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Article The Effect of Content on the Mechanical and Tribological Properties of Hypo-, Hyper- and Eutectoid Ti-Cu Alloys

Yiku Xu 1,*, Jianli Jiang 1, Zehui Yang 1, Qinyang Zhao 1, Yongnan Chen 1,* and Yongqing Zhao 2 1 School of and Engineering, Chang’an University, Xi’an 710064, ; [email protected] (J.J.); [email protected] (Z.Y.); [email protected] (Q.Z.) 2 Northwest Institute for Nonferrous Research, Xi’an 710016, China; [email protected] * Correspondence: [email protected] (Y.X.); [email protected] (Y.C.); Tel.: +86-150-2918-9267 (Y.X.); +86-133-8494-8620 (Y.C.)  Received: 5 July 2020; Accepted: 31 July 2020; Published: 3 August 2020 

Abstract: Titanium alloys are widely used in aerospace, chemical, biomedical and other important fields due to outstanding properties. The mechanical behavior of Ti alloys depends on microstructural characteristics and type of alloying elements. The purpose of this study was to investigate the effects of different Cu contents (2.5 wt.%, 7 wt.% and 14 wt.%) on mechanical and frictional properties of titanium alloys. The properties of titanium were characterized by tensile test, electron , X-ray diffraction, differential scanning calorimetry, reciprocating friction and wear test. The results show that the intermediate that forms the eutectoid structure with α-Ti was identified as FCC Ti2Cu, and no primary β phase was formed. With the increase of Cu content, the Ti2Cu phase precipitation in the alloy increases. Ti2Cu particles with needle structure increase the dislocation pinning effect on grain boundary and improve the strength and of . Thus, Ti-14Cu shows the lowest elongation, the best friction and wear resistance, which is caused by the existence of Ti2Cu phases. It has been proved that the mechanical and frictional properties of Ti-Cu alloys can be adjusted by changing the Cu content, so as to better meet its application in the medical field.

Keywords: Ti-Cu alloys; mechanical properties; tribological properties; microstructure; Ti2Cu precipitates

1. Introduction Titanium alloys have high strength to weight ratio, excellent burn resistant and outstanding among metallic materials, therefore, they become ideal candidates for aerospace, chemical, biomedical and other important fields [1–5]. The physical and mechanical properties of titanium alloys can be adjusted by adding different alloying elements with various contents to them. Generally, titanium alloys can be divided into three catalogues: α, α + β and β alloy. By alloying with β-stabilizing elements in titanium alloys, it is possible to decrease the liquids temperature and lessen the reactivity of the titanium, thus improve their physical and chemical properties. It is known that copper is a β-stabilizing element for titanium alloys. Basically, the addition of Cu to Ti alloys can lower the of the alloy and increase the [6], which facilitates the enhancement of burn resistance and machinability of titanium alloy. Wang also pointed out that with the increase of copper content, the diffusion of elements in the alloy was accelerated and the melting point of the alloy was reduced [7]. Copper as an eutectoid stable element, can enlarge the phase and make the eutectoid reaction speed very fast, so the phase cannot be stabilized to room temperature. According to the phase diagram of Ti-Cu binary alloy, a Ti-Cu alloy with precipitated Ti2Cu phase has a transition temperature of about 1000 ◦C[8].

Materials 2020, 13, 3411; doi:10.3390/ma13153411 www.mdpi.com/journal/materials Materials 2020, 13, 3411 2 of 12

The equilibrium phase diagram of the Ti-Cu binary alloys indicates that adding more than 3% copper element into titanium alloy has two phases, forming Ti2Cu phase and forming needle structure, which can improve the mechanical properties of the material [9,10]. Depending on the specific copper content added to the binary system, Ti-Cu alloys may show distinct microstructures, e.g., eutectoid, hyper- and hypo-eutectoid microstructures, and this allows the possibility for the mechanical behavior of Ti-Cu alloys to be adjusted by controlling the fraction and distribution of Ti2Cu precipitation [11,12]. As a kind of structural material for applications at high temperatures, mechanical properties such as strength and are important for Ti-Cu alloys, and these properties represent high dependence on the addition of copper and processing route [13]. Souza [14] studied the effect of cooling rate on the properties of Ti-Cu alloys, and the results showed that eutectoid grains (α + Ti2Cu) would be produced at any cooling rate. When the cooling rate is greater than 9 C s 1, martensite structure will appear, ◦ · − thus increasing the hardness of Ti-Cu alloys. Andrade [15] studied the influence of composition on mechanical properties of as-cast Ti-Cu alloys with copper content ranging from 0.5 to 10 wt.%. It is pointed out that the hardness of Ti-Cu alloys increases with the increase of Cu content because Cu is beneficial to the formation of Ti2Cu, the hard metal interphase. In addition, Li [16] pointed out that with the increase of annealing temperature, phase segregation would occur and TiCu3 and Ti3Cu4 compounds would appear. This intermetallic compound will make Ti-Cu alloys maintain good mechanical properties. Studies have shown that adding copper to titanium alloys can promote and improve the mechanical properties of products [17–20]. Considering the addition of copper could significantly modify the composition and microstructure of Ti-Cu alloys, the mechanical behavior and chemical properties may be also varied accordingly. Therefore, the role of Cu in alloy design is of great significance to the development of new alloys. Thus, the purpose of this work is to investigate the effect of copper content and evaluate the role of Ti2Cu phase on the mechanical and tribological properties of hypo-, hyper- and eutectoid Ti-Cu alloys.

2. Experiment

2.1. Materials and Preparation The Ti-Cu alloys used in this work were comprised of Cu with different concentrations of 2.5, 7.0 and 14.0 wt.%, which were hypo-eutectoid, eutectoid and hyper-eutectoid structure, respectively. The raw materials were supplied by the Northwest Institute of Nonferrous (China). Ti-Cu alloys were first heated in the resistance furnace with the temperature equal to 800–850 ◦C. Both the heating and holding time were about 30 min. Then the was carried out under a 2000 t hydraulic press with 750 kg air . The was generally 150-200 ◦C below the phase transition temperature. The bar with diameter of 40 mm was obtained and then heat treated. Finally, the alloy was quenched in water to improve its hardness. The chemical compositions are listed in Table1.

Table 1. Compositions of the Ti-Cu alloys.

