Diffusion of oxygen and nitrogen into titanium under laser irradiation in air

Journal Pre-proofs Full Length Article Diffusion of Oxygen and Nitrogen into Titanium under Laser Irradiation in Air Congyuan Zeng, Hao Wen, Boliang Zhang, P.T. Sprunger, S.M. Guo PII: DOI: Reference:

S0169-4332(19)33394-X https://doi.org/10.1016/j.apsusc.2019.144578 APSUSC 144578

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

19 August 2019 28 October 2019 31 October 2019

Please cite this article as: C. Zeng, H. Wen, B. Zhang, P.T. Sprunger, S.M. Guo, Diffusion of Oxygen and Nitrogen into Titanium under Laser Irradiation in Air, Applied Surface Science (2019), doi: https://doi.org/ 10.1016/j.apsusc.2019.144578

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier B.V.

Diffusion of Oxygen and Nitrogen into Titanium under Laser Irradiation in Air *

Congyuan Zeng 1, Hao Wen 1, Boliang Zhang 1, P.T. Sprunger 2, S.M. Guo 1 1

Department of Mechanical & Industrial Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States 2 Department of Physics & Astronomy and Center for Advanced Microstructures and Devices, LSU, Baton Rouge, Louisiana 70803, United States Abstract: This paper examines the dynamic interactions between pure titanium and ambient air under transient laser processing conditions. The microstructures and composition distributions of the reaction products on and within the titanium samples are reported, which lead to the estimation of high-temperature diffusion coefficients for nitrogen and oxygen into the titanium substrate. In-situ synchrotron X-ray diffraction testing is conducted to reveal the high temperature reaction steps between titanium and air. The formation mechanisms of both titanium nitrides and titanium oxides are discussed. Keywords: titanium nitridation; titanium oxidation; laser irradiation; in-situ phase transformation; synchrotron Xray diffraction.

*

[email protected] Tel: +1 225 578 7619

1. Introduction Titanium (Ti) and its alloys are widely used as aerospace and biomaterial materials due to their excellent properties, including high strength-to-weight ratio, high corrosion resistance, and excellent biocompatibility [1-4]. For many applications, the surfaces of Ti parts are deliberately processed to form a coating layer, in which laser gas alloying is one of the most prevailing treatment methods. Laser surface nitriding has been widely studied by researchers, and the TiN layers formed on the surfaces of Ti parts are known to improve surface hardness, tribological performance, corrosion resistance, and biocompatibility [5-10]. Similarly, considerable studies have been conducted on the formation and characteristics of titanium oxide layers. According to Ciganovic’s work [11], a titanium oxide layer not only can improve the corrosion resistance, but also can enhance the surface hardness for improved wear resistance. Moreover, it has been reported that titanium oxides, especially rutile and anatase titania, can improve biocompatibility and bioactivity of Ti based medical devices [11-14]. With laser surface modifications, various surface reaction products with differing colors have been observed by altering the laser energy level and the gas environment [15-21]. Ti surface modification via laser irradiation was studied by Ciganovic et al. [11] under both air and pure oxygen environments. The oxygen incorporated into Ti was found to be similar for both air and pure oxygen cases (~36 wt.% and ~38 wt.%, respectively). Marticorena [22] reported that with a laser energy density of around 2.5 J/cm2, only a TiN thin layer was generated on the Ti surfaces in air. Braga [23] performed surface modifications on Ti dental implants with a laser under ambient air, aiming at producing tailored surface oxides with a given set of laser irradiation parameters. It was revealed that the crystalline phases on the laser processed surfaces could include αTi, βTi, Ti6O, Ti3O and TiO. Meanwhile, Pérez del Pino [24] detected the formation of both rutile and anatase TiO2 on laser processed surfaces in air. To predict the composition and phase distribution of laser irradiated Ti surface, chemical thermodynamic method was used by Ageev et al. [25]. It was concluded that multilayered composite would appear on the Ti surface, with a thin outermost surface layer consisting of Ti dioxide and the lower layers comprising Ti2O3 and TiO oxides together with Ti nitride. Torrent et al. [19] studied the influence of the laser wavelength (532 vs. 1064 nm) on the incorporation of nitrogen, and concluded that a 1064 nm wavelength laser would have a higher efficiency for nitrogen insertion. With laser surface treatment in air, Lavisse et al. [26] found the formation of Ti oxynitride layers, and the insertion amount of light elements (oxygen and nitrogen) increased

with the increase of the laser fluence. The effects of pulsed laser induced plasma plume above the target were also discussed. The complex coating layers were also observed by Antończak et al. [15]. The top transparent layer was found to contain nitrogen and nonstoichiometric compounds, instead of pure TiO2. The composition distribution and phase constituents of laser irradiated Ti in air can be predicted by the chemical thermodynamic method. For example, Ageev et al. [25] replaced the pulsed laser heating (a non-isothermal process) with an equivalent isothermal case. Then the equilibrium phase chemical composition for laser oxidations was predicted and the method was implemented into a software package. In this paper, phase diagrams for titaniumnitrogen and titanium-oxygen systems were calculated using a commercial package (Thermo-Calc). As the real processes for laser-based surface modification are typically non-equilibrium, the experimental results may not perfectly match the theoretical calculations [15, 25, 26]. Contrary to the stationary non-laser heating conditions, which are close to the equilibrium state due to slow heating/cooling rate and uniform temperature distribution, laser heating (both continuous-wave mode (CW) laser scanning and pulsed laser scanning) induces a non-equilibrium process because of the extremely fast heating/cooling rates and non-isothermal feature [27, 28]. The dynamic laser scanning and the non-uniform energy distribution within the laser spot result in a complex temperature distribution (with large thermal gradient). In addition, the reaction products between air and Ti (oxides and nitrides) can modify the absorption of laser energy simultaneously [29, 30], making it harder to quantitatively define the heat absorbed. Finally, due to the extremely fast cooling effect, residual stresses can lead to structural defects (i.e. dislocations and microcracks), which provide diffusion pathways for both oxygen and nitrogen and impact the reaction kinetics of the process [15]. Both laser-nitridation and laser-oxidation of Ti (or its alloys) are diffusion dominated processes [23, 31-34]. Although many studies have been conducted to examine the reactions between Ti and air, almost all have focused on the effects of laser parameters on the resulting products. For example, Dahotre [31] found that nitridation could be achieved with a laser energy density of 2×106 J/m2 and Man [32] achieved nitridation process on Ti-6Al-4V substrates with proper laser parameters, including variation of speed and power. To gain a better understanding of laser induced Ti-air interactions, it is important to understand the diffusion of elements (i.e. oxygen and nitrogen) in Ti during the laser irradiation process, especially at high temperatures. Besides, due to the complexity of the reaction processes of Ti in air, there is a need to probe and better understand the dynamic reaction steps. The diffusion

