Effect of hydrogen on the fretting wear mechanism of a high Nb-TiAl alloy

In this paper, the effect of hydrogen placement on the microdynamic wear mechanism of high niobium-titanium-aluminum alloys is investigated. Corresponding decreases and increases in loading force and displacement amplitude cause the microslip behavior of the alloy to change from partial slip to mixed slip. Slip type in mixed fire complete slip when you, the friction coefficient fluctuates. The average friction coefficient of hydrogen-placed alloys is small compared with that of non-hydrogen-placed alloys, the maximum wear marks are not obvious, and the oxidative wear is weak. The main wear mechanisms of non-hydrogenated alloys are adhesive wear, abrasive wear and oxidative wear. The main wear mechanisms of hydrogen-placed alloys are adhesive wear and abrasive wear.


Introduction
Titanium-aluminum alloy has the advantages of corrosion resistance and light weight, and has great application prospects in the processing of lightweight and high-temperature parts for aircraft.Moreover， TiAl alloys are widely used in the field of human mechanical engineering [1][2][3][4] due to their excellent mechanical properties of metals.The aeronautical engine is known as the heart of the aircraft, providing the necessary power for the flight of the plane.Currently, turbofan engines are one of the significant aero engines.The main function of compressors is to compress the airflow in the inn duct through the high-speed rotating compressor blades.Conditions are created for gas expansion, which will improve the circulation efficiency of the navigation heating force [5] .
Blades are considered to be one of the key components of the compressor.When they are in service, the contact surface between the blade hoe and the wheel groove often causes micro-cracking on its surface due to micro-wear.The cracks are expanding and extending under the joint action of centrifugal force and pneumatic load, which greatly reduces the service life of the blades [6] .It was found that cycle times, displacement amplitude, and frequency can all affect the wear performance of the blade [7][8][9] .Some scholars have subjected the material to atmosphere heat treatment with the expectation of improving its wear properties.Valdes et al. proposed a new model to characterize the properties of AISI 4140 steel after nitriding heat treatment of the alloy.It was found that after nitriding heat treatment, the hardness of alloy steel increased by more than 2.5 times and the amount of wear and specific wear coefficient decreased.It was shown that hydrogen heat treatment of TiAl improves its plasticity and hot workability [10][11][12][13][14] .However, there are few studies on the wear properties of TiAl alloys after hydrogen heat treatment.
In this paper, tangential micromotion experiments were conducted on unhydrogenated and hydrogenated high Nb-titanium-aluminum alloys.A study of the effect on the microdynamic wear mechanism of hydrogen-deposited high Nb-titanium-aluminum alloys.

Materials
Table 1.Content of Ti-45Al-9Nb element.The alloy was hydrogenated using a hot hydrotreating facility.After hydrogenation, SEM and XRD analytical testing of alloys Fig. 1 the thermohydrogen processing equipment, unhydrogenated and hydrogenated (SEM) microstructure images, and XRD patterns.First of all, the vacuum in the hydrogen furnace was pumped to 10 -3 Pa, and then the protective gas argon was charged.The specimen was heated to 800°C.Hydrogen was continuously introduced into the furnace, and when the gas pressure in the furnace reached 0.15MPa, the hydrogen introduction was stopped and stabilized for 2 hours, and finally cooled to room temperature with the furnace.The LECO-ROH600 oxygen/hydrogen analyzer was used to test the hydrogen content of different parts of the specimen, and finally, a TiAl alloy specimen with 0.8 % content and uniform distribution was obtained.At the same time, the vacuum heat treatment of the unhydrogenated alloy was done at the same temperature without hydrogen.As shown in Fig. 1b, microstructure unaffected by hydrogen.The XRD spectra show that hydrogen modifies the phase content in titanium-aluminum alloys，as shown in Fig. 1c.Comparison with γ, α₂ and B₂ in unhydrogenated alloys，the content of the other two phases increased in the alloy with hydrogen addition, except for the α 2 phase, which decreased.