Ti-Cu Alloy Cu wt.% Al wt.% Si wt.% O wt.% N wt.% C wt.% Ti wt.% Ti-2.5Cu 2.5 0.3 0.7 0.05 0.009 0.02 Bal Ti-7Cu 7.0 0.3 0.7 0.05 0.009 0.02 Bal Ti-14Cu 14.0 0.3 0.7 0.05 0.009 0.02 Bal

2.2. Mechanical and Tribological Properties The mechanical properties of the Ti-Cu alloys were evaluated by tensile testing and Vickers hardness testing. Tensile samples with a diameter of 5 mm and gauge length of 45 mm were machined from the center of forged bars, and wet polished using waterproof emery paper. Tensile tests were performed five times for each alloy at a nominal strain rate of 4.2 10 3 s 1 using a microcomputer × − − controlled electronic universal testing machine-5105 (CMT-5105 ) (Jinan, China) universal testing Materials 2020, 13, x FOR PEER REVIEW 3 of 13 machined from the center of forged bars, and wet polished using waterproof emery paper. Tensile tests were performed five times for each alloy at a nominal strain rate of 4.2 × 10−3 s−1 using a Materialsmicrocomputer2020, 13, 3411 controlled electronic universal testing machine-5105 (CMT-5105 )(Jinan, China)3 of 12 universal testing machine at room temperature. The specimens were etched by Keller’s (10 mL HF + 25 mL HNO3 + 15 mL HCl + 500 mL H2O). Microstructure examination was conducted under machine at room temperature. The specimens were etched by Keller’s reagent (10 mL HF + 25 mL an optical microscope (GX71, Olympus, Japan) and a scanning electron microscopy (SEM, JSM-6700, HNO3 + 15 mL HCl + 500 mL H2O). Microstructure examination was conducted under an optical Tokyo, Japan). Vickers hardness measurements were carried out by a MH-5 Digimatic Vickers microscope (GX71, Olympus, Japan) and a scanning electron microscopy (SEM, JSM-6700, Tokyo, hardness tester (Shanghai, China), using a load of 200 g for 30 s. A total of five indents were taken for Japan). Vickers hardness measurements were carried out by a MH-5 Digimatic Vickers hardness tester each sample, and average value of hardness was reported here. (Shanghai, China), using a load of 200 g for 30 s. A total of five indents were taken for each sample, Tribological properties of Ti-Cu alloys (2.5, 7 and 14 wt.%) under -on- contact and average value of hardness was reported here. configuration were evaluated using a reciprocating friction and wear test-rig of MMQ-200. During Tribological properties of Ti-Cu alloys (2.5, 7 and 14 wt.%) under ring-on-block contact the test, the coefficient of friction can be continuously and automatically recorded as a function of configuration were evaluated using a reciprocating friction and wear test-rig of MMQ-200. During the sliding time. The sliding tests were repeated three times for reliability and reproducibility. The phases test, the coefficient of friction can be continuously and automatically recorded as a function of sliding were confirmed using a D8 ADVANCE X-ray diffraction (XRD) (Germany) with Cu-kα radiation at time. The sliding tests were repeated three times for reliability and reproducibility. The phases a scanning rate of 0.05° per second in the 2θ ranges from 30° to 80° and differential scanning were confirmed using a D8 ADVANCE X-ray diffraction (XRD) (Germany) with Cu-kα radiation at a calorimetry (DSC) (EC2000) (USA). After the sliding test, the morphologies and depth profiles of wear scanning rate of 0.05◦ per second in the 2θ ranges from 30◦ to 80◦ and differential scanning calorimetry tracks were analysis by scanning electron microscopy (SEM) (JSM-6700) (Tokyo, Japan) and (DSC) (EC2000) (USA). After the sliding test, the morphologies and depth profiles of wear tracks transmission electron microscopy (TEM) (FEI TalosF200X TEM) (Hillsboro, OR, USA). Energy were analysis by scanning electron microscopy (SEM) (JSM-6700) (Tokyo, Japan) and transmission dispersive X-ray spectroscopy (EDS) (JSM-6700) (Tokyo, Japan) was selected to determine the electron microscopy (TEM) (FEI TalosF200X TEM) (Hillsboro, OR, USA). Energy dispersive X-ray elemental distributions. As the friction progressed, the surface of the substrate was ground to a circle spectroscopy (EDS) (JSM-6700) (Tokyo, Japan) was selected to determine the elemental distributions. with a radius of 5 mm. As the wear time increases, the worn scratch becomes deeper, and the amount As the friction progressed, the surface of the substrate was ground to a circle with a radius of 5 mm. of wear also increases. The hardness (HRC) of (GCr 15) bearing balls (6 mm diameter) friction As the wear time increases, the worn scratch becomes deeper, and the amount of wear also increases. couple was 62–63, and the average surface Ra was about 0.01 μm. All the friction and wear tests were The hardness (HRC) of (GCr 15) bearing steel balls (6 mm diameter) friction couple was 62–63, and the performed using a constant normal load of 5 N with a frequency of 5 Hz and an oscillating amplitude average surface Ra was about 0.01 µm. All the friction and wear tests were performed using a constant of 5 mm for 10 min at room temperature 400 °C, 600 °C and continuous heating at humidity (relative normal load of 5 N with a frequency of 5 Hz and an oscillating amplitude of 5 mm for 10 min at room humidity is about 30%). temperature 400 ◦C, 600 ◦C and continuous heating at humidity (relative humidity is about 30%).

3. Results and discussion

3.1. Mechanical Properties of the Ti-Cu Alloys The ultimate tensile tensile strength strength (UTS), (UTS), yield strength strength (YS), (YS), elongation elongation and and hardness hardness (HV) (HV) of the of theTi- Ti-CuCu alloys alloys are are shown shown in inFigure Figure 1. 1.

Figure 1. Tensile curvescurves and and mechanical mechanical properties properties of Ti-Cuof Ti-Cu alloys alloys at room at room temperature. temperature. True True stress–strain stress– curvesstrain curves of Ti-7Cu of Ti-7Cu alloy (a ),alloy and ( strengtha), and strength (b), elongation (b), elongation (c), hardness (c), (hardnessd) of Ti-Cu (d alloys.) of Ti-Cu alloys.

