Rare Metals2018年第7期

Wear properties of nonhydrogenated,hydrogenated,and dehydrogenated Ti6A14V alloy

Bao-Guo Yuan Hai-Ping Yu Chun-Feng Li Dong-Li Sun

School of Materials Science and Engineering, Hefei University of Technology

School of Materials Science and Engineering,Harbin Institute of Technology

作者简介：*Bao-Guo Yuan e-mail:yuanbaoguo@163.com;

收稿日期：26 April 2013

基金：financially supported by the National Natural Science Foundation of China (No. 51205102);the China Postdoctoral Science Foundation (No. 2012M511401);

Wear properties of nonhydrogenated,hydrogenated,and dehydrogenated Ti6A14V alloy

Bao-Guo Yuan Hai-Ping Yu Chun-Feng Li Dong-Li Sun

School of Materials Science and Engineering, Hefei University of Technology

School of Materials Science and Engineering,Harbin Institute of Technology

Abstract：

Wear properties of the nonhydrogenated,hydrogenated 0.5 wt% and dehydrogenated Ti6 A14 V alloys were studied through dry sliding wear tests using an M-200 type pin-on-disk wear testing machine in ambient air at room temperature to reveal the effects of hydrogen on wear properties of Ti6 A14 V alloy. Morphology and chemical element of worn surface were investigated by means of scanning electron microscope(SEM) and energy dispersive spectroscopy(EDS). Results show that hydrogen decreases the wear resistance of Ti6 A14 V alloy. Wear rate of the Ti6 A14 V alloy increases after hydrogenation. Wear rate increases by 244.3 % when 0.5 wt% hydrogen is introduced into a Ti6 A14 V alloy. Wear rate of the dehydrogenated Ti6 A14 V alloy recovers. Wear mechanisms of the nonhydrogenated, hydrogenated, and dehydrogenated Ti6 A14 V alloys are determined. The nonhydrogenated Ti6 A14 V alloy is controlled by oxidative wear. The hydrogenated Ti6 A14 V alloy is dominated by abrasive wear. Wear mechanism of the dehydrogenated Ti6 A14 V alloys is a mixture of oxidative wear and abrasive wear.

Keyword：

Titanium alloy; Hydrogen; Wear; Mechanism;

Received： 26 April 2013

1 Introduction

Light weight is becoming more important in engineering with the development of industry [ 1] .The use of lightweight materials can help us save economy.Therefore,titanium alloys,magnesium alloys,and aluminum alloys have received much attention in various engineering applications over the recent years [ 2, 3, 4, 5, 6, 7] .Among them,titanium alloys have low density,high specific strength,excellent corrosion resistance,good fatigue properties,and biochemical compatibility,which are widely used as structural components in aerospace,chemical,marine,and orthopedic industries [ 8, 9, 10, 11] .Usage of titanium alloys as engineering tribological components is,however,strongly limited by the serious disadvantages of poor tribological properties during sliding [ 12, 13, 14] ,which are attributed to the low resistance to plastic shearing and the low workhardening,and the low protection exerted by the surface oxide which forms as a consequence of the high flash temperatures induced by friction during sliding [ 15, 16] .

In recent years,research on thermohydrogen processing(THP) of titanium alloys has become a hot research topic [ 17, 18, 19, 20] .THP is a new technology,in which hydrogen is used as a temporary alloying element in titanium alloys to control the micros tructure and improve the mechanical properties [ 21, 22, 23, 24, 25] .After processing,hydrogen should be removed by vacuum annealing to avoid embrittlement during their services [ 26] .Hydrogen can affect wear properties of titanium alloys.However,wear properties of hydrogenated and dehydrogenated titanium alloys and their wear mechanisms were not investigated thoroughly.

Therefore,wear tests were carried out on the nonhydrogenated,hydrogenated,and dehydrogenated titanium alloys using a pin-on-disk-type wear testing machine to reveal the effects of hydrogen on wear properties of Ti6A14V alloy.Wear mechanisms were determined by studying the morphology and chemical elements of specimens using scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS).

2 Experimental

Ti6A14V alloy,4 mm in diameter and 6 mm in height,was used in the present work.The specimens were first hydrogenated in an atmosphere of hydrogen in a tube-type furnace at 1,023 K for 1 h,air cooled to room temperature,then solution treated at 1,123 K for 0.5 h followed by furnace cooling to 973 K,and finally quenched into water at room temperature.The actual hydrogen content in specimens produced in this way was determined by weighing the specimens before and after hydrogenation using a precision balance with a measurement accuracy of0.00001 g.The specimens charged with 0.5 wt%hydrogen were obtained.The hydrogenated specimens were dehydrogenated by vacuum annealing in a vacuum hot press furnace at 1,023 K for 11 h under a vacuum of1×10^-3 Pa.

