The effect has been studied of treatment with nanosecond laser pulses on the fatigue resistance of plate samples of recrystallized (grain size of the order of 2-3 μm) commercially pure titanium (grade VT1-0) under cyclic tensile loading. The results of investigations by scanning and transmission electron microscopy of the subsurface layer microstructure of the alloy under study after exposure to nanosecond laser irradiation and subsequent fatigue tests are presente

Betekhtin, V. I.

Kadomtsev, A. G.

Khramov, G. V.

Kolobov, Yu. R.

Manokhin, S. S.

DSpace at Belgorod State University

Technical Physics Letters, 2022, Vol. 48, No. 106Investigation of the effect of nanosecond laser pulses processing onthe microstructure and fatigue resistance of commercially pure titanium© Yu.R. Kolobov 1, S.S. Manokhin 1,¶, V.I. Betekhtin 2, A.G. Kadomtsev 2, M.V. Narykova 2, G.V. Odintsova 3,G.V. Khramov 41 Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, Russia2 Ioffe Institute, St. Petersburg, Russia3 ITMO University, St. Petersburg, Russia4 Belgorod National Research University, Belgorod, Russia¶ E-mail: manohin@icp.ac.ruReceived September 16, 2021Revised September 28, 2021Accepted September 28, 2021The effect has been studied of treatment with nanosecond laser pulses on the fatigue resistance of plate samplesof recrystallized (grain size of the order of 2−3 µm) commercially pure titanium (grade VT1-0) under cyclic tensileloading. The results of investigations by scanning and transmission electron microscopy of the subsurface layermicrostructure of the alloy under study after exposure to nanosecond laser irradiation and subsequent fatigue testsare presented.Keywords: titanium, surface, pulsed nanosecond laser irradiation, shock−wave action, fatigue properties.DOI: 10.21883/TPL.2022.01.52471.19025One of the promising directions in the field of creat-ing new technologies for processing medical−grade metalmaterials is surface modification using concentrated energyfluxes including laser irradiation [1–3]. The great majorityof widely commercially employed techniques for materialsurface modification involve considerable heating of theirnear−surface layers or entire bulk. This may be toa high extent avoided by using the advanced methodfor processing metal material surfaces with short (pico—, nano—, and subnanosecond) and ultrashort (subpico—and femtosecond) laser pulses which has been intenselydeveloped in recent years[1–3]. Processes evolving un-der such action, being associated with variations in thesurface relief and phase−structural transformations in thin(1−5µm) subsurface layers, enable formation of a uniquecomplex of physical−chemical and mechanical propertiesof the above mentioned regions of materials. By varyingthe laser irradiation parameters, it is possible to performcontrollable macro— to nanoscale modification of the sur-face relief, including creation of multimodal roughness withwhich manifestation of superhydrofobic or superhydrophilicproperties of the surface are associated. It is known thattransition from the mode of the surface material ablationto the mode of shock−wave processing (laser peening)promotes improvement of mechanical characteristics of boththe subsurface and near−surface material layers downto the depth of one micrometer to several hundreds ofmicrometers [1,4]. However, the question about the effect ofnanosecond laser irradiation upon mechanical properties incyclic loading of titanium alloys is so far poorly studied. Thispaper represents investigation of the effect of nanosecondlaser irradiation on the fatigue resistance of recrystallizedcommercially pure titanium and also of peculiar features ofvariations in the recrystallized structure of the studied alloynear−surface layer during the nanosecond laser irradiationand fatigue test.As the test material, commercially pure titanium VT1-0was selected. Its chemical composition is given in the table.For the fatigue test, plane samples with the double−Tcross section with the base thickness of 1mm, widthof 3mm, and length of 40mm were used; the sampleswere fabricated by electrical discharge machining at setup”AQ300L“. Fatigue tests in the cyclic loading mode wereperformed using the equipment of the Nanostructure Ma-terial and Nanotechnologies Center of NRU”BelSU“ atthe electromagnetically driven test machine Instron Elec-tropulse E3000. As known, the loading frequency willbe selected in compliance with the amplitude−frequencycharacteristic (AFC) that depends on the test sample rigidity.In this work, the test frequency of 15Hz was selectedfrom the range where there are no distortions of theAFC sinusoidal character, which are observed at roomtemperature at higher frequencies.Structural investigations were carried out using transmis-sion electron microscopes (TEM) Tecnai G2 F20 S-TWINand FEI TecnaiOsiris at the accelerating voltage of 200 kV.Thin foils (lamellas) that are to be cut normally to thesample surface were prepared for the TEM investigation bythe ion thinning technique with a focused laser beam in thecolumn of the FEI Scios scanning electron−ion microscope.The laser processing of the samples was performedunder a layer of water about 2mm thick using setup”Minimarker-2“ at the following irradiation parameters:wavelength λ = 1064 nm, pulse duration τ ∼ 100 ns, pulse56Investigation of the effect of nanosecond laser pulses processing... 