The true stress–strain curve of Ti-7Cu alloy exhibits a yield point at about 550 MPa and reaches UTS at about 700 MPa, indicating a typical ductility (Figure1a). For Ti-14Cu alloy, YS and UTS rise Materials 2020, 13, x FOR PEER REVIEW 4 of 13 Materials 2020, 13, 3411 4 of 12 The true stress–strain curve of Ti-7Cu alloy exhibits a yield point at about 550 MPa and reaches UTS at about 700 MPa, indicating a typical ductility (Figure 1a). For Ti-14Cu alloy, YS and UTS rise toto about about 600600 MPaMPa andand 790 MPa, MPa, respectively. respectively. Compared Compared with with Ti-2.5 Ti-2.5 Cu Cu alloy alloy shown shown in inFigure Figure 1b,1 b,YS YSand and UTS UTS of ofTi-14Cu Ti-14Cu alloy alloy increased increased by by30% 30% and and 25%, 25%, respectively. respectively. This This is basically is basically consistent consistent with withthe former the former research research that results that results of this of study this studythat the that yield the points yield pointsof Ti-5Cu ofTi-5Cu and Ti-15Cu and Ti-15Cu are 522-592 are 522–592MPa [21]. MPa However, [21]. However, opposite oppositeto the increased to the increased strength, strength,the elongation the elongation decreases decreaseswith the increase with the of increaseCu content, of Cu as content, shown asin shownFigure in1c. Figure A relatively1c. A relatively low ductility low ductility of Ti-14Cu of Ti-14Cu alloy is alloymeasured is measured by 12% byelongation 12% elongation which whichis about is 3% about lower 3% lowerthan that than of that Ti-7Cu of Ti-7Cu and greatly and greatly lower lower than thanTi-2.5Cu Ti-2.5Cu alloy alloy (30% (30%elongation) elongation) [22]. [ 22It ].can It canbe deduced be deduced that that the the addition addition of of Cu Cu not not only only plays plays a a significant significant rolerole ononthe the physiochemicalphysiochemical property property which which improves improves the burn the resistance,burn resistance, but also but alters also the mechanicalalters the mechanical properties. Theproperties. elongation The is elongation tended to decreaseis tendedwith to decrease increasing with the increasing copper concentration. the copper concentration. The lowestelongation The lowest ofelongation Ti-Cu alloys of Ti-Cu is at thealloys alloy is at having the alloy 14% having Cu. The 14% hardness Cu. The testhardness results test in Figureresults1 ind showsFigure 1d that shows the hardnessthat the valuehardness increases value withincreases the increase with the of increa Cu contentse of Cu from content 2.5 wt.% from to 142.5 wt.%. wt.% Theto 14 hardness wt.%. The of Ti-7Cuhardness and of Ti-14Cu Ti-7Cu increasedand Ti-14Cu by 22%increased and 43%, by 22% respectively and 43%, compared respectively with compared Ti-2.5Cu. with Ti-2.5Cu. FigureFigure2 shows2 shows the the fracture fracture morphology morphology of theof the Ti-Cu Ti-Cu alloys. alloys. As canAs becan seen, be seen, the Ti-2.5Cu the Ti-2.5Cu alloy alloy has dimplehas dimple fracture fracture and hasand ductile has ductile fracture fracture characteristics, characteristics, which which is a typical is a typical plastic plastic fracture. fracture.

.

FigureFigure 2. 2.Fracture Fracture morphologymorphology ofof Ti-CuTi-Cu alloysalloys Ti-2.5CuTi-2.5Cu ( a(a),), Ti-7Cu Ti-7Cu ( b(b)) and and Ti-14Cu Ti-14Cu ( c(),c), and and section section morphologymorphology of of Ti-2.5Cu Ti-2.5Cu alloy alloy (d (d),), Ti-7Cu Ti-7Cu alloy alloy (e (),e), Ti-14Cu Ti-14Cu alloy alloy (f ).(f).

TheThe Ti-7Cu Ti-7Cu alloy alloy has has obvious obvious step step and dimpleand dimple structure, structure, which which is ductile is andductile brittle and mixed brittle fracture. mixed Thefracture. Ti-14Cu The alloy Ti-14Cu is dominated alloy is dominated by step structure by step and structure is a typical and is a typical fracture. cleavage The fracture. reduction The ofreduction area increases of areawith increases the increase with the of increase copper of content. copper content. Such a lowSuch elongation a low elongation rate is rate traded is traded with thewith high the strength high strength which which could bringcould challengesbring challenges to the to formability the formability and and resistancecreep resistance of the alloy.of the Asalloy. a result, As a result, typical typical intergranular intergranular fracture fracture and demulsification and demulsification grains grains can becan clearly be clearly observed observed by observingby observing the fracturethe fracture morphology morphology of titanium of titanium alloy alloy with with different different copper copper content content (Figure (Figure2a–c) and2a–c) theand corresponding the corresponding fracture fracture cross-section cross-sectio morphologyn morphology (Figure (Figure2d–f), respectively.2d–f), respectively. InIn order order to to further further explore explore the the influence influence of of section section microstructure microstructure onon thethe propertiesproperties ofof Ti-CuTi-Cu alloys,alloys, thethe grain size size of of section section microstructure microstructure was was measured measured by Image by Image Pro Plus Pro Plus6.0 (1993, 6.0 (1993, 2003, Media 2003, MediaCybernetics, Cybernetics, MD, USA). MD, USA). Each Eachgroup was measured was measured 5 times, 5 times, and andthe theaverage average value value was was taken taken as the as thefinal final grain grain size. size. The The results results are are shown shown in Table in Table 2. 2.

Table 2. The calculated grain size of Ti-Cu alloys (D: µm). Table 2. The calculated grain size of Ti-Cu alloys (D: μm).

Ti-CuTi-Cu AlloyAlloy D1D1 D2D2 D3D3 D4D4 D5D5 AverageAverage Ti-2.5Cu 1.1031.103 1.5161.516 1.1631.163 1.1031.103 1.1041.104 1.198 1.198 Ti-7Cu 9.648 9.648 11.15911.159 7.6147.614 12.56712.567 11.92011.920 10.58210.582 Ti-14Cu 39.585 58.882 56.038 62.846 67.320 56.934 Ti-14Cu 39.585 58.882 56.038 62.846 67.320 56.934