Tribological behaviors of the nonhydrogenated,hydrogenated,and dehydrogenated Ti6A14V alloys were evaluated by dry sliding wear tests using an M-200 type pin-ondisk wear testing machine in ambient air at room temperature.All specimens were ground on 1200 grid emery paper to have uniform standard surface.Pure sliding was obtained by keeping the pin fixed and the counterface disk rotated.Material of the counterface disk was a GCr15 steel of 50 mm in diameter and 5 mm in height with an average hardness of HRC 49.Pins and counterface disks were mechanically ground,ultrasonic ally cleaned in acetone,and dried before the sliding wear tests.Wear tests were performed under 4 N with a sliding velocity of 1 m·s^-1,and the sliding time was 30 min.The data of friction coefficient were recorded continuously as a function of sliding distance by a computer during each wear test.Weights of the nonhydrogenated,hydrogenated,and dehydrogenated alloys before and after dry sliding wear tests were measured using a precision balance with a measurement accuracy of 0.00001 g.Each measurement was preceded by an ultrasonic cleaning and drying.Wear rate(mg·km^-1) was calculated by piding the weight loss by sliding distance.

Wear mechanisms of the nonhydrogenated,hydrogenated,and dehydrogenated Ti6A14V alloys were determined by studying their worn surfaces.Morphology and chemical elements of worn surfaces on the pins were observed and investigated by means of scanning electron microscopy (SEM,Quanta 200) and its energy dispersive spectroscopy (EDS).Thermogravimetric (TG) test of the nonhydrogenated Ti6A14V alloy was carried out using a NETZSCH STA-449C thermal analysis system from 25 to1,200℃with a rising temperature rate of 10℃·min^-1 in argon (flow rate is 45 ml·min^-1).

3 Results and discussion

3.1 Friction property

Friction coefficients of the nonhydrogenated,hydrogenated,and dehydrogenated Ti6A14V alloys are shown in Fig.1.Friction coefficients fluctuate with the sliding time and are relevant with hydrogen.Friction coefficient of the nonhydrogenated Ti6A14V alloy increases with the sliding time at the beginning of wear test,decreases after reaching its maximum,and then becomes stable after unstable state.Friction coefficient of the nonhydrogenated Ti6A14V alloy is about 0.3 at the beginning of wear and increases with the increase of sliding time;the stable value is in the range of0.37-0.45.After hydrogenation,fluctuation range of friction coefficient of the hydrogenated Ti6A14V alloy is larger than that of the nonhydrogenated Ti6A14V alloy.Friction coefficient becomes stable with the increasing sliding time.There is no peak at the beginning of wear for the hydrogenated Ti6A14V alloys.Among the nonhydrogenated,hydrogenated,and dehydrogenated Ti6A14V alloys,friction coefficient of the dehydrogenated Ti6A14V alloy is the highest.There is a peak at the beginning of wear for the dehydrogenated Ti6A14V alloy,which is similar to that of the nonhydrogenated Ti6Al4V alloy.However,its friction coefficient is higher than that of the nonhydrogenated Ti6A14V alloy.Medium friction coefficients of the nonhydrogenated,hydrogenated,and dehydrogenated Ti6A14V alloys are 0.379,0.369,and 0.471,respectively.The above results indicate that hydrogen decreases the friction coefficient of Ti6A14V alloy.Medium friction coefficient of the dehydrogenated Ti6Al4V alloy is the highest.

Fig.1 Friction coefficients:a nonhydrogenated Ti6A14V alloy,b hydrogenated Ti6A14V alloy,and c dehydrogenated Ti6A14V alloy

3.2 Wear property

Wear rates of the nonhydrogenated,hydrogenated,and dehydrogenated Ti6A14V alloys are shown in Fig.2.Wear rate increases by 244.3%when 0.5 wt%hydrogen is introduced into a Ti6A14V alloy,which indicates that hydrogen decreases the wear resistance of Ti6A14V alloy.Wear rate of the dehydrogenated Ti6A14V alloy is lower than that of the corresponding hydrogenated Ti6A14V alloy and higher than that of the nonhydrogenated Ti6A14V alloy,and increases by 100.0%compared with the nonhydrogenated Ti6A14V alloy.Results indicate that wear rate decreases not only for the hydrogenated Ti6Al4V alloy,but also for the dehydrogenated Ti6A14V alloy.

Fig.2 Wear rate of nonhydrogenated,hydrogenated,and dehydro-genated Ti6A14V alloys