Experiment procedure
The tangential micro-motion experiments for ball and plane contact were performed on the MFT-5000 micro-motion friction and wear tester.SEM and white light interferometry were used to study the wear morphology and abrasion depth.,  d, and g are for the unhydrogenated alloy.This indicates that the friction types are total slip region (GSR), mixed slip region (MSR), and partial slip region (PSR), respectively.When D was constant, the larger F n , the greater the contact area between the grinding pairs.The contact surface was more inclined to produce elastic deformation to adjust, and the relative displacement between the grinding pairs hardly occurred.When F n =200 N, and D=50, 80, and 110 μm, the F t -D curve is shown in Fig. 2g, h, and i.The fretting regimes were respectively PSR, MSR, and GSR.When F n was constant, the larger the D, the larger the relative displacement between the grinding pairs.Fig. 3 shows the Ft-D curves of hydrogenated titanium-aluminum alloys with the number of cycles for different D values.In general, the slip type changes sequentially from PSR to MSR and GSR with decreasing Fn and increasing D. However, the trend of hydrogenated alloys is relatively flat.

Running condition fretting maps
Figure 4 shows the run state friction map (RCFM) for both alloys.All the tangential fretting showed a tendency to run to GSR with increasing D. However, when D was constant, all the tangential fretting showed a tendency to run to PSR with increasing F n .
The RCFM exhibits three states, namely, partial, mixed, and complete slip, which respectively correspond to the slightly degraded regime, bond cracking area, and wear area in the fretting diagram of material response.But the wear is predominant in the GSR.

Coefficients of friction
Figure 5 shows the coefficient of friction (COF) for both alloys.The COF fluctuated slightly when tangential fretting ran in a part of the GSR.When the tangential fretting ran in the MSR and GSR, the COF fluctuated dramatically.This phenomenon was because the relative displacement between the grinding pairs was relatively large when the fretting ran in MSR and GSR.
When F n was constant, the COF increased with increasing D. There may be two reasons for that: (1) at the same frequency, the larger D, the faster the relative motion between the grinding pairs, which meant the greater the friction resistance between them; (2) wear debris was generated in the friction process of TiAl alloys.Under a large D, wear debris was easily squeezed by the grinding pairs, which caused adhesive wear, so the COF was large.
When D was constant, the COF decreased with increasing F n .It was because increasing the load increased the tangential stiffness, which was conducive to sliding.At the same time, the increasing load caused increasing tangential force, resulting in large elastic deformation [15] .relative sticking of the two contact surfaces.
At the same wear parameters, the hydrogenated alloy will reach the peak COF earlier.After the hydrogenated alloy reaches the smooth fluctuation stage, the frequency and range of fluctuations are smaller than those of the unhydrogenated alloy.

Wear volume
Fig. 6 shows the wear values of unhydrogenated and hydrogenated alloys after performing microfriction wear experiments.When Fn is fixed, the wear value of the unhydrogenated alloy will show the following variation pattern.When D=80 μm or 110 μm, the wear value increases with the increase of Fn.When D=50 μm, as Fn increases, there is a tendency for the alloy wear value to decrease.When D is too small, the larger Fn is, the relative slip between the grinding subsets is smaller and the wear is less.
Fig. 6b shows the wear values of hydrogenated alloys after microfriction wear experiments.The trend of the wear values of hydrogenated alloys is not obvious, and the wear volume of GSR and PSR is larger than PSR.In general, the wear volume of hydrogenated alloys is relatively small and the wear resistance is excellent.