57Chemical composition of the titanium−based alloy VT1-0Content of elements, wt.%Al Fe Si O C N HRequired < 0.7 < 0.25 < 0.1 < 0.2 < 0.07 < 0.04 < 0.006Actual < 0.01 < 0.12 < 0.01 < 0.14 < 0.01 < 0.01 < 0.01300 mmFigure 1. Surface microstructure of the alloy VT1-0 samplesin the initial recrystallized state after processing with nanosecondlaser pulses under a layer of water. The arrow points to an isolatedcrater characteristic of the single−pulse impact. The single pulsepower density is ∼ 1GW/cm2 .energy E ≈ 10−3 J, pulse repetition rate ν = 50 kHz, laserbeam scanning rate v = 1500mm/s, single pulse powerdensity F ∼ 1GW/cm2. The samples were irradiated inthe scanning mode with the preset by the control systemmaximally dense arrangement of surface craters withouttheir superposition (only one pulse per point, free ofoverlapping). The spot of the laser beam focused on thesurface was 35µm in diameter.In fabricating thin foil (lamella) from the subsurfacelayer of the material under study by the”cross-section“technique, scanning electron−ion microscope FEI SciosDual Beam (Common Use Center of FRC”Crystallographyand Photonics“) was used. The lamellas were cut fromthe center of isolated craters in the form of blurred moltenalloy droplets, which arose under the action of a singlepulse. Along the crater periphery there is a clearly observedconvex bead with elliptic edges additionally evidencing forlocal melting.At the rate of laser beam scanning over the materialsurface (under a layer of water), which has been selectedin this work, both isolated and overlapping craters maybe observed by a metallographic microscope (Fig. 1). Asknown, emergence of the craters is connected with thefact that, when the laser pulse power exceeds the ablationthreshold, a microexplosion occurs and gives rise to acrater on the sample surface and to glowing plasma jointlywith solid and liquid particles flying apart. The areaoccupied by overlapping craters was determined by the freeanalytical program code”ImageJ“. For this analysis, imageswere processed by increasing the contrast and essentiallyenhancing the brightness of midtones. This operation leavesvisible only the craters, which enables calculation of thesurface area occupied by the craters; this area appeared tobe about 80%. The relief of the laser−processed regionwas studied at the crater cross section obtained by using afocused ion beam in the column of the scanning electronmicroscope (SEM). The crater depth and height of the beadsurrounding it which were measured with respect to theinitial surface level were 6 and 0.5 µm, respectively.To determine the fatigue limit via the experimental data,dependences were plotted (Fig. 2) of the cycle maximalstrain on the number of cycles till destruction (Woellerdiagram) for the samples of recrystallized titanium VT1-0in the initial state and after the surface preprocessing with alaser. Fig. 2 shows that in the entire range of preset strainssamples withstand on the average a considerably higherstrain after processing with a nanosecond laser irradiationthan in the initial state prior to laser processing. Thus,lgN2004003004.0 6.05.03.5 5.54.5100500s, MPa R2 = 0.94R2 = 0.96Figure 2. Fatigue curves for the alloy VT1-0 samples in the initialrecrystallized state (squares) and after processing with nanosecondlaser pulses (triangles) in water (at the single pulse power densityof ∼ 1GW/cm2).Technical Physics Letters, 2022, Vol. 48, No. 158 Yu.R. Kolobov, S.S. Manokhin, V.I. Betekhtin, A.G. Kadomtsev, M.V. Narykova...1 mm 1 mmSubsurface modifiedlayerInitialstructure1 mm( 100)1( 010)1[0001]a bc dFigure 3. Microstructure of the commercially pure titanium (VT1-0) sample in the initial recrystallized state after pretreatment with ananosecond laser beam under water (at the single pulse power density of ∼ 1GW/cm2) and fatigue test (1 · 106 cycles, the cycle strainof 100MPa). a — bright−field TEM image; b — dark−field TEM image ; c — microdiffraction (the region it has been obtained in ismarked in part a); d — a dark−field image of the initial recrystallized sample (prior to irradiation) given for comparison (transmissionscanning electron microscopy).modification of subsurface layers of the studied sampleswith the nanosecond laser irradiation results in an essentialincrease in the resistance to fatigue destruction.TEM images obtained after the nanosecond laser irradi-ation and fatigue test exhibit in the near−surface layer amodified layer about 1−2µm