Study of oxygen content in tanium alloys a�er exposure at elevated temperature

Isothermal oxida on tes ng of near α tanium alloys VT18U, VT20, Ti6Al7Nb and Ti6242S was performed in air at 560 °С for 1000 hours. Parameters of diffusion layer on the alloy surfaces were studied by microhardness indenta ons, op cal microscopy, X-ray diffrac on analysis and nuclear microanalysis. It was established that concentra on of oxygen in diffusion layer of tested alloys a�er oxida on differs significantly. An approach was demonstrated and validated by nuclear microanalysis data that allows compara ve evalua on of the total concentra on of inters al impuri es in the diffusion layer by the X-ray diffrac on method.


Introduc on
A breakthrough development of high temperature tanium alloys in 1960-1970s inspired advancement of aircra�-and enginebuilding. Well-known alloys such as IMI685, B120, Ti6242, and Russian alloys VT18 and VT20 have been developed during this me period [1][2][3][4][5][6]. The new high temperature tanium alloys allowed crea on of new aerospace technologies, e.g. fully 93 percent of the legendary Blackbird's structural weight consisted of tanium alloys [7]. This aircra� was designed and put into opera on in 1960s.
The maximum opera ng temperature of near α tanium alloys is limited to 600 °C. Higher temperatures dras cally reduce high temperature strength, creep resistance [1,3,5]. Moreover, long-term exposure of tanium alloys to air at temperatures over 400°C leads to oxidezes and forma on diffusion layer due to of high sulubility of oxygen in the matrix [8][9][10]. The resultant diffusion layer is characterized by high strength, low duc lity and toughness, it significantly reduces fa gue strength. It has been shown that high temperature alloys IMI834, Ti6242S, Beta21S, and Ti1100 exhibited deteriorated duc lity and fa gue strength a�er opera on at elevated temperatures, when tested in the oxidized state [10][11][12][13][14][15].
At the moment, there are s ll some open issues concerning rela onship between the main parameters influencing the metal (temperature, me, atmosphere, pressure) with its structural characteris cs (oxygen concentra on in metal, gas-rich layer thickness, material hardness, thickness, density and oxide adhesion, weight gain, diffusion coefficients, etc.) and mechanical proper es in the oxidized state (fa gue strength, duc lity, tensile strength). Therefore, further study of air oxida on behavior of tanium alloys is a vital scien fic and engineering task. The objec ve of this work is to demonstrate new data that facilitates be�er understanding of the nature of deteriora on of duc lity and other mechanical proper es a�er exposure of tanium alloys at elevated temperatures.

Material and experiments
The studied materials were sheets made of tanium alloys VT18U, VT20, Ti6Al7Nb and Ti6242S by VSMPO-AVISMA Corpora on. Sheets were cut to 20x20 mm samples using electrical discharge machining, therea�er, all samples were mechanically ground (P240), degreased with petrol and cleaned with ethyl alcohol.
Oxida on of samples was performed in air at 560 °C for 1000 hours. Oxida on was done in an electric laboratory furnace with a fan for air mixing and temperature maintenance within ± 5 ° C.
X-ray diffrac on analysis (XRD) of samples was performed on a Bruker D8 Advance diffractometer with a LynxEye detector. XRD pa�erns were taken in copper radia on in the following condi ons: tube voltage of 40 kV, tube current of 40 mA, step size 0.02°, exposure 0.5 sec per point. The la ce parameters were refined using TOPAS 3 so�ware by a whole pa�ern modeling in the angle range of 34 to 43 º 2θ [16]. The qualita ve phase analysis of the annealed samples was performed using crystallographic database PDF2007 and so�ware package EVA13.0.0.3. The diffractometer was aligned using the corundum (NIST SRM1976b).
Metallographic analysis and microhardness measurements were performed on samples that were cross-sec oned from the ones for XRD and NMA. Vickers microhardness measurements were made using a diamond square pyramid indenter and 10 gr load by Shtruers Duramin tester. The observa on of the surface oxide was performed by scanning electron microscope (SEM) Quanta 3D FEG. To quan fy the thickness of diffusion layer, the microsec ons were etched in 5 % aqueous solu on of hydrofluoric acid. The thickness of gas-rich layer was taken as the average thickness of near-surface structure with a lighter contrast compared to the base metal. The obtained thickness values were averaged over 60-70 measurements taken in different places on the sample surface. Figure 1 shows a diffusion layer on the oxidized alloy samples a�er oxida on. The average thickness of oxygen penetra on measured on microsec ons a�er examina on by op cal microscopy (OM) is given in Table 1. The minimum thickness of diffusion layer is observed in Ti6Al7Nb, the maximum thickness -in Ti6242S.   Figure 2 shows the oxide formed on the surface of Ti6242S as a result of oxida on, this oxide has a typical needle-like morphology [19]. The results of qualita ve XRD analysis demonstrate that the oxide scale consists of TiO2 ru le type, Fig. 3. The density of pure ru le is less than that of tanium and is 4.2 g/cm 2 [20]. Alloyed tanium alloys may have varying phase composi on and density of the oxide scale due to the presence of alloying elements in the oxide scale [21]. Table 1 presents the data on the thickness of the oxide scale on sample surfaces. The thickness of the oxide scale on the studied samples is less than 500 nm, therefore the X-ray diffrac on allows to obtain informa on on the structure of gas-rich layer and oxide. The calculated depth of copper radia on penetra on contribu ng 80% to the diffracted radia on is ≈ 3 µm at 43º 2θ (so� -AbsorbDX V1.1.4).