Wear mechanism analysis
The SEM images of the unhydrogenated alloy under F n =200 N and D=50, 80, and 110 μm are shown in Fig. 7a-c.When D=50 μm, there was much spalling on the surface of the wear scar.It was because D was too small, and this area was located in the middle of the entire wear scar.Fine plastic deformation begins to appear on the surface of the alloy wear marks.It was because the abrasive particles were not easily discharged and were compacted to produce hardening.D=80 μm, the wear surface is uneven, and many spalling pits, flaky abrasive chips and furrows along the tangential wear direction appear in the wear center.The wear morphology at D=110 μm was similar to that at D=80 μm.However, it can be seen that when D=110 μm, the lamellar spalling caused by adhesion and furrow caused by abrasive wear were more obvious.
The SEM images of the hydrogenated alloy under F n =200 N and D=50, 80, and 110 μm are shown in Fig. 7d-f.Slight spalling pits were observed in the central area when D=50 μm.More severe spall pits, furrows, and microcracks occurred in that edging area.When D=80 μm, many flaking pits appear in the middle area of the abrasion marks, and the adhesion effect inside the abrasion marks is intensified.In the center of the wear, a small number of cracks parallel to the direction of wear were produced.Grooves are caused by slight sliding and fatigue cracks appear at the wear edge.The elastic deformation generated by TiAl alloys resisted the action of the alternate load and finally generated fatigue cracks.Fig. 7f shows the central morphology of the wear scar when D=110 μm.Plastic deformation, in the center area of the wear, spalling pits and layers of wear debris appeared.The surface of the TiAl alloy was more prone to adhesion under high pressure.In the overall wear edge section non-discharged debris was continuously ground, compacted, and covered on the surface of the wear mark.When D = 50 μm, both alloys enter PSR.As shown in Fig. 7a and d, cracking is relatively larger at the corners of hydrogenated alloy wear marks.Therefore, the hydrogenated alloys are more prone to cracking compared to each other under the same micro-frictional wear parameters.When D=80 μm (Fig. 7e), the fretting of hydrogenated alloy still ran in PSR.A plastic line bar appears at the corners of the grinding marks and the cracks further expand.The above phenomenon was because hydrogen reduced the movement resistance of dislocations and promoted the emission, proliferation, and movement of dislocations.Hydrogen reduced the critical stress intensity factor of crack growth and promoted crack growth.At D = 110 μm, a plow groove parallel to the direction of micro-motion wear appears at the corners of the hydrogenated alloy wear marks.It was because hydrogen activated more slip systems and promoted the mechanical twin deformation of the γ phase and dislocation movement of the α 2 phase.
Table 2 shows the energy spectra of the central and edge regions of the two alloys.In this case, the oxygen content in the same region of the unhydrogenated alloy increases with D. This phenomenon was because, with increasing D, the relative displacement between grinding pairs increased.The wear part of the grinding pairs was more easily exposed to the air, and the exposure frequency at the edge was higher, which aggravated the oxidation degree.There was almost no oxidative wear when D=50 μm.When D=80 and 110 μm, there was a slight oxidation wear.D=50 and 80 μm, hydrogenated alloy ran in PSR.The variation tendency of oxygen content was the same as that of unhydrogenated alloy.However, the oxygen content in the same area decreased obviously, and there was no oxidation wear.When D=110 μm, hydrogenated alloys ran in MSR, oxygen content in the edge area was 20.4 % lower than that in the center area.This phenomenon was because when D was large, the central area was exposed to the air.At the same D value, oxidative wear is more severe for unhydrogenated alloys.This is because hydrogen slows down the oxidation of the alloy.

Conclusions
(1) The COF decreased with increasing F n and increased with increasing D for both the hydrogenated and unhydrogenated alloys.With increasing F n and decreasing D, the fretting regime gradually changed from PSR to MSR and GSR.
(2) Under the same parameters, the COF curve of the hydrogenated alloy fluctuates less and the wear surface is less susceptible to oxidation.
(3) For the wear mechanisms of the unhydrogenated alloys, oxidation and adhesive wear dominated at PSR, and oxidation and adhesive wear dominated at MSR and GSR.For hydrogen-placed alloys, the wear mechanism is dominated by slight fatigue and abrasive, adhesive wear at PSR, and slight oxidation and abrasive, adhesive wear at MSR and GSR.

Fig. 2
shows the tangential friction Ft-D (force-displacement amplitude) curves for unhydrogenated titanium-aluminum alloys at different loading forces (Fn) and displacement amplitudes (D).The curves vary with the increase of cycle time.The friction loop curves of the unhydrogenated alloy are shown in Fig. 2a, d and g for D=50 μm and Fn=100, 150 and 200 N.The friction ring curves shown in Figs.2a

Figure. 2
Figure. 2 Friction ring curves of unhydrogenated titanium-aluminum alloys under different Fn and D conditions.

Figure. 3
Figure. 3 Friction ring curves of hydrogenated titanium-aluminum alloys under different Fn and D conditions

Figure. 6
Figure.6 Wear volume of (a) alloys without hydrogen and (b) alloys with hydrogen

Table 2
Energy spectra of elements in unhydrogenated and hydrogenated alloy.