthe loss angle.Figure 6 presents the dependence of the tangent ofloss angle on the relative deviation of the maximumstress per cycle from the yield point of the materialunder tensile stress (σ – σmax)/σ0.2 for the domain inTECHNICAL PHYSICS  Vol. 63  No. 3  2018EFFECT OF PROCESSING BY FEMTOSECOND PULSED LASER 389Table 1. Results of mechanical tension and fatigue tests for the original plates with a thickness of 0.5–0.6 mm made of theSMC VT1-0 titanium alloy and the laser-processed platesSampleTension Fatigue limit σRN for 106 cycles, MPaσRN/σBσ0.2, MPa σB, MPa σB/σ0.2Original 642 ± 7 805 ± 7 1.25 385 0.47Laser-processed 652 ± 7 802 ± 5 1.23 422 0.53which the tangent of loss angle is independent of thenumber of cycles (Fig. 4). It is seen that the tangent ofloss angle decreases with a decrease in the maximumstress per cycle relative to the yield point. Such a resultcorresponds to a decrease in the intensity of micro-plastic deformation with a decrease in the maximumstress per cycle, which is in agreement with an increasein the number of cycles preceding destruction with adecrease in the maximum stress per cycle.To determine the fatigue limit, we use the experi-mental results to plot the maximum stress per cycleversus number of cycles preceding destruction (fatiguecurve) for the original VT1-0 titanium plates (opensquares) and laser-processed samples (closedsquares). The results of Fig. 7 are obtained for thestress of the curves of cyclic strengthening that aresimilar to the curves of Fig. 5. For the experimentalstresses, the FLR-processed samples exhibit greaternumber of cycles prior to destruction in comparisonwith the original samples.The above results on the effect of surface process-ing with the aid of femtosecond laser pulses on themechanical properties of the material under study (inparticular, improvement of the fatigue limit and anincrease in the plasticity in the quasi-static breakingtests) are in agreement with each other, since the prop-agation of fatigue microcracks in materials withenhanced plasticity is slower than the propagation ofsuch cracks in fragile materials. Thus, the surfaceFLR-modification of implants that leads to the for-mation of relief with complicated structural hierarchyleads to improvement of both biocompatibility andmechanical properties. Note that an increase in thefatigue resistance and plasticity of the SMC VT1-0titanium resulting from the femtosecond laser process-ing shows the advantages of the method in comparisonwith alternative methods for surface processing of tita-nium alloys. In particular, the formation of bioactiveand bioinert coatings on the surface of titanium alloysusing microarc oxidation leads to lower fatigue limit inthe fatigue tests of the Ti–Al–Zr titanium alloy withcontrolled load amplitude in the tension–compres-sion procedure [14] and undoped titanium (Grade 4alloy) in the test using four-point bending (yield pointand mechanical strength) [15]. The microarc oxida-tion also leads to a decrease in the mechanical strength ofthe SMC VT1-0 titanium under quasi-static load [16].Table 1 presents the results of mechanical tensionand fatigue tests of the original SMC plates of theTECHNICAL PHYSICS  Vol. 63  No. 3  2018VT1-0 titanium alloy with a thickness of 0.6 mm andplates processed with the aid of the femtosecond laserpulses.It is seen that the yield points and mechanicalstrengths of the laser-processed and original samplesare almost identical and the conditional fatigue limitfor 106 cycles is significantly higher for the laser-pro-cessed samples.CONCLUSIONSSurface processing of the SMC VT1-0 titaniumplates with a thickness of 0.6 mm by femtosecond laserpulses leads to an increase in the conditional fatiguelimit (for the tests consisting of 106 cycles). The tensiletests of the SMC VT1-0 titanium plates with thick-nesses of 0.5 and 0.2 mm show that the laser process-ing weakly affects the measured yield points andmechanical strength but leads to a significant increasein the prefracture deformation.The study using the DMA method shows that themicroplastic deformation of the samples under studydecreases with a decrease in the maximum stress percycle in the cyclic tests. The destruction of the samplesunder cyclic loads is accompanied by a sharp increasein the microplastic deformation (tangent of lossangle). Such a result can be used as a diagnostic fea-ture of the fatigue limit of the material.ACKNOWLEDGMENTSThe study of the effect of processing with the aid offemtosecond laser pulses on the mechanical propertiesof the SMC VT1-0 titanium alloy in the fatigue testsusing the cantilever bending was supported by the Rus-sian Science Foundation (project no. 15-12-30010).The study of the regimes of laser irradiation was sup-ported by the Program for Improvement of Competi-tiveness of the National Research Nuclear UniversityMEPhI.REFERENCES1. Yu. R. Kolobov, Nanotechnol. Russ. 4, 758 (2009).2. R. Le Harzic et al., Appl. Phys. Lett. 80, 3886 (2002).3. Yu. R. Kolobov, A. G. Lipnitskii, M. B. Ivanov,I. V. Nelasov, and S. S. Manokhin, Russ. Phys. J. 54,918 (2012).390 KOLOBOV et al.4. Yu. R. Kolobov, M. B. Ivanov, S. S. Manokhin, andE. Erubaev, Inorg. Mater. 52, 128 (2016).5. E. Fadeeva et al., Langmuir 27, 3012 (2011).6. R. J. Crawford et al., Adv. Colloid Interface Sci. 179,142 (2012).7. E. Luong-Van, I. Rodrigue, et al., J. Mater. Res. 28,165 (2013).8. Yu. R. Kolobov, M. B. Ivanov, E. V. Golosov, andA. V. Penkin, RF Patent No. 2389568 (2010).9. S. A. Saltykov, Stereometric Metallography (Metallur-giya, Moscow, 1986).10. M. N. Stepnov, Statistical Methods for Processing theResults of Mechanical Tests (Mashinostroenie, Moscow,1985).11. A. Ionin et al., Laser Phys. Lett. 10, 045605 (2013).12. N. E. Fomin, V. A. Yudin, and A. A. Kireev, Innovat-sionnoe Obraz., No. 4, 129 (2013).13. Internal Friction Method in Metallography, Ed. byM. S. Blanter and Yu. V. Piguzov (Metallurgiya, Mos-cow, 1991).14. Y. M. Wang, P. F. Zhang, L. X. Guo, J. H. Ouyang,Y. Zhou, and D. C. Jia, Appl. Surf. Sci. 255, 8616(2009).15. J. M. Wheeler, C. A. Collier, J. M. Paillard, andJ. A. Curran, Surf. Coat. Technol. 204, 3399 (2010).16. E. A. Korneeva, A. N. Skomorokhov, Yu. R. Kolobov,I. N. Kuz’menko, G. V. Khramov, and V. V. Raki-tyanskii, Kompoz. Nanostrukt. 4, 32 (2011).Translated by A. 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Effect of processing by femtosecond pulsed laser on mechanical properties of submicrocrystalline titanium

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Effect of processing by femtosecond pulsed laser on mechanical properties of submicrocrystalline titanium

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