Effect of shotpeening on sliding wear and tensile behavior of titanium implant alloys

Effect of shotpeening on sliding wear and tensile behavior of titanium implant alloys

Materials and Design 56 (2014) 480–486 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matd...

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Materials and Design 56 (2014) 480–486

Contents lists available at ScienceDirect

Materials and Design journal homepage: www.elsevier.com/locate/matdes

Effect of shotpeening on sliding wear and tensile behavior of titanium implant alloys B.K.C. Ganesh a, W. Sha b,⇑, N. Ramanaiah a, A. Krishnaiah c a

Department of Mechanical Engineering, Andhra University, Visakhapatnam, India School of Planning, Architecture and Civil Engineering, Queen’s University Belfast, UK c Department of Mechanical Engineering, Osmania University, Hyderabad, India b

a r t i c l e

i n f o

Article history: Received 26 August 2013 Accepted 22 November 2013 Available online 1 December 2013 Keywords: Shotpeening Microhardness Wear rate Ultimate tensile strength Osseointegration

a b s t r a c t Titanium has good biocompatibility and so its alloys are used as implant materials, but they suffer from having poor wear resistance. This research aims to improve the wear resistance and the tensile strength of titanium alloys potentially for implant applications. Titanium alloys Ti–6Al–4V and Ti–6Al–7Nb were subjected to shotpeening process to study the wear and tensile behavior. An improvement in the wear resistance has been achieved due to surface hardening of these alloys by the process of shotpeening. Surface microhardness of shotpeened Ti–6Al–4V and Ti–6Al–7Nb alloys has increased by 113 and 58 HV(0.5), respectively. After shotpeening, ultimate tensile strength of Ti–6Al–4V increased from 1000 MPa to 1150 MPa, higher than improvement obtained for heat treated titanium specimens. The results confirm that shotpeening pre-treatment improved tensile and sliding wear behavior of Ti–6Al–4V and Ti–6Al– 7Nb alloys. In addition, shotpeening increased surface roughness. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The introduction of compressive residual stress in the surface layer by surface modification technique such as shotpeening can mitigate wear and improve mechanical properties. Shotpeening is a method of cold working in which compressive stresses are induced in the exposed surface layers of metallic parts by the impingement of stream of shots, directed at the metal surface at high velocity under controlled conditions. It differs from blast cleaning (shot blasting), the purpose of which is to clean and remove impurities on the surface. It can also improve the surface roughness to develop the osseointegration of the materials to be implanted. Although shotpeening cleans the surface being peened, this function is incidental. A major purpose of shotpeening is to increase the fatigue life of the components. The immediate effect of bombarding high velocity shots onto a metallic target is the creation of a thin layer of high magnitude compressive residual stress at or near the metal surface, which is balanced by a small tensile stress in the deeper core, as shown in Fig. 1 [1]. When individual particles of shot in a high velocity stream contact a metal surface, they produce light and rounded depressions in the surface, stretching it radially and causing plastic flow of surface metal at the instant of contact. The effect usually extends to about 0.13–0.25 mm, but may extend as much as

⇑ Corresponding author. Tel.: +44 28 90974017. E-mail address: [email protected] (W. Sha). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.11.052

0.5 mm below the surface. The metal beneath the layer is not plastically deformed. In the stress distribution that results, the surface metal has induced residual compressive stress parallel to the surface, while metal beneath has reaction induced tensile stress. This compressive stress offsets any service imposed tensile as encountered in rolling or bending, and improves fatigue life of parts in service markedly. Peening action improves the distribution of stresses in surfaces that have been caused by grinding, machining, or heat treating. It is particularly effective on ground or machined surfaces, because it changes the undesirable residual tensile stress condition that these processes usually impose in a metal surface to a beneficial compressive stress condition. Shotpeening is especially effective in reducing the harmful stress concentration effects of notches, fillets, forging pits, surface defects, and the low strength effects of decarburization, and the heat affected zones of weldments. The magnitude of this compressive residual stress is a function of the mechanical properties of the target material and may reach values as high as 50–60% of the material’s ultimate tensile strength. Its depth is largely dependent on the peening intensity and the relative hardness of the impinging shots and target material. For a relatively soft target material (230–300 HV), it is feasible to produce a compressive layer of 0.8–1 mm, whilst for a harder material (700 HV), it can be difficult to produce a compressive layer of much more than 0.2–0.25 mm. The introduction of this compressive residual stress at the metal surface layer brings one major benefit. It reduces and can negate any residual or

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Fig. 1. Effects of shotpeening [1].