3.3 Wear mechanism

Morphology and chemical element of worn surface of the nonhydrogenated,hydrogenated,and dehydrogenated Ti6A14V alloys are shown in Figs.3 and 4,respectively.From Fig.3a,it can be seen that much debris and transfer on worn surface of the nonhydrogenated Ti6Al4V alloy can be found,and only a few tracks are found.From Fig.4a,it can be seen that there are large amounts of O and Fe on the surfaces of debris and transfer,which indicates that GCr15steel wears during the process of wear test,and Fe transfers from GCr 15 steel to the surface of Ti6AI4V alloy pin.The higher amount of O on the worn surface of Ti6AI4V alloy is attributed to the oxidation of Ti and O induced by the rising temperature during the process of sliding.Therefore,the nonhydrogenated Ti6A14V alloy is controlled by oxidative wear.After hydrogenation,many tracks can be found on worn surface of the hydrogenated Ti6Al4V alloy(Fig.3b),which are parallel to the direction of sliding.The appearance of more tracks is caused by the decreasing hardness of Ti6A14V alloy induced by the addition of hydrogen.The amounts of debris and O on worn surface of the hydrogenated Ti6Al4V alloy decrease obviously(Fig.4b).The above results show that the hydrogenated Ti6A14V alloy is dominated by abrasive wear.After dehydrogenation of the hydrogenated Ti6A14V alloy,typical tracks can also be found on worn surface of the dehydrogenated Ti6Al4V alloy (Fig.3c),indicating that wear mechanism of the dehydrogenated Ti6Al4V alloy consists of abrasive wear.In addition,the amounts of debris and transfer on worn surface of the dehydrogenated Ti6A14V alloy are higher than those of the hydrogenated Ti6A14V alloy and less than those of the nonhydrogenated Ti6A14V alloy.The amounts of O in debris and transfer of the dehydrogenated Ti6A14V alloy are higher than those of the hydrogenated Ti6A14V alloy,indicating that oxidative wear exists during the process of sliding.Therefore,wear mechanism of the dehydrogenated Ti6A14V alloy is a mixture of oxidative wear and abrasive wear.

Fig.3 Worn surfaces:a nonhydrogenated Ti6A14V alloy,b hydrogenated Ti6A14V alloy,and c dehydrogenated Ti6A14V alloy

Fig.4 SEM images and EDS analysis of worn surfaces:a nonhydro-genated Ti6A14V alloy,b hydrogenated Ti6A14V alloy,and c dehy-drogenated Ti6A14V alloy

It can be concluded from the above experimental results that wear mechanisms of the nonhydrogenated,hydrogenated,and dehydrogenated Ti6A14V alloys consist of oxidative wear and abrasive wear.The resistance to wear is mainly attributed to the properties of the alloys.Temperature on the worn surface of the nonhydrogenated Ti6A14V alloy is higher when the nonhydrogenated Ti6A14V alloy slides against a GCr15 steel at air atmosphere due to its lower thermoconductivity.Oxidation of Ti6A14V alloy becomes severer with the increase of temperature,which can be seen from its TG results,as shown in Fig.5.Therefore,the nonhydrogenated Ti6A14V alloy is dominated by oxidative wear.Ti on the surface of Ti6Al4V alloy reacts easily with O into TiO₂ film.Wear appears between the oxidation film and the counterface disk.Oxidation film can slough off easily from the alloy under the contact stress because of the severer brittlement of the TiO₂film.Naked surface continues to generate new oxidative film,and therefore,oxidative wear occurs.Wear rate of oxidative wear is usually smaller.During the process of abrasive wear,wear rate is inverse with the hardness of titanium alloy.Typical tracks occur easily on surface of the hydrogenated Ti6Al4V alloy,which is attributed to the decreased hardness of the hydrogenated Ti6A14V alloy.Temperature on worn surface of the hydrogenated Ti6A14V alloy is lower than that of the nonhydrogenated Ti6AI4V alloy during the process of sliding,which is attributed to the improving heat-sinking capability because hydrogen improves the thermoconductivity of titanium alloy [ 27] .Then,the nonhydrogenated Ti6A14V alloy is dominated by oxidative wear,while the hydrogenated Ti6A14V alloy is dominated by abrasive wear.Hardness recovers after dehydrogenation and is still lower than that of the nonhydrogenated Ti6A14V alloy.Therefore,wear rate of the dehydrogenated Ti6A14V alloy is between those of the nonhydrogenated Ti6A14V alloy and hydrogenated Ti6A14V alloy.Wear mechanism of the dehydrogenated Ti6A14V alloy is a mixture of oxidative wear and abrasive wear.

Fig.5 TG curve of nonhydrogenated Ti6Al4V alloy

From the experimental results of wear,wear properties of the hydrogenated and dehydrogenated Ti6A14V alloys are worse than that of the nonhydrogenated Ti6A14V alloy.Therefore,the hydrogenated and dehydrogenated Ti6A14V alloys require a surface treatment when they are used in wear region in order to improve their resistance to wear.

4 Conclusion

Friction coefficient of Ti6A14V alloy decreases after hydrogenation.Friction coefficient of the dehydrogenated Ti6A14V alloy is the highest among those of the nonhydrogenated,hydrogenated,and dehydrogenated Ti6A14V alloys.Hydrogen decreases the wear resistance of Ti6A14V alloy.Wear rate increases by 244.3%,when 0.5 wt%hydrogen is introduced into a Ti6A14V alloy.Wear rate of the dehydrogenated Ti6A14V alloy is lower than that of the corresponding hydrogenated Ti6A14V alloy and higher than that of the nonhydrogenated Ti6A14V alloy.The nonhydrogenated Ti6A14V alloy is controlled by oxidative wear.The hydrogenated Ti6A14V alloy is dominated by abrasive wear.Wear mechanism of the dehydrogenated Ti6A14V alloys is a mixture of oxidative wear and abrasive wear.

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