Materials 2020, 13, x FOR PEER REVIEW 5 of 13 Materials 2020, 13, 3411 5 of 12 The higher the Cu content is, the larger the grain size of the Ti-Cu alloys would be. Based on SEM characterization of the microstructure in Figure 2, it can be concluded that the grain boundary The higher the Cu content is, the larger the grain size of the Ti-Cu alloys would be. Based on of Ti-2.5Cu alloy is not obvious, with the smallest grain size of about 1.198 μm. The grain boundary SEM characterization of the microstructure in Figure2, it can be concluded that the grain boundary of of Ti-7Cu alloy is obvious, and the grain size is about 10.582 μm. Displaying typical large-grain Ti-2.5Cu alloy is not obvious, with the smallest grain size of about 1.198 µm. The grain boundary of boundary structure, the grain size of Ti-14Cu alloy increases significantly to 56.934 μm, which is Ti-7Cu alloy is obvious, and the grain size is about 10.582 µm. Displaying typical large-grain boundary 81.41% and 97.90%, respectively compared with Ti-7Cu alloy and Ti-2.5Cu alloy. It is deduced that structure, the grain size of Ti-14Cu alloy increases significantly to 56.934 µm, which is 81.41% and the higher Cu content may bring higher diffusion, and give rise to larger eutectoid lamellar space and 97.90%, respectively compared with Ti-7Cu alloy and Ti-2.5Cu alloy. It is deduced that the higher Cu grain size. The results are consistent with Ref. [21]. It can be concluded that the fracture is governed content may bring higher diffusion, and give rise to larger eutectoid lamellar space and grain size. by cleavage mechanism, with the appearance of crack propagation due to the rupture of atomic bonds The results are consistent with Ref. [21]. It can be concluded that the fracture is governed by cleavage close to some crystallographic planes. Then intergranular fractures occur when length of cracks mechanism, with the appearance of crack propagation due to the rupture of atomic bonds close to propagating along the grain boundaries reach a critical limit. some crystallographic planes. Then intergranular fractures occur when length of cracks propagating along the grain boundaries reach a critical limit. 3.2. Tribological Properties of the Ti-Cu Alloys 3.2. TribologicalFigure 3 shows Properties the XRD of the patterns Ti-Cu Alloys and DSC heating curves of titanium alloys with different Cu contents.Figure 3 shows the XRD patterns and DSC heating curves of titanium alloys with di fferent Cu contents.

Figure 3. (a) XRD patterns of titanium alloys with different Cu contents; (b) differential scanning

calorimetry (DSC) heating curves of titanium alloys with different Cu contents. Figure 3. (a) XRD patterns of titanium alloys with different Cu contents; (b) differential scanning calorimetryIt is proved (DSC) that heating the phases curves of of Ti-2.5Cu, titanium Ti-7Cualloys with and different Ti-14Cu Cu are contents. mainly α-Ti and Ti2Cu, and the precipitation amount of Ti2Cu increases with the increase of Cu content. This indicates that Cu content increasesIt is proved the volume that the fraction phases of of eutectoid Ti-2.5Cu, structure, Ti-7Cu thusand Ti-14Cu increasing are the mainly content α-Ti of and Ti2Cu Ti2 intermetallicCu, and the precipitationphase. The phase amount transition of Ti2Cu trend increases is consistent with withthe increase the DSC resultsof Cu incontent. Figure 3Thisb. Due indicates to melting that heat Cu contentabsorption, increases titanium the alloysvolume appear fraction as pits of ateutectoi aroundd 690structure,◦C. The thus eutectoid increasing transformation the content of theof Ti Ti-Cu2Cu intermetallicbinary alloy fromphase.β-Ti The to phaseα-Ti and transition Ti2Cu can trend be foundis consistent at 800 ◦ withC. The the peritectic DSC results reaction in Figure is proved 3b. toDue be tooccurring meltingat heat about absorption, 1000 ◦C. The titanium results arealloys also a consistentppear as withpits theat phasearound transition 690 °C. processThe eutectoid of Ti-Cu transformationbinary alloy phase of the diagram. Ti-Cu Basedbinary on alloy the above,from β it-Ti can to be α-Ti concluded and Ti2 thatCu can the additionbe found of at Cu 800 in titanium°C. The peritecticalloys eff ectivelyreaction promotedis proved to the be eutectoid occurring transformation at about 1000 °C. process, The results thus increasingare also consistent the precipitation with the phaseamount transition of Ti2Cu process phase. of Ti-Cu binary alloy phase diagram. Based on the above, it can be concluded that theThe addition friction of coe Cuffi cientsin titanium of Ti-Cu alloys alloys effect at roomively promoted temperature, the400 eutectoid◦C and transformation 600 ◦C with continuous process, thusheating increasing condition the are precipitation exhibitedin amount Figure 4ofa–c, Ti2Cu respectively. phase. TheThe friction friction coefficients coefficients of curves Ti-Cu ofalloys different at room Ti-Cu temperature, alloys exhibit 400 °C similar and 600 friction °C with behavior continuous at the heatingsame temperature condition are which exhibited shows in aFigure trend 4a–c, of increasing respectively. at first and then stabilizing later. The Ti-7Cu alloy exhibits a relatively high friction coefficient with a relatively stable value of 0.315, while the friction coefficient of the Ti-14Cu is 68%, which is lower than that of Ti-7Cu at room temperature. After a pre-grinding , the surface is gradually worn flat and the friction coefficient between the grinding surfaces increases and finally reaches the steady state. Copper added to titanium alloy will be preferentially precipitated at the grain boundary to form the copper-rich region, and then form Ti2Cu with a BCC structure. As a high hardness intermetallic compound, Ti2Cu increases the hardness

Materials 2020, 13, 3411 6 of 12

of alloys. Under the condition of friction, the high load of rigid Ti2Cu has a certain bearing capacity, which improves the wear resistance of the materials. As the temperature reaches 400 ◦C and 600 ◦C, the friction coefficient of the Ti-14Cu fluctuates around a relative lower level than that of Ti-2.5Cu and Ti-7Cu alloys. This may be caused by the alloying of copper in titanium alloys. Copper alloying inhibits grain coarsening in terms of kinetics and thermodynamics so that the Ti-14Cu alloy has higher thermalMaterials stability2020, 13, x andFOR PEER thus REVIEW reduces the amplitude of the friction coefficient during friction. 6 of 13

FigureFigure 4. 4.Friction Friction coecoefficientfficient ofof Ti-CuTi-Cu alloysalloys atat (a(a)) room room temperature, temperature, ((bb)) 400400◦ °C,C,( c(c)) 600 600◦ °CC and and ( d(d)) a a pin-diskpin-disk wear wear machine machine (MMQ)-reciprocating (MMQ)-reciprocating friction friction and and wear wear test test diagram. diagram.