coefficients of oxygen and nitrogen into Ti (or its alloys) substrate at high temperatures, such as the molten state, are very limited since previous studies are mainly focused on the diffusion of oxygen and nitrogen in Ti (or its alloy) below the melting points [35-45]. Therefore, the motivation of this paper is to investigate oxygen and nitrogen diffusion into Ti substrate at temperatures above melting point and to investigate Ti-air dynamic reaction steps under typical laser processing conditions. 2. Materials and methods 2.1 Transient laser processing The oxidation and nitridation processes between Ti and air are studied under representative laser surface modification and additive manufacturing process conditions, with a laser lateral translation speed ranging from tens of millimeters per second to several meters per second. With a slow laser scanning speed, the melt-pool size can be one order of magnitude larger than that of a fast scanning case, which in turn gives a significantly different meltpool dwelling time. To subject Ti pieces under different laser processing parameters, a custom laser processing system is used, which has a 200 W IPG ytterbium fiber laser (model YLR-200-AC-Y11, CW mode, Gaussian energy distribution, wavelength 1064 nm). With a fixed laser power of 100 W and a fixed laser spot size of 58 µm, four different scanning speeds are tested, namely 25, 100, 400 and 1600 mm/s. The scanning patterns are controlled by a ProSeries II scan head and the laser hatch spacing for the scanning pattern is determined for each scanning speed to achieve the whole surface coverage. Commercially pure Ti pieces (10×10×3 mm 3, 99.2 wt% purity) are used in this set of testing. Before laser processing, the sample surfaces are ground successively with SiC grinding papers (320 and 600 grit). Then the samples are ultrasonically rinsed in acetone, ethanol, and deionized water in sequence for 20 min each. 2.2 Synchrotron X-ray diffraction based in-situ testing setup The dynamic high-temperature reaction steps of Ti in an ambient air environment is studied using in-situ synchrotron X-ray diffraction (SXRD) with spatially fixed laser irradiation over time span of several seconds, in which the laser power is temporally increased to a maximum of 200 W and then decreased. The Ti target is made of Ti powders (~325 mesh, 99.5 wt% purity, Alfa Aesar), cold pressed into a disk-shaped sample (Φ25.4×2.5 mm3). Insitu synchrotron X-ray diffraction tests were conducted on the Protein Crystallography beamline at the Center for

Advanced Microstructures and Devices (CAMD), which is a 1.3 GeV synchrotron-radiation research center at Louisiana State University. In this study, the X-rays with a wavelength of 1.3808 Å were used, and the X-ray beam size was adjusted to 0.3 mm×0.3 mm using slits. Over the X-ray beam/sample interaction zone, full laser irradiation is applied using an IPG fiber laser (type YLS-2000, CW mode, Gaussian energy distribution). To record the diffraction data, a PILATUS 100K detector system (Dectris AG, Switzerland), with an active area of 83.833.5 mm2, is used at a framing rate of 10 Hz. SXRD obeys Bragg’s equation, which is described as d,

and

, where

are interplanar spacing, diffraction angle, and wavelength of the synchrotron X-rays, respectively. The

sequential SXRD data files taken during the laser processing stages are processed using the program FIT2D, giving the diffraction information (diffraction angles and intensities) of the entire process. During the test, the detector and the synchrotron X-ray are enabled prior to laser heating. Approximately 1 second later, the laser is turned on with a prescribed heating profile with ramp-up, soaking, and ramp-down stages. For the ramp-up stage, laser power is increased to 200 W in 5 second; for the soaking stage, the power is kept at 200 W for one second; and for the rampdown stage, the power is reduced to 0 W with an approximate fixed rate in 1 second. The X-rays and the PILATUS detector are kept working for about four more seconds after the shutdown of the laser. 2.3 Sample characterization After both laser transit processing and synchrotron in-situ studies, the samples are prepared for subsequent characterization. Cross sections of the laser processed samples are exposed using a BUEHLER low-speed saw. Upon mounting with the mixture of SamplKwick powder and SamplKwick liquid (volume ratio 2:1), the sample cross sections are ground successively with SiC grinding papers (320, 600, 800 and 1000 grit), polished with MetaDiTM Supereme polycrystalline diamond suspension (1 μm), and ultrasonically rinsed in acetone, ethanol, and deionized water in sequence for 20 min each. Subsequently, the polished sample cross sections are etched with the Kroll’s Reagent purchased from PACE Technologies for 15 s, and then cleaned using deionized water to wash off the etching solution. The microstructures of the cross-sections are examined by a Quanta™ 3D Dual Beam™ FEG FIB-SEM scanning electron microscope with an accelerating voltage of 20 KV. 3. Results and discussions 3.1 Diffusion of oxygen and nitrogen into Ti under laser irradiation

To investigate the laser-activated diffusion of oxygen and nitrogen into Ti substrates, four laser scanning speeds (25, 100, 400 and 1600 mm/s) are selected for laser irradiation in an ambient air environment. These samples are named Ti-A25, Ti-A100, Ti-A400 and Ti-A1600, respectively, where “A” refers to air. The following formula is applied to determine the laser energy density,

, where

,

,

and

stand for laser energy

density, laser power, laser-sample interaction time, and laser beam diameter, respectively. The laser-sample interaction time is calculated by

, where

is laser scanning speed [31]. The calculated laser energy densities

for Ti-A25, Ti-A100, Ti-A400 and Ti-A1600 are 87.81×106, 21.95 ×106, 5.49 ×106 and 1.37 ×106 J/m2, respectively. According to previous studies [31, 46, 47], for laser energy densities of 1.7 ×106 and 2.12 ×106 J/m2, peak temperatures of the irradiated Ti-6Al-4V surface would reach around 4400 and 4900 ℃, respectively. The laser tracks were also examined under a microscope and melting was confirmed for all laser scanning speeds. 3.1.1 The determination of hatch spacing for laser scanning The optimal hatch spacing for laser scanning must be determined to achieve the full surface coverage. The tests are performed on commercially pure Ti pieces with the custom laser processing system. To quantify the molten track geometries, single laser scanning tracks are performed on Ti samples for each targeted laser scanning speed. The processing chamber of the custom laser processing system is filled with atmospheric air with a pressure of one bar. Figure 1 shows a set of single-track morphology processed with different laser scanning speeds, and the measured results are listed in Table 1. Based on the measured average widths of the single tracks, the hatch spacing to achieve a full molten track coverage (laser overlapping 50 %) on the sample surfaces can be determined. It is worth noting that ripple shaped structures are observed on all the single tracks in Fig. 1, which originate from the fluctuation of the melt pool volume [48], indicating melt formation on titanium surfaces during laser irradiation. 3.1.2 Effect of laser scanning speed on the diffusion of oxygen and nitrogen into Ti Figure 2 shows the SEM image and the energy-dispersive X-ray spectroscopy (EDS) mapping results of the cross section of the representative Ti-A100 sample, which is irradiated with full molten track coverage. Figures 2(b) and 2(c) illustrate that oxygen (O) preferentially remains outermost surface region (< 500 nm), while nitrogen (N) extends much deeper and mainly enriches underneath the outermost surface region (> 500 nm), which is consistent