deep (Fig. 3, a, b). This layerwas formed as a result of processing by nanosecond laserpulses under a layer of water since no differences weredetected in its microstructure as compared to that afterlaser irradiation and prior to the fatigue test (Fig. 3, d).The studied alloy microstructure consists in crystallites ofthe elongated plate shape about 300 nm wide and 2−3µmlong. The crystallites are elongated mainly in the directionperpendicular to the sample surface just as it should bein crystallization with heat removal in the given direction.Based on the microdiffraction pattern (Fig. 3, c) it ispossible to conclude that the crystallites misalignment is ofthe small−angle character (the misalignment angle below15◦). The arrangement of reflexes does not fit to thetwin misalignment since in the case of twinning in theconsidered crystallite orientation [0001̄] (Fig. 3, c) thereshould be observed twin reflexes located symmetrically withrespect to the (1010) plane [5]. Thus, closely locatedreflexes from planes of the (1̄010), (1̄100), (01̄10) and othertypes evidence for the presence of small−angle boundariesbetween neighboring crystallites. We may assume that sucha misalignment of the crystallites is connected with theirnucleation in the molten alloy on a polycrystalline substratewith a well pronounced crystallographic texture. Thus, thecrystalline substrate on which the elongated monocrystallinestructural elements (crystallites) are crystallized and grow,prescribes the crystalline−structure matching between theTechnical Physics Letters, 2022, Vol. 48, No. 1Investigation of the effect of nanosecond laser pulses processing... 59substrate and observed blocks with small−angle mutualmisalignment. This is typically observed during the growthof blocked monocrystals from liquid phase on a solid−phasecrystalline substrate. Probably, it is possible to obtainvarious crystalline structures with a preset crystallographicorientation depending on the rolling texture that mainlyprescribes the type of the crystallographic plane (on whichthe crystallization nucleus will be formed).It has been revealed that processing of experimentalsamples of recrystallized commercially pure plate−shapedtitanium VT1-0 by nanosecond laser pulses results inessential elevation of the conditional fatigue limit under acyclic tensile load at the cycle number of up to 106. Wehave established that this effect is connected with formationin the thin (about 2µm) subsurface layer under the impactof nanosecond laser pulses of a plate−type microstructurewith elongated (mainly normally to the sample surface)microstructure elements about 300 nm in transverse size and2−3µm in length. Formation of the above−characterizedsubmicrocrystalline structure confirms the earlier observedmanifestation of crystallization of a thin subsurface layermelted with a laser beam in the direction perpendicular tothe sample surface (in the heat removal direction).Structural grinding of metals and alloys to the nanoscalelevel is known to prevent in at least one direction nucleationand evolution of cracks that occur in thin near−surfacelayers in the case of fatigue in the studied range of loadingcycles.AcknowledgementsStructural studies were performed using equipment ofthe Common Use Center of FRC”Crystallography andPhotonics“RAS (Moscow).Financial supportThe main part of the work concerning investigation ofthe alloy VT1-0 structure after nanosecond laser irradiationwas supported by the Russian Scientific Foundation (project 19-12-00221); the additional part devoted to determiningthe surface relief parameters, including geometrical param-eters of isolated craters on the surfaces of samples ofthe alloy under study was accomplished in the frameworkof State Assignment (state registration  AAAA-A19-119111390022-2 and AAAA-A19-119022690098-3).Conflict of interestsThe authors declare that they have no conflict of interests.References[1] Yu.R. Kolobov, Russ. Phys. J., 61 (4), 611 (2018).DOI: 10.1007/s11182-018-1440-4.[2] V.P. Veiko, Yu.Yu. Karlagina, E.E. Egorova, E.A. Zernitskaya,D.S. Kuznetsova, V.V. Elagin, G.V. Odintsova, J. Phys.: Conf.Ser., 1571 (1), 012010 (2020).DOI: 10.1088/1742-6596/1571/1/012010[3] A.A. Ionin, S.I. Kudryashov, A.A. Samokhin, Phys. Usp., 60(2), 149 (2017). DOI: 10.3367/UFNe.2016.09.037974.[4] W. Jia, Q. Hong, H. Zhao, L. Li, D. Han, Mater. Sci, Eng. A,606, 354 (2014). DOI: 10.1016/j.msea.2014.03.108[5] P.G. Partridge, Met. Rev., 12 (1), 169 (1967).DOI: 10.1179/mtlr.1967.12.1.169Technical Physics Letters, 2022, Vol. 48, No. 1

Investigation of the effect of nanosecond laser pulses processing on the microstructure and fatigue resistance of commercially pure titanium

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Investigation of the effect of nanosecond laser pulses processing on the microstructure and fatigue resistance of commercially pure titanium

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