Effect of Silver Content on the Wear and Mechanical Properties of Powder Metallurgical Ti-5Al-2.5Fe-xAg Alloys

In the current research, the effect of Ag on the mechanical properties of Ti5Al2.5Fe alloy has been investigated. The Ti5Al2.5Fe alloy, with different amounts of Ag ranged from 1 to 5 wt. % was prepared by mechanical mixing and then fabricated by hot pressing at 950 o C for 15 min under 50 MPa. Three holding steps were applied to the powder compacts to restrain the liquid phases inside graphite die before reaching the maximum sintering temperature. The sintered samples were subjected to hardness, bending and wear tests to study the effect Ag on the mechanical properties of Ti5Al2.5Fe alloy. The microstructural characterization was carried out by means of optical and scanning electron microscope. The results showed that Ag played a differential role on the mechanical properties supported by microstructural constituents. The bending strength and hardness of the produced samples increased with the addition of Ag, the hardness of the alloys then tends to decreased with increasing Ag content but still remained above the hardness of Ti5Al2.5Fe alloy. Wear test also showed similar trends with hardness test results. Finally, the optimum Ag content for the Ti5Al2.5Fe alloy has been determined as 1 wt.%. XRD analysis showed that unsolved Ag content was the main reason for the decrease in the mechanical properties.


Introduction
Low density and high mechanical properties make Ti alloys one of the most preferred materials in military and civil applications [1]. Beside these advantages, high corrosion resistance and biocompatibility properties are other ideal properties to create orthopedic prostheses, orthodontics and joint replacements with Ti alloys [2]. However, there are some restrictive reasons in the production of Ti alloys such as high melting point and high chemical reactivity to oxygen and nitrogen at high temperatures. The applied possible solutions are to change the production technique or decrease melting temperature by alloying with other elements [3]. Ti alloys generally are produced by casting, forging and powder metallurgical techniques. Powder metallurgical production techniques of producing Ti alloys provide advantages such as faster and near net shape production cycle [4].
Recently, many studies have been carried out on titanium alloys containing silver. The addition of Ag on the titanium alloys has an intensive effect on both mechanical and antibacterial properties. Ti-Ag alloys exhibit better mechanical properties than pure Ti [5].
Wen et al., has studied Ti-Nb-Ag alloys, produced by powder metallurgical techniques and observed that Ag precipitation around β phase significantly affects the fracture characteristics of the alloy [6]. Xiao et al., reported that Ag addition in Ti-5Al-4Mo-4V alloy has increased the relative density and compression properties [7]. Takahashi et al., investigated mechanical properties of 22.5, 25, 27.5 and 30 wt.% Ag addition in Ti and they announced that a great enhancement in hardness and tensile strength occurred because of precipitation of Ti2Ag and TiAg intermetallics [8].
CP-Ti, known as one of the most important biomedical material, exhibit poor wear properties [9]. Therefore, it is suggested from many researchers that alloying of Ti with copper and silver can increase tribological properties, while antibacterial properties still remain [10-13].
In our previous study [14] we investigated different content of Cu addition in Ti-5Al-2.5Fe and the results showed that Cu addition affects mechanical properties in a small range whilst still showing good antibacterial properties and cytotoxicity. In this study, the effect of Ag addition in Ti-5Al-2.5Fe alloy was studied. Hot pressing was used to consolidate initial powders. This technique, as a fast sintering route of powder metallurgy, provides some advantages such as better mechanical properties and easy control the intermetallic formation in a shorter time at lower temperatures [15]. The sintered samples were examined and the results discussed in detail.