with previous observations [15]. The TiN region is composed of a TiN layer and TiN dendrites embedded in the Ti substrate [49], which is clearly revealed in Figs. 2(a) and 2(c). Based on the EDS mapping results of the cross section, the diffusion depths of O and N can be determined. The average thickness of the top layer and the average penetration depth of TiN dendrites are considered the effective diffusion depths of O and N, respectively. As illustrated in Figs. 2(b) and 2(c) for the sample Ti-A100, the diffusion depths of O and N for all four laser scanning speed samples are estimated. For compositional analysis, the average diffusion depths of oxygen and nitrogen were used. To obtain the averaged data, more than 15 diffusion depth values for each element (i.e. oxygen or nitrogen) were measured over the range of several melt pools. Figure 3 displays the SEM images of the cross sections of the Ti-A25, Ti-A100, Ti-A400 and Ti-A1600 samples, which are, in general, composed of an outermost surface layer (an O enrichment layer) and a TiN region. As seen, the diffusion depths of O and N decrease as the laser scanning speed increase, i.e., the determined diffusion depth (x) of O decreases from 0.79 m to 0.10 m when the laser scanning speed increases from 25 mm/s to 1600 mm/s. This phenomenon can be explained by the fact that a slower scanning speed results in a longer time, and hence longer laser heating time, for O and N to diffuse into the substrate and react with Ti, leading to a thicker diffusion depth. According to the results acquired above, the diffusion coefficients of O and N into Ti substrate, denoted as and

, are calculated, where Ti(O,N) represents the Ti substrate with already diffused O and N. The

laser-sample interaction time (diffusion time) t is calculated by laser scanning speed. Based on the formula the diffusion time, the With this simple model,

and

, where d is the laser spot size, and v is the

[50], where D is the average diffusion coefficient and t is values are calculated from the measured diffusion depths x, Table 2.

values for the tested laser speeds are found to be very similar. However,

increases prominently as laser scanning speed decreases. Closer observation reveals a dramatical increase of (from 3.49×10-4 to 4.24×10-2 cm2/s) when laser scanning speed decreases from 100 mm/s to 25 mm/s. This is also reported by other researchers [51]. Because of the much larger diffusion depth, magnitude larger than

is around three orders of

. Similar observations were also obtained in previous studies [15, 19, 51, 52], where

TiO2 appeared over TiN. The formation enthalpies of TiN and TiO2 are -338 and -944 kJ/mol, respectively [52]. The

stable TiO2 (with more negative enthalpy of formation) leads to a small diffusion coefficient for oxygen into Ti and forms a thin TiO2 layer near the surface [51, 52]. 3.2 Synchrotron X-ray diffraction test to understand the diffusion processes To help better understanding the diffusion processes of O and N into the Ti substrate, in-situ SXRD test is conducted to reveal the high-temperature reaction steps between Ti and air. The laser irradiation has a fixed location and a duration over several seconds. The results processed by the software FIT2D, which radially integrates the corresponding XRD pattern into crystallographic peaks. This data is shown in Fig. 4 and gives detailed information of XRD diffraction angles and intensities at different time frame numbers. Taking into consideration that the sampling rate is 10 Hz, the total 120 frames indicate 12 seconds for the whole data collecting process. The laser irradiation is initiated approximately 0.7 s after the start of data recording, and the transient laser heating process endures from time-frames 7 to 77, seven seconds in total. Based on the time and the generated phases during the process, the Ti and air interactions can be divided into four stages. Stage 1 (y = 0-29) occurs at around room temperature, and the constant diffraction patterns, intensity and angle, indicate αTi structural phase on the tested sample surface. This stage can be further divided into two periods, time-frames 0-7 (period 1, before programmed laser initiation) and time-frames 7-29 (period 2, after programmed laser initiation). Interestingly, the diffraction patterns remain the same after the preset laser initiation. This can be attributed to the characteristics of the used YLS-2000 IPG fiber laser, which cannot be program-controlled accurately at a low power setting (below 10% of the maximum power (200 W)). Careful observation of the whole SXRD results (Fig. 4) reveals that the laser was effectively initiated at time-frame 29, and then raised to 200 W at around time-frame 57. The dotted line in the left image of Fig. 4 illustrates the intended power ramp-up rate. Based on the observed SXRD results, the Ti sample remained at/near room-temperature before time-frame 29 because of the non-linear irradiation power; moreover, the only phase can be detected is αTi. Stage 2 (y = 29-37, 37-41 and 41-49) clearly shows the rising temperature of the laser irradiated area on the Ti sample surface. Diffraction peaks of αTi continuously shift to the smaller angle side (time frames 29-37), which is due to the sample expansion caused by the increasing temperature. An abrupt variation of the diffraction angles and intensities occur at time frame 37, which is attributed to the phase transformation from αTi [close packed hexagonal (HCP) structure] to βTi [body centered cubic (BCC) structure]. To assist the understanding to the