FigureThe friction4d shows coefficients a schematic curves diagram of different of reciprocating Ti-Cu alloys friction exhibit and similar wear friction test. Based behavior on above, at the thesame Ti-14Cu temperature alloy exhibits which superiorshows a frictiontrend of reducing increasing than at first Ti-2.5Cu and andthen Ti-7Custabilizing alloys. later. It isThe probably Ti-7Cu duealloy to exhibits the reason a relatively that Cu is high a kind friction of good coefficient lubricant with for titaniuma relatively alloys. stable Adding value copperof 0.315, into while Ti can the efrictionffectively coefficient form a hard of the Ti2Cu Ti-14Cu phase is in 68%, the friction which process.is lower The than hard that phase of Ti-7Cu (Ti2Cu) at isroom mainly temperature. involved inAfter the abrasivea pre-grinding wear during period, the the friction surface process is gradually under theworn condition flat and of the friction friction because coefficient the combination between the ofgrinding Ti2Cu phase surfaces and increases matrix is veryand finally weak andreaches easy the to peelsteady off. state. Further, Copper the furrow added is to easily titanium generated alloy will by frictionbe preferentially between friction precipitated pairs, whileat the thegrain titanium boundary alloy to is form easy the to produce copper-rich adhesive region, wear and due then to form low surface hardness and insufficient plastic shear resistance. When the copper content is low, the wear Ti2Cu with a BCC structure. As a high hardness intermetallic compound, Ti2Cu increases the hardness process is mainly adhesive wear due to low Ti Cu content and the wear degree is relatively large. of alloys. Under the condition of friction, the high2 load of rigid Ti2Cu has a certain bearing capacity, Thewhich wear improves resistance the of wear titanium resistance alloy canof the be improvedmaterials. byAs producingthe temperature hard metal reaches intermetallic 400 °C and phase 600 °C, in titaniumthe friction alloy coefficient and introducing of the Ti-14Cu abrasive fluctuates wear to o ffaroundset part a ofrelative adhesive lower wear. level than that of Ti-2.5Cu and TheTi-7Cu average alloys. friction This may coeffi becients caused of Ti-Cuby the alloys alloying are of present copper in in Figure titanium5a. alloys. Copper alloying inhibitsIt can grain be found coarsening that the in average terms of friction kinetics coe andfficient thermodynamics increases as the so temperature that the Ti-14Cu increase alloy for thehas Ti-Cuhigher alloys thermal with stability the same and Cu thus content. reduces The the change amplitude of mechanical of the fricti andon physical coefficient properties during is friction. attributed to theFigure increase 4d of shows intermolecular a schematic distance diagram caused of reciprocating by , friction and which wear leads test. to Based the increase on above, of viscositythe Ti-14Cu and alloy the adhesion exhibits frictionsuperior coe frictionfficient reducing of titanium than alloys. Ti-2.5Cu It has and also Ti-7Cu been alloys. reported It is that probably high temperaturesdue to the reason promote that theCu diisff ausion kind of oxygengood lubricant atoms, leadingfor titanium to the alloys. oxidation Adding of Ti copper and Cu into atoms Ti can in the metal [23]. TiO is also a kind of hard phase which may aggravate abrasive wear. It may effectively form a hard2 Ti2Cu phase in the friction process. The hard phase (Ti2Cu) is mainly involved alsoin the be aabrasive reason that wear the during Ti-14Cu the alloy friction produces process less titaniumunder the oxide condition within theof samefriction frictional because area the reducing the abrasive wear process. combination of Ti2Cu phase and matrix is very weak and easy to peel off. Further, the furrow is easily generated by friction between friction pairs, while the titanium alloy is easy to produce adhesive wear due to low surface hardness and insufficient plastic shear resistance. When the copper content is low, the wear process is mainly adhesive wear due to low Ti2Cu content and the wear degree is relatively large. The wear resistance of titanium alloy can be improved by producing hard metal intermetallic phase in titanium alloy and introducing abrasive wear to offset part of adhesive wear. The average friction coefficients of Ti-Cu alloys are present in Figure 5a.

Materials 2020, 13, 3411 7 of 12 Materials 2020, 13, x FOR PEER REVIEW 7 of 13

Figure 5.5. (a(a)) Average Average friction friction coe coefficienfficient oft the of threethe three Ti-Cu Ti-Cu alloys atalloys different at different temperatures; temperatures; (b) grinding (b) ratiogrinding of Ti-Cu ratio alloys. of Ti-Cu alloys.

InIt can addition, be found the friction that the properties average friction of the three coeffici alloysent under increases the condition as the temperature of continuous increase temperature for the riseTi-Cu were alloys also with recorded the insame Figure Cu5 .content. It can be The seen change that at roomof mechanical temperature, and thephysical friction properties coe fficients is ofattributed Ti-2.5Cu to and the Ti-7Cuincrease alloys of inte arermolecular very high distance and maintained caused by thermal a relatively expansion, similar which trend leads showing to the a relativelyincrease of large viscosity fluctuation. and the When adhesi theon temperature friction coefficient reaches of to titanium 400 ◦C and alloys. 600 ◦ ItC, has the also friction been coe reportedfficient isthat observed high temperatures to decrease andpromote the value the diffusion of the Ti-2.5Cu of alloy atoms, is much leading lower to than the oxidation the Ti-7Cu of alloy. Ti and As Cu is known,atoms in it the is easy metal to [23]. form TiO titanium2 oxide oxide is also which a kind is aof hard hard particle phase which but not may wear-resistant. aggravate abrasive When adding wear. copperIt may also to the be titaniuma reason that alloy, the copper Ti-14Cu oxide alloy would produc bees formedless titanium which oxide is a gaswithin volatilization. the same frictional It will eareaffectively reducing consume the abrasive the oxygen wear during process. the friction process especially during high temperature friction. As theIn temperature addition, the increases, friction the properties oxygen in of the the copper three oxide alloys volatilizes under inthe the condition form of a gas,of continuous so that the remainingtemperature copper rise were element also willrecorded take oxygenin Figure from 5. It the can titanium be seen oxidethat at to room form temperature, copper oxide, the and friction then thecoefficients amount of theTi-2.5Cu hard particleand Ti-7Cu titanium alloys oxide are very is reduced. high and In themaintained process of a increasingrelatively similar temperature, trend theshowing tendency a relatively of titanium large oxide fluctuation. to convert When to copper the temperature oxide always reaches exists, to while 400 the°C and amount 600 of°C, hard the particlesfriction coefficient becomes less is observed and less, to thus decrease the friction and the coe ffivacientlue of is the also Ti-2.5Cu getting lower.alloy is As much the copperlower than content the increases,Ti-7Cu alloy. the amountAs is known, of titanium it is easy oxide to deceases,form titanium thereby oxide improving which is the a wearhard resistance.particle but not wear- resistant.The grindingWhen adding ratios copper of the Ti-xCuto the titanium (x = 2.5, 7,alloy, 14) alloycopper are oxide shown would in Figure be formed5b. Compared which is witha gas Ti-2.5Cuvolatilization. (3.5%), It thewill grinding effectively ratio consume of Ti-7Cu the (4.2%)oxygen and during Ti-14Cu the (6.9%)friction increases process especially by 20% and during 97%, respectively,high temperature which friction. proves theAs the addition temperature of the copper increases, markedly the oxygen improves in the the copper grindability oxide volatilizes of the Ti-Cu in alloys.the form The of grindinga gas, so that ratio the here remaining refers to copper the element change will of thetake alloy oxygen before from and the after titanium the wear oxide test. to Itform can copper be expressed oxide, byand the then following the amount formula, of the Equation hard particle (1): titanium oxide is reduced. In the process of increasing temperature, the tendency of titanium oxide to convert to copper oxide always exists, ma mb while the amount of hard particles becomesGrinding less ratio and= less,− thus the friction coefficient is also getting(1) Ma Mb lower. As the copper content increases, the amount of titanium− oxide deceases, thereby improving wherethe wearma resistance.is the mass of the grinding wheel to be tested before the friction and wear test, and mb refersThe to thegrinding mass of ratios the grinding of the Ti-xCu wheel (x to = be 2.5, tested 7, 14) after alloy the are friction shown and in wearFigure test. 5b.M Compareda is the mass with of theTi-2.5Cu material (3.5%), to be the tested grinding before ratio the frictionof Ti-7Cu and (4.2%) wear and test, Ti-14CuMb is the (6.9%) mass increases of the material by 20% to and be tested 97%, afterrespectively, the friction which and proves wear test.the addition It can be of seen the fromcopper the markedly test results improves that Ti2 Cuthe precipitationgrindability of is the easier Ti- toCu form alloys. in Ti-CuThe grinding alloys with ratio higher here refers copper to concentrationthe mass change (7 orof 14the wt.%) alloy alongbefore the and grain after boundary. the wear Thistest. phenomenonIt can be expressed may be by caused the following by the better formula, enrichment Equation of (1): copper in the grain boundary due to the lamellar structure of the Ti2Cu phase. The Ti2Cu precipitate𝑚 𝑚 enriched at the grain boundaries increases Grinding ratio (1) the number of grain boundaries, increasing the strength𝑀 of𝑀 the material and thus reducing the ductility. Therefore, the Ti-14Cu alloy had better grindability compared to Ti-2.5Cu and Ti-7Cu alloys. where 𝑚is the mass of the grinding wheel to be tested before the friction and wear test, and 𝑚 refers to the mass of the grinding wheel to be tested after the friction and wear test. 𝑀is the mass of 3.3. Effect of Ti2Cu on Mechanical Properties and Tribological Properties the material to be tested before the friction and wear test, 𝑀is the mass of the material to be tested afterFigure the friction6 shows and the wear structures test. It can of the be Ti-Cuseen from alloys. the test results that Ti2Cu precipitation is easier to form in Ti-Cu alloys with higher copper concentration (7 or 14 wt.%) along the grain boundary. This phenomenon may be caused by the better enrichment of copper in the grain boundary due to the lamellar structure of the Ti2Cu phase. The Ti2Cu precipitate enriched at the grain boundaries increases the number of grain boundaries, increasing the strength of the material and thus reducing the