Materials and Experimental Works
The scanning electron microscopy images (SEM) of the commercially pure titanium, aluminum, iron and silver powders used in the current study are shown in Fig. 1   To investigate the effect of the Ag content on the wear resistance of the Ti5Al2.5Fe alloy, wear tests were performed at room temperature under dry sliding conditions. Dry sliding wear tests were conducted using a ball-on-disk setup (Nanovea MT/60/NI) according to the ASTM G99 standard against 52100 steel ball bearings (6 mm in diameter) as a counter-body in the tests. The sliding speed of 0.13 m s -1 and distance of 500 m were kept constant during all the sliding tests. The applied load used in the wear tests was 25 N.
The equation below was used to obtain the wear rate of the specimens: where W is the wear rate (mm 3 m -1 ), M denotes mass loss (g), and ρ (g.mm -3 ) and D (m) are the density and sliding distance, respectively [19]. SEM type JEOL 6060 was used to investigate the worn and fracture surfaces of the sintered samples.   Ti2Al5 (ICSD-98-010-6253). Jia et al. [20] repported also the presence of the detected phases such as α phase, Ti3Al and a small amount of transformed β phase in the Ti-5Al-2.5Fe alloy sintered at 1250 °C. With addition of 1 wt.% Ag, pure Ag (ICSD-98-005-6269) and