oxidation and nitridation processes, Ti-O and Ti-N binary phase diagrams are calculated using Thermo-Calc software with the TCBIN database, shown in Fig. 5. According to both the calculated Ti-N and Ti-O binary phase diagrams, the HCP to BCC allotropic phase transformation temperature for pure Ti is 822 °C at atmospheric pressure. Four distinct, although intensity attenuated, diffraction peaks exist within time frames 37 to 41, namely a, b, c and d. Peaks a and b are attributed to the emergence of a βTi structural phase, while peaks c and d indicated a phase-separated αTi lattice-expanded phase. Note that the diffraction intensity of αTi is gradually enhanced over the time-frames 37-41, while the diffraction intensity of βTi is gradually attenuated. Pure Ti undergoes the following phase transformation αTi → βTi → Liquid when it is heated up. The observed unique coexistence of αTi and βTi can be explained with the help of the Ti-N and Ti-O binary phase diagrams in Fig. 5. Antończak [15] pointed out that when the temperature was over 600 °C, both αTi and βTi would absorb interstitially large quantities of O and N. Specifically, αTi has the capability of dissolving 23 at% (8 wt%) of N and 33 at% (14 wt%) of O. This is also indicated on the Ti-N and Ti-O binary phase diagram, marks A and B in Fig. 5(a) and (b), respectively. Wang et al. [53] reported that laser heating could induce N and O plasma, which would accelerate the absorption of N and O into the Ti sample. Our in-situ SXRD results indicate that under laser irradiation, Ti is first transformed from αTi to βTi, and then from a βTi to αTi+βTi two phase state because of the fast concurrent absorption of N or O into the selvage region. At time frame 41, βTi disappears entirely. This could be caused by the conversion to αTi due to the further absorption of N or O. In the meantime, TiN signal begins to emerge while the diffraction peaks of αTi can still be detected. The coexistence of αTi and TiN phases can be explained from the following three aspects. First, the composition and temperature of the laser irradiated area reach the transient nitridation curve (TNC) marked in Fig. 5(a), which has a temperature of 2350 °C and represent the coexistence of αTi, TiN and Liquid phase. Second, the energy distribution of the laser spot is not uniform (Gaussian distribution), which leads to the uneven temperature distribution of the X-ray illuminated area; the center of the laser irradiated area, having a Gaussian width, has a higher temperature because of the higher energy density, wherein the Liquid+TiN two-phase area occurs. Third, considering the complex temperature and composition distribution on the laser irradiated area, some regions on the sample surface may go directly into the αTi+TiN two-phase region. Between the time-frames 41 and 49, a new diffraction peak (e) appears, which belongs to TiNx, a sub-stoichiometric phase. However, this diffraction peak becomes almost invisible at around time-frame 49 as it reacts to completion of TiN. By time frame 49, TiN becomes

the dominate phase. What is noteworthy is that before time-frame 49, no diffraction pattern of titanium oxides can be detected strongly suggesting that only a liquid phase of this component exists. Stage 3 (y = 49-67) indicates the diffraction patterns of TiN. During this stage, the temperature of the laser irradiated area continuously increases because the diffraction patterns of TiN further shifts the smaller angles. In addition, the diffraction intensities of TiN are further enhanced over this stage, which means more TiN is generated. At time-frame 67, the maximum temperature is reached because the diffraction angle of TiN phase reaches the minimum value. It is also evident that with the laser profile in this study, the maximum temperature under laser irradiation does not exceed the melting point of TiN, which would be indicated by a strongly increased peak width and intensity drop off. Stage 4 (y = 67-71, 71-77 and 77-80) demonstrates the cooling process. Obviously, from time-frames 67 to 71, the diffraction peaks of TiN continuously shift to larger angles, which indicates the thermal contraction of TiN phase because of the reducing temperature. During this period, only discernable diffraction peaks corresponding to TiN are identified. At time-frame 71, two new diffraction peaks are identified with XRD angles around 55 and 56, indicated by “f”. According to Esaka et al. [54], these diffraction peaks belong to TiOxNy like structure, which is resulted from the early reaction stage between TiN and oxygen. TiO xNy-like structure is formed when O atoms diffuse into the TiN lattice and replace N. The intensity of TiN is gradually attenuated over the time-frames 71-77, while the TiOxNy diffraction peaks disappear and rutile TiO2 peaks emerge at time-frame 77. This agrees with Esaka’s discovery that TiOxNy is a metastable, transitional phase. Based on other researchers [55-58], although TiN is a thermodynamically stable phase, when exposes to air or O 2, it undergoes the following reaction: TiN + O2 → TiO2 + ½N2. The standard Gibbs energy difference (ΔG) of this process is -581.6 KJ/mol, meaning the process is thermodynamically favorable. Based on the observed diffraction patterns from time-frames 67 to 77, the phase changes presumably follow TiN → TiOxNy → TiO2, and the process is rate-limited by the slower diffusion of atomic oxygen into the TiN/Ti substrate. Based on the finding above, to obtain a thick TiO 2 layer, enough diffusion time for O atoms is necessary. However, what calls for special attention is that TiO2 may indeed exist in liquid form since its melting point, 1843 C, is much lower than that of TiN, 2930 C. Liquid state TiO2 (LTO) would not yield any diffraction pattern in the SXRD test. Therefore, the TiO2 phase appeared at time frame 77 may come from two possibilities, (i) the solidification from LTO, (ii) the reaction products between TiN and O 2. From time-frames 77 to

120, the diffraction intensity of TiN, both (111) and (200) peaks, continues to decrease slightly while the diffraction intensity of rutile TiO2 increases to some extent. The diffraction peaks continue to shift to the larger angle side, but with much smaller angle changing rate compared with the period from time-frame 67 to 77, indicating a much smaller cooling rate. Based on the in-situ synchrotron X-ray diffraction test, the phase transformation steps of Ti during a laser irradiation process under air are as follows, αTi→ βTi→ αTi+βTi→ αTi+TiN+TiN x (+LTO)→ liquid+TiN (+LTO)→ TiN+TiOxNy (+LTO)→ TiO2+TiN. 3.3 Microstructures and compositions of the sample after the in-situ SXRD test and the calculation of diffusion coefficients in a prolonged laser heating process Figure 6 shows the SEM images and EDS mapping results of the laser irradiated area after the in-situ SXRD test. Based on Fig. 6(a), the cross section can be divided into two regions, the solid layer, with a thickness around 240 μm, and the heat affected layer. The apparent separation of the two layers must be caused by the induced stress in the laser process of the cold pressed ~ 40m Ti powder. What is noteworthy from Fig. 6(e) is that the top half of the re-solidified layer contains much more pores than the bottom half, which is explained as follows. On one hand, the cold pressed Ti disk is originally loosely packed, large amounts of gaps between powders exist in the Ti disk. On the other hand, as explained in the previous section, oxygen might react with the already formed TiN and N2 is released. Besides, during the in-situ SXRD test, the cooling process lasts for around 1 second (from time frame 67 to 77). Considering a maximum temperature over 2000 ℃, the average cooling rate would be at least 2.0×103 K/s. Due to the quick cooling rate, the generated N2 gas could be trapped in the sample. The cross section of the solid layer can then be treated into two regions, the porous solid layer nearer the surface and the underlying solid layer at the bottom. The porous solid layer and the bottom solid layer have almost the same thickness, 120 μm each. Figures 6(f), 6(g), and 6(h) are the EDS mapping results of the area shown in Fig. 6(e). These spatially composition images reveal that O is mainly enriched in the porous solid layer, especially on the outermost surface layer, while N has higher concentration in the bottom of the porous solid layer. O is hardly discernable in the bottom solid layer. However, an N signal can still be identified in this region, but with a relatively weaker intensity than that in the porous solid layer. On the other hand, EDS shows that Ti exists everywhere in the cross section, though the signal intensity in the porous solid layer is relatively weaker than the bottom solid layer. The fact that O enriches distinctly