Materials 2020, 13, x FOR PEER REVIEW 8 of 13

ductility. Therefore, the Ti-14Cu alloy had better grindability compared to Ti-2.5Cu and Ti-7Cu alloys.

3.3. Effect of Ti2Cu on Mechanical Properties and Tribological Properties Materials 2020, 13, 3411 8 of 12 Figure 6 shows the structures of the Ti-Cu alloys.

Figure 6. Microstructure of Ti-Cu alloys; (a) Ti-2.5Cu, ((b)) Ti-7Cu,Ti-7Cu, ((cc)) Ti-14Cu.Ti-14Cu. RedRed frame:frame: SEM images of grain boundary for SEM images.

Based on the XRD measurement, the intermediate phase that forms the eutectoid structure with α-Ti waswas identified identified as as FCC FCC Ti 2TiCu,2Cu, and and no primaryno primaryβ phase β phase wasformed. was formed. With theWith increase the increase of Cu element of Cu inelement Ti-Cu alloys,in Ti-Cu the alloys, morphology the morphology of Ti2Cu phase of Ti changes2Cu phase from changes granular from through granular mixing through granular mixing and shortgranular strip and and short then strip to acicular and then structure. to acicular The sizestructure. of the TiThe2Cu size phase of the was Ti measured2Cu phase using was Imagemeasured Pro Plususing 6.0 Image and shownPro Plus in 6.0 Table and3. shown in Table 3.

Table 3. Size of the Ti2Cu phase of Ti-Cu alloys (D: µm). Table 3. Size of the Ti2Cu phase of Ti-Cu alloys (D: μm).

Ti-CuTi-Cu Alloy Alloy D1 D1 D2 D2 D3 D4 D5D5 AverageAverage Ti-2.5CuTi-2.5Cu (granular) (granular) 1.470 1.470 1.765 1.765 1.213 1.213 1.470 1.9731.973 1.578 1.578 Ti-7CuTi-7Cu (granular) (granular) 1.7651.765 1.7651.765 1.6641.664 2.371 1.7151.715 1.8561.856 Ti-7Cu (short strip) 11.070 9.602 6.056 9.300 6.990 8.604 Ti-7CuTi-14Cu (short (acicular) strip) 11.070 15.498 9.602 15.261 21.6906.056 19.4309.300 22.4936.990 18.8748.604 Ti-14Cu (acicular) 15.498 15.261 21.690 19.430 22.493 18.874

It is worth noting that the Ti Cu phase of Ti-14Cu reaches 18.874 µm, which increases by 54.413% It is worth noting that the Ti2Cu phase of Ti-14Cu reaches 18.874 μm, which increases by 54.413% and 91.639%, respectively, comparedcompared withwith Ti-7CuTi-7Cu andand Ti-2.5Cu.Ti-2.5Cu. It also can be found that the Ti Cu precipitates of the Ti-7Cu alloy distribute in grain and along It also can be found that the Ti22Cu precipitates of the Ti-7Cu alloy distribute in grain and along grain boundaries,boundaries, while while in thein Ti-14Cuthe Ti-14Cu alloy, alloy, the precipitates the precipitates only distribute only distribute along the grainalong boundaries, the grain whichboundaries, is all which existing is inall theexisting form in of the layer form structure. of layer structure. Under the Under action the of action shear of force shear and force friction and conditions, Ti Cu phase with rigidity can improve the plastic shear resistance of Ti-Cu alloys, so as to friction conditions,2 Ti2Cu phase with rigidity can improve the plastic shear resistance of Ti-Cu alloys, improveso as to improve its wear its resistance. wear resistance. Ti-14Cu showsTi-14Cu the shows best wearthe best resistance wear resistance could be causedcould be by caused the existence by the of layered Ti Cu. existence of layered2 Ti2Cu. In order toto furtherfurther determinedetermine thethe distributiondistribution of Cu elements, EDS was used forfor determinationdetermination and the results are shownshown inin TableTable4 4.. It is found that the distribution in Ti-2.5Cu, Ti-7Cu and Ti-14Cu alloys was similar in grain boundary. The Cu content in the grain boundary is higher than that in intracrystalline. It is known that copper as a β-stable element, can promote the precipitation of Ti2Cu hard phase in titanium alloy. The higher copper content the alloy has, the greater fraction of the Ti2Cu hard phase precipitation in the alloy. Especially the needle-like Ti2Cu structure increases the effect of Ti2Cu particles, through increasing dislocation , thus effectively preventing grain boundary migration, further improving strength and hardness. The interwoven and alternating lamellar Ti2Cu structure, which greatly increases the enrichment of Ti2Cu phase at the grain boundary, making the number of Ti2Cu phase at Materials 2020, 13, x FOR PEER REVIEW 9 of 13

Table 4. Concentration of Cu on grain boundaries and within grains.