Results
Oxygen, nitrogen and carbon are inters al atoms located in the inters ces of the crystal la ce of tanium [22]. During the oxida on of tanium alloys in air, oxygen is the main element that forms a diffusion layer, but we failed to find accurate informa on on the distribu on of gases in the diffusion layers of various tanium alloys. Further to the oxida on theory, oxygen distribu on in diffusion layer a�er annealing has a certain gradient; this gradient depends on the temperature and me of oxida on, and also on the oxide characteris cs (presence of defects, phase composi on, thickness, etc.), oxygen solubility in the alloy, oxygen concentra on near oxide/metal interface, diffusion rate and ac va on energy [8][9][10][22][23][24][25][26]. Increasing oxygen concentra on leads to the increase of the la ce parameters and thus to the expansion of the volume of tanium unit cell [22,27]. Studies [28][29][30] demonstrate approaches that allow the assessment of oxygen concentra on in tanium by the parameters of its crystal la ce. Capability of the XRD analysis of diffusion layer is limited to the depth of X-ray penetra on and is substan ally complicated by the presence of oxygen concentra on gradient, which forms a diffrac on pa�ern resul ng from the superposi on of the diffracted radia on. Nevertheless, use of the whole pa�ern modeling of diffrac on pa�erns allows compara ve evalua on of oxygen concentra on in diffusion layer. Figure 3 shows diffrac on pa�erns taken from the surface of oxidized alloys, as can be seen, loca on of lines significantly differs for studied alloys. As men oned above, increasing oxygen concentra on leads to the expansion of the volume of the unit cell, which results in a shi� of diffrac on lines on the diffrac on pa�ern towards small angles, propor onal to the oxygen concentra on. Slight asymmetry of diffrac on lines is associated with the gradient of oxygen concentra on.
The profile of the diffrac on pa�ern (Fig. 3) fi�ed using α phases of tanium with hexagonal symmetry (P63/mmc), β phase of tanium with cubic symmetry (Im-3m) and tetragonal phase of tanium oxide TiO2 (P42/mnm) was used to calculate the la ce parameters of the alloys before and a�er oxida on. Data on the rela ve varia on of the volume of the unit cell of α phase of tanium ΔV = (V α -V α0 ) x 100 / V α0 , where V α is the cell volume near the oxide/metal interface a�er oxida on and V α0 is the ini al value for Ti6Al7Nb, VT20, Ti6242S and VT18U alloys: 0.97, 2.07, 3.93 and 4.14 % respec vely. Minimum increment of the volume of the unit cell of tanium α phase a�er oxida on was obtained for Ti6Al7Nb, while maximum increment was obtained for VT18U. The obtained XRD data indirectly characterize the concentra on of oxygen in the diffusion layer of the alloys and, as can be seen, is confirmed by the results of nuclear microanalysis, Figure 4 (a). According to the obtained data, the oxygen concentra on at 0.5 μm from the surface for Ti6Al7Nb, VT20, Ti6242S and VT18U is 9.32, 13.18, 13.95, 14.42 wt % respec vely and decreases towards the metal core approximately with one gradient. Thus, there is a good correla on of XRD and NMA data tes fying to the fact that the concentra on of oxygen in diffusion layer of studied alloys differs significantly.