in the outermost surface layer may be due to two possible reasons, first the reaction between TiN and O 2 and second the enrichment of LTO at the solidification front. Due to the limited diffusion time, only limited quantity of O and N is absorbed by the sample, leading to limited diffusion depth of O and N. In this meantime, N diffuses faster than O into Ti substrate (as discussed in section 3.1.2). The above discussions explain why the bottom solid layer contains little O, reduced concentration of N, and fewer pores. Figure 6(b) is the cross-section image with larger magnification. The top surface layer has a thickness of approximately 2.5 μm, which is comparable to the Angéline Poulon-Quintin’s discovery [1]. Based on the result of SXRD, the outermost surface layer should be a mixture of TiO 2 and TiN, or TiO2 over TiN. Figures 6(j), 6(k), and 6(l) are the EDS mapping results of the area shown in Fig. 6(i). With larger magnification, it is noticeable that the outermost surface layer is mainly composed of titanium-oxides species, while the N signal is negligible in the layer. In Fig. 6(b), pores, with the sizes of several hundred nanometers, are also observed. Figures 6(c) and 6(d) are top surface SEM images of the laser irradiated area with different magnifications. Although speculative, both microsized and nano-sized pores exist, which could originate from both the low-packing-density powder bed and the release of the reaction product N2 (pores circled with white dashed ellipse in Figs. 6(b) and 6(d)). Chen [57] and Pérez del Pino [24] also observed the existence of nano-sized pores, which are likely due to the reaction between O 2 and TiN. The

and

values can be estimated based on the observed after SXRD experiment results.

According to Fig. 6(j), the thickness of the O-enriched outermost surface layer is around 2.5 μm. However, O signal is still clear at the bottom of the porous solid layer, which indicates a diffusion depth about 120 μm. Based on the discussion above in the in-situ SXRD test section, the outermost O-enriched layer originated from the oxidation of TiN, in other words, the diffusion depth of O in TiN layer is around 2.5 μm. According to Fig. 6(g), N has strongerintensity signal at the bottom of the porous solid layer, that is, its signal can still be detected in the bottom solid layer. Therefore, the diffusion thickness of N is estimated to be 240 μm. According to the formula ,

and

(diffusion coefficient of O in TiN) values can be calculated. The relationship between the

diffusion coefficient (D) and absolute temperature ( ) is described as activation energy and

, the

, where

is the

represents gas constant. Evidently, the higher temperature yields larger diffusion coefficient.

Besides, considering the fast heating and cooling rate of the laser processing condition, high-temperature diffusion

plays the dominant role in the O and N intake. Based on the SXRD test results in Fig. 4, the high temperature period ranges from time frame 41 to 77, 3.6 seconds in total. The reason to choose this time frame range is that, at time frame 41, TiN starts to generate, while at time frame 77, TiO2 appears. Taking into consideration that Ti has a stronger affinity for oxygen than nitrogen [59], the temperature at time frame 41 must be higher than the melting point of titanium oxide (or TiO2). Therefore, the high-temperature range is estimated to be over the melting point of TiO2 but below the melting point of TiN, from 1843 ℃ to 2930 ℃. Based on this, the calculated results are listed in is around 4.0×10-5 cm2/s, while

Table 3.

is approximately 1.0×10-5 cm2/s.

4.34×10-9 cm2/s, which is about four orders of magnitude less than that of

and

is found to be

.

3.4 Discussion of Diffusion coefficients of O and N in Ti To date, several studies have been published regarding the diffusion coefficients of O and N in Ti, titanium oxides, and titanium nitrides, including diffusion coefficients of O in αTi [35-37], TiO2 [41],

, N in αTi [42, 43],

, N in βTi [44],

, N in TiN [45],

, O in βTi [38-40],

, O in

, and N in Ti2N [44],

.

However, these studies are limited to temperatures below 1700 ℃. The data reported in Tables 2 and 3 provide diffusion coefficients far over the melting point of Ti, which are needed for the design of laser-based surface modifications or additive manufacturing of Ti alloys. Evidently,

is much higher than

over the high

temperature range, which, as discussed above, is ascribed to the high solubility and high diffusion coefficient of N in Ti at high temperatures. As shown in Fig. 7, the diffusion coefficients reported in literature generally increase monotonically with the increase of temperature, and the linear relation between the logarithmic function of diffusion coefficients and the inverse absolute temperature indicates the agreement with the Arrhenius equation the experimentally determined

(B range in Fig. 7) and

. Notably,

(A range in Fig. 7) of this paper are located

on the extrapolated lines of the previous literature data, indicating a strong consistency of the results acquired in this study with the prior data. The calculated

in Table 2 of 8.00×10-7 cm2/s for the Ti samples is much smaller than that obtained

under the SXRD condition, specifically 1.0×10-5 cm2/s. This is likely caused by the different levels of localized laser energy density. For the tests summarized in Table 2, the laser energy density is much higher than that of the SXRD

process due to the necessity to defocus the laser over the SXRD probe zone; this fact is supported by the much enhanced rate of heating to melting/evaporation. Because of this, the consequent evaporation of Ti vapor may form a localized oxygen deprived zone and thus inhibit the available oxygen diffusion into the Ti substrate. Although the on Fig. 7 shows from 8.00×10-7 to 1.0×10-5 cm2/s, the true value is more likely close to 1.0×10 -5

range of

cm2/s. As discussed above,

depends on laser energy density (temperature). A larger laser energy density

would yield a higher temperature. This is the reason why the calculated

in Table 2 is larger than that under

the SXRD condition. And because of the huge laser energy density difference, the calculated

varies

significantly, from 4.0×10-5 to 4.24×10-2 cm2/s. 4. Conclusions In this paper, the reactions between commercially pure Ti and atmospheric air are investigated under typical laser surface modification and laser additive manufacturing conditions. The composition distributions and microstructures of the cross sections of the pure Ti samples, after laser irradiation with different laser parameters, are examined. A synchrotron X-ray diffraction test is performed to investigate the dynamic reaction processes between pure Ti and atmospheric air under laser heating, which helps to better elucidate the diffusion processes of oxygen and nitrogen into Ti throughout a large temperature range. Based on material characterizations, hightemperature diffusion coefficients of nitrogen and oxygen into Ti under air environment are estimated. And the following conclusions can be reached. (1) The synchrotron X-ray diffraction test reveals the in-situ dynamic reaction steps between pure Ti and air. The phase transformation steps during the laser melting and cooling process are found to be αTi→ βTi→ αTi+βTi→ αTi+TiN+TiNx (+LTO)→ liquid+TiN (+LTO)→ TiN+TiOxNy (+LTO)→ TiO2+TiN. Titanium nitrides appear first, while TiO2 is only observed during the cooling process, resulting from the solidification of the pre-existing liquid titanium oxides or the reaction between TiN and O2. (2) Commercially pure Ti samples are laser irradiated under an atmospheric environment with typical laser surface modification processing conditions. After laser irradiation, oxygen is found to enrich only in the outermost surface layer, while nitrogen is mainly existed beneath the oxygen rich layer. (3) For the first time, the high-temperature diffusion coefficients of oxygen and nitrogen into laser irradiated Ti samples in air are estimated. Over the temperature range 1843 ℃ - 2930 ℃, the diffusion coefficients of nitrogen and