Concentration of Cu Grain Boundaries Intracrystalline (wt.%) 1 2 3 4 Average Ti-2.5Cu 4.32 3.18 3.76 3.38 3.66 2.21 Materials 2020Ti-7Cu, 13, 3411 8.58 7.91 9.44 9.17 8.78 7.3 9 of 12 Ti-14Cu 22.17 18.23 18.34 23.02 20.44 12.7

1–4 means different EDS points on grain boundaries. the grain boundary much larger than that inside the grain. Due to the higher copper content in the Ti Cu phase, serious segregation phenomenon can be found in the Ti Cu phase. With the increase of 2 It is found that the distribution in Ti-2.5Cu, Ti-7Cu and Ti-14Cu2 alloys was similar in grain Ti Cu phase segregation, the wear depth generated in the friction process becomes shallower, all as to boundary.2 The Cu content in the grain boundary is higher than that in intracrystalline. It is known withstand greater loads under normal friction-resistant pressure. that copper as a β-stable element, can promote the precipitation of Ti2Cu hard phase in titanium alloy. The higher copper content the alloy has, the greater fraction of the Ti2Cu hard phase precipitation in Table 4. Concentration of Cu on grain boundaries and within grains. the alloy. Especially the needle-like Ti2Cu structure increases the effect of Ti2Cu particles, through increasingConcentration dislocation ofdensity, thus effectivelyGrain preventing Boundaries grain boundary migration, further Intracrystalline improving strengthCu (wt.%) and hardness. 1The interwoven 2 and 3 alternating 4 lamellar Average Ti2Cu structure, which greatly increasesTi-2.5Cu the enrichment of 4.32 Ti2Cu phase 3.18 at the 3.76 grain boundary, 3.38 making 3.66 the number 2.21 of Ti2Cu phase at the grainTi-7Cu boundary much 8.58 larger than 7.91 that inside 9.44 the grain. 9.17 Due to 8.78 the higher copper 7.3 content in the Ti2Cu phase,Ti-14Cu serious segregation 22.17 phenomenon 18.23 18.34 can be 23.02 found in 20.44 the Ti2Cu phase. 12.7 With the increase of Ti2Cu phase segregation,1–4 means dithefferent wear EDS depth points ongenerated grain boundaries. in the friction process becomes shallower, all as to withstand greater loads under normal friction-resistant pressure. AA schematic schematic diagram diagram of of tensile tensile specimens specimens of of a aTi-Cu Ti-Cu alloy alloy is is show show in in Figure Figure 7a,7a, and and cross cross sectionalsectional TEM TEM images images and and the theCu distribution Cu distribution of the of Ti-14Cu the Ti-14Cu and Ti-2.5Cu and Ti-2.5Cu alloys are alloys shown are in shown Figure in 7b,c,Figure respectively.7b,c, respectively.

FigureFigure 7. 7. (a()a Schematic) Schematic diagram diagram of of tensile tensile specimens specimens of of a Ti-Cu alloy;alloy; ((bb)) crosscross sectionalsectional TEM TEM image image of ofTable Table4. Cu4. Cu and and distribution distribution diagram diagra ofm the of correspondingthe corresponding Cu element;Cu element; ( c) cross (c) cross sectional sectional TEM TEM image imageof Ti-2.5Cu of Ti-2.5Cu and distribution and distribution diagram diag ofram the of corresponding the corresponding Cu element. Cu element.

AsAs can can be be seen seen from from the the Cu Cu element element distribution distribution diagram diagram in purple in purple , color, the hypereutectoid the hypereutectoid Ti- 14CuTi-14Cu has hasa darker a darker color color and and long long strip strip structur structure,e, comparing comparing with with the the granular granular structure structure of of subeutectoidsubeutectoid Ti-2.5Cu. Ti-2.5Cu. It It can can be be concluded concluded that that Ti Ti2Cu2Cu phase phase precipitation precipitation increases increases with with the the increase increase ofof copper copper content. content. The The mechanical mechanical behavior behavior of of Ti-C Ti-Cuu alloys alloys depends depends directly directly on on their their microstructural microstructural characteristics,characteristics, especially especially thethe naturenature of of stabilized stabilized phases phases of α -Tiof andα-Ti Tiand2Cu Ti precipitates,2Cu precipitates, which dependswhich dependson the Cu on contentthe Cu andcontent processing and processing routes applied. routes applied. TwoTwo assumptions assumptions have have been been proposed proposed to to explai explainn the the strengthening strengthening mechanism mechanism of of the the Ti-Cu Ti-Cu alloys.alloys. One One is is related related to to the the formation formation of of a alarger larger volume volume fraction fraction of of martensitic martensitic structure structure under under high high coolingcooling rate. rate. It Itis is known known that that the the increase increase of of the the content content for for ββ stabilizingstabilizing element element causes causes hexagonal hexagonal martensitemartensite (α (’α) ’to) to transform transform into into orthorhombic orthorhombic martensite martensite (α”) ( α[24,25],”)[24, 25which], which ordinarily ordinarily occurs occurs in the in alloythe alloyquenched quenched from from high-temperature high-temperature with with differe diffnterent processing processing routes. routes. The The other other is isrelated related to to precipitates strengthened by the Ti2Cu precipitates with various volume fraction, morphologies and distributions [25,26].