Fig. 4. Distribu on of oxygen (a) and nitrogen (b) concentra ons near the oxide/metal interface in tanium alloys a�er oxida on in air at 560°С
for 1000 hours Figure 4 (b) shows the dependences of nitrogen concentra on in diffusion layer; samples all alloys exhibit a very thin layer with the increased concentra on of nitrogen near the oxide/metal interface. Forma on of this layer was also observed in other studies [31]. Nitrogen concentra on decreases to values below 0.1 wt. % in all alloys at of 2.3 microns from the oxide surface. Thus, it is clear that oxygen is the main contributor to the forma on of diffusion layer during the oxida on of alloys in air.
Measurement of hardness of diffusion layer is an effec ve tool for analyzing oxygen gradient in diffusion layer [13,27,30]. It is obvious that the observed difference in oxygen concentra on near the oxide/metal interface of the studied alloys determines the oxygen gradient in the en re diffusion layer, Fig. 5 (a). Figure 5 (b) shows indenta ons of various sizes on the studied alloys Ti6242S and Ti6Al7Nb.

Discussion
We believe it can be a�ributed to the following differences in behaviors of tanium alloys. Titanium alloy surface a�er oxida on exhibits an oxide layer which thickness was found to be different in the studied alloys. The oxide surface layer acts as a par al barrier limi ng the interac on of metal with atmospheric gases at elevated temperatures. Various parameters of oxides such as phase composi on, thickness and density influence the rate of diffusion processes. Between VT20 and Ti6242S alloys with a diffusion layer of the same thickness, VT20 alloy having significantly thicker oxide demonstrates lower rate of diffusion through the oxide, which probably determines the final concentra on of oxygen dissolved in metal which is lower than that of Ti6242S alloy. Ti6Al7Nb alloy with the thinnest oxide and diffusion layer exhibits the minimum oxygen concentra on a�er oxida on. The oxide diffrac on lines of Ti6Al7Nb alloy are much more intense than diffrac on lines of VT18U and Ti6242S alloys, which have just a slightly thicker oxide, which suggests that the density of the Ti6Al7Nb alloy oxide is higher and diffusion rate through the oxide is probably lower than in other alloys. One more parameter of the alloys which could obviously influence the oxygen concentra on in the studied alloys is oxygen solubility, which, as known, depends on the alloy chemical composi on [26].
In this study, we have shown a good correla on of data obtained by XRD and NMA methods. Ul mately, the difference in oxygen concentra on in gas-rich layer determines the deforma on behavior of the alloys [32,33], increasing oxygen concentra on in gas-rich layer leads to reduc on of duc lity of the oxidized alloys un l completely bri�le fracture in the elas c por on of load diagram. For example, VT18U and Ti6Al7Nb alloys in the oxidized condi on studied herein demonstrate tensile elonga on of 4.6 % and 13.2 % [32] respec vely due to the significant difference in oxygen concentra on in gas-rich layer (difference of concentra on near the oxide/metal interface 5 wt. %).

Conclusion
Titanium alloys VT18U, VT20, Ti6Al7Nb and Ti6242S were studied a�er oxida on in air at 560 °С for 1000 hours. Parameters of diffusion layer on the alloy surfaces were studied by microhardness measurements, op cal microscopy, X-ray diffrac on analysis and nuclear microanalysis. It was determined that concentra on of oxygen in diffusion layer of tested alloys a�er oxida on differs significantly.
The study of the effect of oxida on on the la ce parameters of metal and concentra on of oxygen in diffusion layer can facilitate understanding of their effect on the α phase plas city and mechanical proper es of tanium alloys a�er exposure at elevated temperature.