oxygen into Ti are estimated to be 4.00×10-5 - 4.24×10-2 cm2/s and 8.00×10-7-1.00×10-5 cm2/s, respectively, while the diffusion coefficient of oxygen in TiN is 4.34×10 -9 cm2/s. Through controlling laser processing parameters, TiO2/TiN layers with controllable thickness can be obtained. Acknowledgements This work is supported by the NSF-Consortium for innovation in manufacturing and materials (CIMM) program (grant number # OIA-1541079). We gratefully acknowledge the expertise and experimental help of H. Bellamy and, in addition, the entire CAMD staff.

References [1] A. Poulon-Quintin, I. Watanabe, E. Watanabe, C. Bertrand, Microstructure and mechanical properties of surface treated cast titanium with Nd:YAG laser, Dent. Mater. 28(9) (2012) 945-951. [2] E. Uhlmann, R. Kersting, T.B. Klein, M.F. Cruz, A.V. Borille, Additive Manufacturing of Titanium Alloy for Aircraft Components, Procedia CIRP 35 (2015) 55-60. [3] N. Taniguchi, S. Fujibayashi, M. Takemoto, K. Sasaki, B. Otsuki, T. Nakamura, T. Matsushita, T. Kokubo, S. Matsuda, Effect of pore size on bone ingrowth into porous titanium implants fabricated by additive manufacturing: An in vivo experiment, Mater. Sci. Eng. C 59 (2016) 690-701. [4] Y.-L. Hao, S.-J. Li, R. Yang, Biomedical titanium alloys and their additive manufacturing, Rare Metals 35(9) (2016) 661-671. [5] K. Katahira, Y. Tanida, S. Takesue, J. Komotori, Rapid surface nitriding of titanium alloy by a nanosecond fiber laser under atmospheric conditions, CIRP Ann. 67(1) (2018) 563-566. [6] C.-W. Chan, S. Lee, G. Smith, G. Sarri, C.-H. Ng, A. Sharba, H.-C. Man, Enhancement of wear and corrosion resistance of beta titanium alloy by laser gas alloying with nitrogen, Appl. Surf. Sci. 367 (2016) 80-90. [7] S.N. Dahotre, H.D. Vora, R.S. Rajamure, L. Huang, R. Banerjee, W. He, N.B. Dahotre, Laser Induced Nitrogen Enhanced Titanium Surfaces for Improved Osseo-Integration, Ann. Biomed. Sci. 42(1) (2014) 50-61. [8] S. Sathish, M. Geetha, N.D. Pandey, C. Richard, R. Asokamani, Studies on the corrosion and wear behavior of the laser nitrided biomedical titanium and its alloys, Mater. Sci. Eng. C 30(3) (2010) 376-382. [9] N. Ohtsu, K. Kodama, K. Kitagawa, K. Wagatsuma, Comparison of surface films formed on titanium by pulsed Nd:YAG laser irradiation at different powers and wavelengths in nitrogen atmosphere, Appl. Surf. Sci. 256(14) (2010) 4522-4526. [10] V.K. Balla, J. Soderlind, S. Bose, A. Bandyopadhyay, Microstructure, mechanical and wear properties of laser surface melted Ti6Al4V alloy, J. Mech. Behav. Biomed. Mater. 32 (2014) 335-344. [11] J. Ciganovic, J. Stasic, B. Gakovic, M. Momcilovic, D. Milovanovic, M. Bokorov, M. Trtica, Surface modification of the titanium implant using TEA CO2 laser pulses in controllable gas atmospheres – Comparative study, Appl. Surf. Sci. 258(7) (2012) 2741-2748.

[12] A. Hindy, F. Farahmand, F.s. Tabatabaei, In vitro biological outcome of laser application for modification or processing of titanium dental implants, Lasers Med. Sci. 32(5) (2017) 1197-1206. [13] R.Y.O. Honda, M. Mizutani, H. Ohmori, J.U.N. Komotori, BIOCOMPATIBILITY EVALUATION OF NANOSECOND LASER TREATED TITANIUM SURFACES, Int. J. Mod. Phys.: Conf. Ser. 06 (2012) 682-687. [14] M.D. Paz, J.I. Álava, L. Goikoetxea, S. Chiussi, I. Díaz-Güemes, J. Usón, F. Sánchez, B. León, Biological response of laser macrostructured and oxidized titanium alloy: An in vitro and in vivo study, J. Appl. Biomater. Biom. 9(3) (2011) 214-222. [15] A.J. Antończak, Ł. Skowroński, M. Trzcinski, V.V. Kinzhybalo, Ł.K. Łazarek, K.M. Abramski, Laser-induced oxidation of titanium substrate: Analysis of the physicochemical structure of the surface and sub-surface layers, Appl. Surf. Sci. 325 (2015) 217-226. [16] E. Akman, E. Cerkezoglu, Compositional and micro-scratch analyses of laser induced colored surface of titanium, Opt. Lasers Eng. 84 (2016) 37-43. [17] T. Jwad, S. Deng, H. Butt, S. Dimov, Laser induced single spot oxidation of titanium, Appl. Surf. Sci. 387 (2016) 617-624. [18] Ł. Skowroński, A.J. Antończak, M. Trzcinski, Ł. Łazarek, T. Hiller, A. Bukaluk, A.A. Wronkowska, Optical properties of laser induced oxynitride films on titanium, Appl. Surf. Sci. 304 (2014) 107-114. [19] F. Torrent, L. Lavisse, P. Berger, J.M. Jouvard, H. Andrzejewski, G. Pillon, S. Bourgeois, M.C. Marco de Lucas, Wavelength influence on nitrogen insertion into titanium by nanosecond pulsed laser irradiation in air, Appl. Surf. Sci. 278 (2013) 245-249. [20] R. Ocaña, A. Calatayud, Laser-induced Coloring on Small Titanium Surfaces, Physics Procedia 83 (2016) 225232. [21] C.-C. Yang, Y.-C. Lin, C.-C. Yang, Y.-H. Lin, K.-C. Huang, K.-m. Lin, W.-T. Hsiao, Laser-induced coloring of titanium alloy using ultraviolet nanosecond pulses scanning technology, J. Alloys Compd. 715 (2017) 349-361. [22] M. Marticorena, G. Corti, D. Olmedo, M.B. Guglielmotti, S. Duhalde, Laser surface modification of Ti implants to improve osseointegration, J. Phys.: Conf. Ser. 59(1) (2007) 662-665. [23] F.J.C. Braga, R.F.C. Marques, E.d.A. Filho, A.C. Guastaldi, Surface modification of Ti dental implants by Nd:YVO4 laser irradiation, Appl. Surf. Sci. 253(23) (2007) 9203-9208. [24] A. Pérez del Pino, P. Serra, J.L. Morenza, Laser surface processing of titanium in air: Influence of scan traces overlapping, J. Laser Appl. 15(2) (2003) 120-123. [25] E. Ageev, Y.M. Andreeva, Y.Y. Karlagina, Y.R. Kolobov, S. Manokhin, G. Odintsova, A. Slobodov, V. Veiko, Composition analysis of oxide films formed on titanium surface under pulsed laser action by method of chemical thermodynamics, Laser Phys. 27(4) (2017) 046001. [26] L. Lavisse, M.C. Sahour, J.M. Jouvard, G. Pillon, M.C. Marco de Lucas, S. Bourgeois, D. Grevey, Growth of titanium oxynitride layers by short pulsed Nd:YAG laser treatment of Ti plates: Influence of the cumulated laser fluence, Appl. Surf. Sci. 255(10) (2009) 5515-5518. [27] X. Zhao, S. Li, M. Zhang, Y. Liu, T.B. Sercombe, S. Wang, Y. Hao, R. Yang, L.E. Murr, Comparison of the microstructures and mechanical properties of Ti–6Al–4V fabricated by selective laser melting and electron beam melting, Mater. Des. 95 (2016) 21-31. [28] X.P. Li, M. Roberts, Y.J. Liu, C.W. Kang, H. Huang, T.B. Sercombe, Effect of substrate temperature on the interface bond between support and substrate during selective laser melting of Al–Ni–Y–Co–La metallic glass, Mater. Des. (1980-2015) 65 (2015) 1-6.