In this work, the main results show that the strength of the material is related to the precipitated strengthening effects caused by Ti2Cu precipitates having different volume fractions and morphologies. Higher Cu concentrations contribute to segregation phenomenon of Ti2Cu. A modified Orowan Materials 2020, 13, x FOR PEER REVIEW 10 of 13 precipitates strengthened by the Ti2Cu precipitates with various volume fraction, morphologies and distributions [25,26]. MaterialsIn this2020 ,work,13, 3411 the main results show that the strength of the material is related to the precipitated10 of 12 strengthening effects caused by Ti2Cu precipitates having different volume fractions and morphologies. Higher Cu concentrations contribute to segregation phenomenon of Ti2Cu. A mechanism for Ti-Cu alloys to evaluate Ti2Cu precipitate effects was proposed [27,28] as the modifiedformula, EquationOrowan mechanism (2): for Ti-Cu alloys to evaluate Ti2Cu precipitate effects was proposed [27,28] as the formula, Equation (2): √ GGb b 0.886 d0.886t tt dttt ∆τ =Δτ =  q × In  In (2)  d tdttt  × b b 2π2√π 11−ν 0.8250.825 t t − 0.3930.393d −d0t.8860.886t  tt (2) −  f f − t − t    where Δ∆τ is the increment inin yieldyield strengthstrength for for the the alloy alloy due due to to precipitate precipitate hardening, hardening,b isb is magnitude magnitude of ofBurgers-vector, Burgers-vector,dt is d uniformt is uniform diameter diameter of precipitates, of precipitates,tt is thickness tt is thickness of precipitates, of precipitates,G is shear G modulus,is ,f is volume f is fraction volume of fraction precipitates, of precipitates,ν is the Poisson ν is the ratio. Poisson Considering ratio. Considering that copper elementthat copper will element increase willthe precipitationincrease the precipitation amount of Ti amount2Cu phase, of Ti the2Cu addition phase, the of Cuaddition in this of study Cu in will this leadstudy to will an increaselead to an in increasethe volume in the fraction volume of Tifraction2Cu precipitates, of Ti2Cu precipitates, thus leading thus to an leading increase to inan the increase intensity. in the Furthermore, intensity. Furthermore,the increased strengththe increased shows strength a linear relationshipshows a linear with relationship Cu concentration with Cu due concentration to the difference due in to grain the differencesize and defect in grain density size and which defect can density also aff ectwhich the material’scan also affect strength. the material’s Overall, strength. the predictions Overall, are the in predictionsgood agreement are in with good those agreement obtained with in thethose present obtained work. in the present work. TheThe wear wear scratch scratch morphology morphology of Ti-2.5Cu, Ti-7CuTi-7Cu and Ti-14Cu areare exhibitedexhibited inin FigureFigure8 8..

FigureFigure 8. 8. SEMSEM images images showing showing the the microstructure microstructure and and morphology morphology of of the the worn worn scratch scratch ( (aa)) Ti-2.5Cu, Ti-2.5Cu, ((bb)) Ti-7Cu Ti-7Cu and ( c) Ti-14Cu.

ItIt was foundfound thatthat the the titanium titanium alloy alloy of of Ti-2.5Cu Ti-2.5Cu had had deep deep wear wear marks marks and and obvious obvious adhesive adhesive wear. wear.The surface The surface film would film would break break and present and present a new a metalnew metal surface surface under under the action the action of normal of normal force force and andtangential tangential force force during during the the friction friction process process due due to to the the low low content content of of Ti Ti2Cu.2Cu. ItIt isis knownknown that under under thethe action action of of load, load, there there will will be be a acertain certain amount amount of of wear. wear. When When the the amount amount of ofwear wear accumulating accumulating to ato certain a certain extent, extent, it would it would accumulate accumulate in the in scratch, the scratch, where where larger larger wear wearparticles particles will impact will impact the wear the surfacewear surface and cracks and cracks occur, occur, as shown as shown in Figure in Figure 8b.8 b.With With the the increase increase of of Cu Cu content, content, the the wear depth becomesbecomes shallower shallower due to the increase of Ti 2CuCu phase. phase. Ti Ti22CuCu can can resist resist the the normal normal compressive compressive stress stress ofof the the friction pair pair under under the the condition condition of of friction, friction, and and bear bear a a larger larger load accompanied by by a a little adhesiveadhesive wearwear to to resist resist the the wear wear of the of hard the particles.hard particles. It has been It provedhas been that proved the gradual that accumulationthe gradual accumulationof the Ti2Cu phase of the results Ti2Cu inphase a lower results degree in a oflower wear degree reduction. of wear reduction.

4. Conclusions 4. Conclusions In this work, the effect of different copper contents (2.5 wt.%, 7 wt.% and 14 wt.%) on the In this work, the effect of different copper contents (2.5 wt.%, 7 wt.% and 14 wt.%) on the mechanical properties and tribological properties of titanium alloys was studied. The main conclusion mechanical properties and tribological properties of titanium alloys was studied. The main are as follows: conclusion are as follows: The tensile strength and hardness of Ti-Cu alloys is increased with the increase of copper content. Compared with the tensile strength of Ti-2.5Cu alloy, YS of Ti-14Cu alloy is increased by 30% and UTS

Materials 2020, 13, 3411 11 of 12 is increased by 25%. Ti-7Cu has 22% higher hardness than Ti-2.5Cu alloy, and Ti-14Cu has 43% higher hardness than Ti-2.5Cu alloy. Compared with Ti-2.5Cu alloy, the grinding comparison of Ti-14Cu alloy is increased by 97%. It is proved that the addition of copper improves the wear resistance of Ti-Cu alloys. The elongation of Ti-Cu alloys decreased with the increase of Ti2Cu precipitation amount with the Ti-14Cu alloy exhibiting the lowest elongation. The fracture morphologies of Ti-Cu alloys are intergranular fracture and cleavage fracture, and the dimples appeared in 2.5% and 7% Ti-Cu alloys. Copper as a β-stable element in titanium alloys can effectively promote the precipitation of the Ti2Cu hard phase. The higher the copper content, the greater the precipitation of the Ti2Cu hard phase. EDS results show that Cu concentration at the grain boundary is much higher than that inside the grain. The copper-rich zone at the grain boundary will greatly increase the segregation of Ti2Cu phase at the grain boundary. Ti2Cu phase with copper-rich region and needle-like structure improves the properties of titanium alloys to some extent. This work proves that adding proper amount of copper to titanium alloys can improve their mechanical and frictional properties, which might meet their application requirement in different fields such as biomedical and aerospace.

Author Contributions: Conceptualization, J.J. and Q.Z.; funding Acquisition, Y.X., Y.C. and Y.Z.; investigation, Y.X. and J.J.; methodology, Y.X., J.J., Z.Y. and Q.Z.; project administration, Y.X., Y.C. and Y.Z.; data curation, J.J., Z.Y. and Q.Z.; writing—original draft, Y.X., Z.Y. and Y.C.; writing—review and editing, J.J., Q.Z. and Y.Z. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by Natural Science Basic Research Plan of China (Grant no. 5201019 and 51401033), Natural Science Cooperation Foundation of Shaanxi Province (2016KW-051), China Postdoctoral Science Foundation (2016M590909), the Fundamental Research Funds for the Central Universities (No. 300102318205; 310831161020; 310831163401; 300102319304) and Xi’an Science and Technology Project (2020KJRC0128). Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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