Results and Discussion
intermetallic Ag3Al (ICDD-00-028-0033) were detected and Ti2Al5 dissapeared in 22 and 24° angles but formed again in 62° angle. Results showed that the addition of Ag increased β phase in the alloy and unresolved pure Ag was detected. Observation of pure Ag phase indicates that reaction between Ag and other elements was not completed. Increasing Ag addition to 3 and 5 wt.%, did not change the types of phases but they were appeared in new angles. Xiao et al. [7], examined Ti-5Al-4Mo-4V alloy with addition different content of Ag.
The XRD results showed that the addition of Ag increases β structure in the alloy and it affects mechanical properties. Moreover, Daoush et al. [21], confirmed that increasing content of β phase can be attributed to the lower hardness results. In addition, the presented microstructure in Fig. 3 shows that the Ag addition to Ti-5Al-2.5Fe alloy increase the amount β phase, which is in agreement with the results of the XRD analysis.
Intensity (a, u)   5 shows the density measurement results as a function of Ag content added to Ti-5Al-2.5Fe alloy. The density of silver is relatively higher than titanium and its alloys. Therefore, the final density values of the produced compacts have increased gradually with increasing Ag content. This result is similar to Xiao et al. [7] study as they added 0, 2, 5 and 10 wt.% Ag to Ti-5Al-4Mo-4V alloy. They found that Archimedes' density increased with the addition of increasing content of Ag. Kikuchi et al. [22], showed also that addition of increasing amounts of Ag (5, 10, 20 and 30 wt.%) has increased the final densities of the material. In this study, the densities of the produced compacts have a good correlation with theoretical densities of the desired compositions. In addition to the final densities of the produced alloys, there is a small decrease that has been observed in the relative densities, this may be due to the change of the crystal structure of the titanium alloy from HCP to BCC and may be due to the formation different intermetallics within the final compact after sintering. Silver is also a beta stabilizing element in titanium alloys. HCP alfa (α) titanium has densely packed atoms compared to BCC titanium atoms [23].  Their results showed that small reductions in hardness observed between 3.5 and 5 at.% Ag addition. Figure 6. Hardness measurement of Ti-5Al-2.5Fe-xAg. Fig. 7 shows the bending tests results. The results show that the addition of Ag has increased bending strength and displacement. This result can be correlated to the hardness results [26].
In the reference sample, bending result showed the lowest (1190 MPa) bending strength and Ti-5Al-2.5Fe-5Ag showed the highest bending strength and displacement (1750 MPa and 1.3 mm). Similar to XRD results, that could be the reason for the increasing appearance of β phases in the structure [27]. It could also be described with hardness results, which have continuously reduced with Ag addition, whereas bending strength and elongation have improved. Figure 7. Bending strength-deflection profiles of Ti-5Al-2.5Fe and Ti-5Al-2.5Fe-xAg alloys.
SEM images of fracture surfaces after bending test can be seen in Fig. 8. Fracture surface of the Ti5Al2.5Fe alloy as a reference sample has consisted of brittle and ductile fracture modes.
Bright areas showing brittle fracture and dimples showing ductile behavior can be clearly seen on the Fig. 8a. It was found that the main fracture mechanism for the reference samples is a transgranular fracture. Similar mixture of fracture modes was reported in literature for Tibased alloys [28,29]. Brittle behavior of the reference sample was also supported by the bending test results, as shown in Fig. 7, showing lower displacement compared to the Ti5Al2.5Fe-xAg alloys. The addition of silver to the reference alloy changed the fracture mode from mainly brittle behavior to the combination of brittle and ductile fracture (Fig. 8bd). Planar faces and small dimples are seen on the Fig. 8b while elongated dimples can be seen on Fig. 8c-d. Fig. 8b, shows that 1 wt.% Ag addition caused more homogenous distribution of equiaxed dimples between cleavage facets, compared to reference sample.
Increasing the amount of Ag addition to 3 wt.% has enhanced cleavage facets and homogenous distribution in the structure. The sample with 5 wt.% Ag addition exhibited more rough dimples and less cleavage facets. According to Shi et al. [30], the main difference between fracture characteristic is related with plasticity. Figure 8. SEM images of fractured surfaces after bending test of (a) Ti-5Al-2.5Fe, (b) Ti-5Al-2.5Fe-1Ag, (c) Ti-5Al-2.5Fe-3Ag, (d) Ti-5Al-2.5Fe-5Ag alloys Fig. 9 shows the wear rates of produced samples. Ti-5Al-2.5Fe showed the lowest wear rate among the sintered samples. With the addition of 1 and 3 wt.% Ag, a small increase in wear rate was observed. However, the addition of more Ag content (5 wt.%) resulted in a reduction in the wear rate. It is important to note that; highest wear rate was observed in Ti-5Al-2.5Fe-3Ag alloy which showed the most unresolved Ag peak, according to XRD results. As is mentioned before, hardness results of 1 and 3 wt.% Ag added samples were the same, but there is big difference in wear rate between the two samples. This is in coincide with the research of Lee et al. [31], who confirms that there is not a direct relationship between hardness and tribological properties. Similar results were also reported about the hardness/wear relationship [13,32]. Such phenomenon can be explained by the fact that wear process is a complex combination of multiple factors while the surface hardness presents only one of those factors. Figure 9. Variation of wear rate according to Ag addition in Ti-5Al-2.5Fe alloy SEM images of worn surfaces of the produced alloys are shown in Fig. 10. Typical adhesive and abrasive wear mechanisms were dominant in all samples. In Fig. 10a, abrasive wear indicating grooves and adhesion, which caused material transfer, were observed in the sliding direction. Sampiao et al. [33] suggested that the main reason for the groove interruption could be due to the formation of abrasive particles caused three-body abrasion. Based on that study, the newly created abrasive particles roll on the surface and scratch it in a limited zone. Fig.   10b consists of long abrasive grooves between ridges; material transfer caused debris and plowing zones. Lee et al. [34], defined that plowing mechanism consisted of pushing material to the side of the grooves during sliding. Fig. 10c shows noticeable amount of wear debris accumulation, which are most wear loss according wear rate results given in Fig. 9. High wear rate zones and deep abrasive grooves were observed from Fig 10d, which show that both the abrasive and adhesive mechanisms worked instantaneously. Moreover, Yang et al. [35], found materials transfer in a very small load condition such as 5N during investigation the wear properties of a Ti-6Al-4V alloy. Figure 10. SEM images of worn surfaces of (a) Ti-5Al-2.5Fe, (b) Ti-5Al-2.5Fe-1Ag, (c) Ti-5Al-2.5Fe-3Ag and (d) Ti-5Al-2.5Fe-5Ag.

Conclusions
1. The sintered Ti-5Al-2.5Fe-xAg alloys exhibited a typical basket weave (α+β) structure in all the samples after production process 2. Addition of 1 wt.% Ag has increased hardness but the addition of more Ag caused a small decrease in hardness results.
3. A significant improvement in bending strength was obtained with Ag addition.
5. The elongated dimples structure of the facture behavior of the alloys increased with increasing silver content in the alloy.
6. Wear rate increased with 1 and 3 wt.% Ag addition, however 5 wt.% Ag addition showed a slight decrease. This result has been associated with the occurrence of unresolved Ag in the structure and these results were supported by the XRD analysis.