[29] L. Nánai, R. Vajtai, T. F. George, Laser-induced oxidation of metals: state of the art, Thin Solid Films 298(1) (1997) 160-164. [30] A.I.P. Nwobu, R.D. Rawlings, D.R.F. West, Nitride formation in titanium based substrates during laser surface melting in nitrogen–argon atmospheres, Acta Mater. 47(2) (1999) 631-643. [31] S.N. Dahotre, H.D. Vora, K. Pavani, R. Banerjee, An integrated experimental and computational approach to laser surface nitriding of Ti–6Al–4V, Appl. Surf. Sci. 271 (2013) 141-148. [32] H.C. Man, M. Bai, F.T. Cheng, Laser diffusion nitriding of Ti–6Al–4V for improving hardness and wear resistance, Appl. Surf. Sci. 258(1) (2011) 436-441. [33] D. Höche, S. Müller, G. Rapin, M. Shinn, E. Remdt, M. Gubisch, P. Schaaf, Marangoni Convection during Free Electron Laser Nitriding of Titanium, Metall. Mater. Trans. B 40(4) (2009) 497-507. [34] A. Pérez del Pino, P. Serra, J.L. Morenza, Coloring of titanium by pulsed laser processing in air, Thin Solid Films 415(1) (2002) 201-205. [35] P. Kofstad, High-temperature oxidation of titanium, J. Less Common. Met. 12(6) (1967) 449-464. [36] D. David, G. Beranger, E. A. Garcia, Study of the diffusion of oxygen in α-titanium oxidized in the temperature range 460°-700°C, J. Electrochem. Soc. 130 (1983) 1423-1426. [37] D. David, E.A. Garcia, X. Lucas, G. Beranger, Etude de la diffusion de l'oxygene dans le titane α oxyde entre 700 °C et 950 °C, J. Less Common Met. 65 (1979) 51-69. [38] P. Roe, Diffusion of oxygen in alpha and beta titanium, Met. Prog. 76 (1959) 237-239. [39] F. Claisse, H.P. Koenig, Thermal and forced diffusion of oxygen in β-titanium, Acta Mater. 4(6) (1956) 650654. [40] O. Carlson, F. Schmidt, R. Lichtenberg, Investigation of reported anomalies in the electrotransport of interstitial solutes in titanium and iron, Metall. Trans. A: Phys. Metall. Mater. Sci. 6(4) (1975) 725. [41] R.N. Shenoy, J. Unnam, R.K. Clark, Oxidation and embrittlement of Ti-6Al-2Sn-4Zr-2Mo alloy, Oxid. Met. 26(1) (1986) 105-124. [42] R.P. Cardoso, G. Arnoult, T. Belmonte, G. Henrion, S. Weber, Titanium Nitriding by Microwave Atmospheric Pressure Plasma: Towards Single Crystal Synthesis, Plasma Process. Polym. 6(S1) (2009) S302-S305. [43] B.S. Yilbas, S.Z. Shuja, M.S.J. Hashmi, A numerical solution for laser heating of titanium and nitrogen diffusion in solid, J. Mater. Process. Tech. 136(1) (2003) 12-23. [44] F.W. Wood, O.G. Paasche, Dubious details of nitrogen diffusion in nitrided titanium, Thin Solid Films 40 (1977) 131-137. [45] G.G. Maksimovich, V.N. Fedirko, I.N. Pogrelyuk, Nitriding of titanium alloys at the atmospheric pressure of nitrogen, Sov. Mater. Sci. 23(6) (1989) 566-568. [46] H.D. Vora, R. Shanker Rajamure, S.N. Dahotre, Y.-H. Ho, R. Banerjee, N.B. Dahotre, Integrated experimental and theoretical approach for corrosion and wear evaluation of laser surface nitrided, Ti-6Al-4V biomaterial in physiological solution, J. Mech. Behav. Biomed. Mater. 37 (2014) 153-164. [47] S.N. Dahotre, H.D. Vora, R.S. Rajamure, L. Huang, R. Banerjee, W. He, N.B. Dahotre, Laser Induced Nitrogen Enhanced Titanium Surfaces for Improved Osseo-Integration, Ann. Biomed. Eng. 2014, pp. 50-61. [48] A. Temmler, E. Willenborg, K. Wissenbach, Design Surfaces by Laser Remelting, Physics Procedia 12 (2011) 419-430.

[49] K.J.A. Brookes, Introduction to surface engineering Peter A. Dearnley, Met. Powder Rep. 73(3) (2018) 126-127. [50] T.P. Thorpe, A.A. Morrish, S.B. Qadri, Effect of grain size on the oxidation kinetics of sputtered titanium nitride films, J. Vac. Sci. Technol., A 6(3) (1988) 1727-1729. [51] A.L. Thomann, A. Basillais, M. Wegscheider, C. Boulmer-Leborgne, A. Pereira, P. Delaporte, M. Sentis, T. Sauvage, Chemical and structural modifications of laser treated iron surfaces: investigation of laser processing parameters, Appl. Surf. Sci. 230(1) (2004) 350-363. [52] E. Carpene, P. Schaaf, M. Han, K.P. Lieb, M. Shinn, Reactive surface processing by irradiation with excimer laser, Nd:YAG laser, free electron laser and Ti:sapphire laser in nitrogen atmosphere, Appl. Surf. Sci. 186(1) (2002) 195-199. [53] X. Wang, S. Zhang, X. Cheng, E. Zhu, W. Hang, B. Huang, Ion kinetic energy distributions in laser-induced plasma, Spectrochim. Acta, Part B 99 (2014) 101-114. [54] F. Esaka, K. Furuya, H. Shimada, M. Imamura, N. Matsubayashi, H. Sato, A. Nishijima, A. Kawana, H. Ichimura, T. Kikuchi, Comparison of surface oxidation of titanium nitride and chromium nitride films studied by xray absorption and photoelectron spectroscopy, J. Vac. Sci. Technol., A 15(5) (1997) 2521-2528. [55] M. Wittmer, J. Noser, H. Melchior, Oxidation kinetics of TiN thin films, J. Appl. Phys. 52(11) (1981) 66596664. [56] N.C. Saha, H.G. Tompkins, Titanium nitride oxidation chemistry: An x-ray photoelectron spectroscopy study, J. Appl. Phys. 72(7) (1992) 3072-3079. [57] H.-Y. Chen, F.-H. Lu, Oxidation behavior of titanium nitride films, J. Vac. Sci. Technol. A 23(4) (2005) 10061009. [58] H.G. Tompkins, Oxidation of titanium nitride in room air and in dry O2, J. Appl. Phys. 70(7) (1991) 3876-3880. [59] H. Wang, R. Zhang, Z. Yuan, X. Shu, E. Liu, Z. Han, A comparative study of the corrosion performance of titanium (Ti), titanium nitride (TiN), titanium dioxide (TiO2) and nitrogen-doped titanium oxides (N–TiO2), as coatings for biomedical applications, Ceram. Int. 41(9, Part B) (2015) 11844-11851.

Figure and table captions

Fig. 1. Widths of single tracks with different laser scanning speeds under an atmospheric environment (100 W) (a) 25 mm/s (b) 100 mm/s (c) 400 mm/s (d) 1600 mm/s. Arrows indicate the approximate edges of the laser single track, on which ripple shaped structures are observed.

Fig. 2. EDS mapping results of the cross section of sample Ti-A100. (a) indicates the cross-section area where the EDS test was conducted, (b) and (c) show the distribution of element O and N on the cross section.

Fig. 3. (a), (b), (c) and (d) represent the SEM images of the cross sections for samples Ti-A25, Ti-A100, Ti-A400 and Ti-A1600, respectively. The insert image in (a) shows the cross section of Ti-A25 with small magnification.

Fig. 4. In-situ X-ray diffraction angles and intensities as a function of time frames, the left image shows the programmed laser profile. 1, 2, 3, 4 demonstrate the diffraction peaks of αTi(100), αTi(002), αTi(101) and αTi(102), respectively. a, b are the peaks of βTi. c, d are the peaks of αTi. e represents the peak of TiN x. f shows the peaks of the phase TiOxNy. ① and ② are diffraction peaks of TiN(111) and TiN(200), respectively. Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅴ and Ⅵ indicate the diffraction peaks of rutile TiO2(101), TiO2(200), TiO2(111), TiO2(210), TiO2(211) and TiO2(220), respectively.

Fig. 5. Phase diagrams calculated with the software Thermo-Calc (a) Ti-N phase diagram (b) Ti-O phase diagram.

Fig. 6. Microstructures and EDS mapping results of the laser irradiated pure Ti surface under air atmosphere after in-situ SXRD testing. (a), (b) are the cross-section images of the sample with different magnification. (c), (d) show the top surface images of the laser irradiated area. (f), (g) and (h) display the EDS mapping results, representing O, N and Ti respectively, of the sample cross section of (e). (j), (k) and (l) again exhibit the EDS mapping results of the sample cross section with larger magnification of (i). Fig. 7. Diffusion coefficients of O in Ti, βTi and TiO2, and the diffusion coefficients of N in Ti, βTi, TiN and Ti2N (literature), together with the experimentally estimated diffusion coefficients of O and N into laser irradiated Ti in air (this study).

Table 1 Laser parameters Parameter

Value

Laser power (W)

100

100

100

100

Spot size (µm)

58

58

58

58

Scanning speed (mm/s)

25

100

400

1600

Width of single track (μm)

362

225

114

75

Hatch spacing (µm)

181

112.5

57

37.5

Table 2 The calculated diffusion coefficients of O and N into laser irradiated Ti Element

Sample

Diffusion depth x (μm)

Ti-A25 Ti-A100

Spot size d (μm)

Scanning speed v (mm/s)

Interaction time t (s)

D (cm2/s)

0.79

25

2.32×10-3

6.73×10-7

0.50

100

5.80×10-4

10.78×10-7

-4

O

58 Ti-A400

0.21

400

1.45×10

TiA1600

0.10

1600

3.63×10-5

6.89×10-7

Ti-A25

198.4

25

2.32×10-3

4.24×10--2

Ti-A100

9.00

100

5.80×10-4

3.49×10-4

Ti-A400

2.87

400

1.45×10-4

1.42×10-4

TiA1600

1.06

1600

3.63×10-5

7.73×10-5

N

58

7.60×10

-7

Average D (cm2/s)

Standard Deviation (cm2/s)

8.00×10-7

1.89×10-7

/

/

Table 3 The estimated diffusion coefficients of O and N in a slow process based on synchrotron testing Diffusion type

Diffusion depth (μm)

Temperature range (oC)

t (s)

D (cm2/s)

O in TiN

2.5

1843-2930

3.6

4.34×10-9

O in Ti(O,N)

120

1843-2930

3.6

1.0×10-5

N in Ti(O,N)

240

1843-2930

3.6

4.0×10-5

Highlight 

In-situ synchrotron X-ray diffraction is used to study titanium-air reactions.



The high-temperature reaction steps between titanium and air are revealed.



High-temperature diffusions of oxygen & nitrogen in Ti are investigated.

Declaration of interest: None