subsequently imposed tensile stress at the metal surface. It is well known that most fatigue failures and stress corrosion failures start at or near the surface stressed in tension. Therefore, by reducing the net tensile stresses at and near the surface of the component, fatigue crack initiation and stress corrosion can be delayed, improving the fatigue life of the component treated [1]. Media selection plays an important role to obtain the desired properties by the process. Many kinds of cast steel shots, cut wire shots, glass beads, and zirconium shots are available with various sizes. Depending upon the amount of pressure exerted through the blast nozzle and the surface being processed, each type of media can achieve different results. The resultant properties produced by the application of this process are almost limitless. Change in a few variables can alter various microstructural and mechanical properties of the peened specimens dramatically. It is important therefore to select the optimal variables after the right combination has been found for consistent and high quality results. There are many ways to deliver the working medium to the surface being treated including compressed air, mechanical and water slurry. The most popular way is compressed air. Air blast is categorized into two methods of media delivery, suction blast and pressure blast. Suction blast systems are selected for light to medium amounts of production and moderate budgets. Suction is not as efficient as pressure, so the range of applications is more limited, but it often yields comparable results. Suction systems have the ability to blast continuously without stopping for media refills. They are also simpler to use and have fewer wear parts, making them inexpensive and easy to maintain. Suction systems work on the principle that air passing over an orifice will create vacuum at that point. This action takes place in the hand held suction gun, with a media hose connected from the vacuum area to media storage hopper. Compressed air is piped into the back of the gun and causes the lifted media to be blown out of a nozzle on the front of the gun. Energy is expended indirectly to lift the media and then mix it with the compressed air, making suction less efficient than a pressure system. Pressure blasting feeds media into the compressed air stream at a pressurized storage vessel. The media then accelerates in the air stream as it is routed by a blast hose to the nozzle. Resulting media velocity is often several times that of a suction system, resulting in a common fourfold increase in production. Direct pressure uses force, rather than suction, so it offers much more control at very high and very low operating pressures. Low pressure is used for

delicate or fragile substrates, such as removing carbon from aluminum pistons or flash from integrated circuits. Direct pressure systems are especially useful for finishing hard-to-reach recessed areas and odd shapes. The shotpeening process has to be precisely controlled and repeatable for optimum benefit. To achieve this, all its process variables must be identified and controlled [2]. There are many fundamental parameters affecting the shotpeening processes. The most common are as follows: (1) (2) (3) (4) (5) (6) (7)

Shot density Hardness and size of the shots Nozzle characteristics (diameter, deflection angle, length) Air pressure Impact angle Exposure time Linear and rotational speed of work piece relative to the nozzle.

To specify all these variables, every shotpeening job would require time consuming investigations and industrially impractical procedures. To overcome this problem, a concept was introduced, of peening intensity measurement based on the curvature induced in a thin test strip, by which most of the listed process parameters can automatically be incorporated into one process variable called the Almen peening intensity. With the peening intensity known, one has to only define the shot type and size and peening coverage desired to fully define the peening process. Despite important progress in understanding the process, some areas are not totally mastered yet and difficulties are still hard to avoid. Being able to predict the effect of process in set conditions is indeed the key to gaining complete control over the process and to making it much more reliable. Surface hardening by shotpeening is one of the upcoming research areas that requires much attention. This process of surface hardening is an important application for improving various mechanical properties which have a poor response to heat treatment process. The application of shotpeening is very vastly studied in terms of improvement of fatigue life for the components working in a cyclic loading environment including biomaterials where the compressive residual stress is induced into the component to prevent crack initiation and propagation. Nowadays this process is also used to improve the microhardness of the peened surfaces.

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This process is more elaborately done for improving surface hardness of 316L stainless steel orthopedic biomaterial and also for improving surface hardness of various aluminum alloys. Only one paper [3] has studied the effects of shotpeening on wear behavior of Ti–6Al–4V alloy. Shotpeening is an effective method of surface treatment for the introduction of residual compressive stress in the surface and subsurface layers and improving the fatigue strength. Surface modifications produced by the shotpeening treatment are (a) roughening of the surface, (b) an increased, near surface, strain hardening and (c) the development of a characteristic profile of residual stress. Considering fatigue damage, surface roughening will accelerate the nucleation and early propagation of cracks, strain hardening will retard the propagation of cracks, by increasing the resistance to plastic deformation and residual stress profile will provide a corresponding crack closure stress that will reduce the driving force for crack propagation. It also in some cases can introduce large amount of defects and interfaces into the surface layers and transform the microstructure of surface to include nano-sized crystals [4]. Shotpeening is one of surface modification processes widely used in industry, inducing severe plastic deformation on surface region of materials. It has been reported that severe plastic deformation on surface region leads to surface nanocrystallization and its following improvements of mechanical, corrosion, wear properties and fatigue strength. It has been suggested that density of dislocations and density and size of precipitates could influence the surface properties due to shotpeening [5]. Cvijovic´-Alagic´ et al. have recently studied the wear and corrosion behavior of Ti–13Nb–13Zr and Ti–6Al–4V alloys in simulated physiological solution [5]. The Ti6Al7Nb alloy belongs to the group of a/b alloys. The microstructure and mechanical behavior of Ti–6Al–7Nb alloy produced by selective laser melting have been investigated by Chlebus et al. [6]. Geetha et al. reviewed Ti based biomaterials, the ultimate choice for orthopedic implants, including Ti–6Al–7Nb Wrought and titanium–aluminum–vanadium (Ti–6Al–4V) alloy [7]. Wear resistance of experimental titanium alloys for dental applications has been studied by Faria et al. [8]. Mechanical properties and biocompatibility of titanium alloys were tested, including a + b alloys (Ti–6Al–7Nb and Ti–6Al–4V). 2. Materials and methods The Ti–6Al–4V material was procured from South Asia Metal Corporation, Mumbai, India. Ti–6Al–7Nb was imported from Boaji Litai Corporation, Baoji, Shanxi, China. The chemical compositions of both alloys are given in Table 1. Wear test was conducted on the above specimens according to ASTM: G-99 specifications on a pin-on-disc tribometer. Three tests were made to arrive at a final reading for each condition. A Ducom TR 20LE pin-on-disc wear testing machine was used, with a linear sliding speed of 1 m/s and a sliding distance of 500 m. The titanium alloy pin materials were tested while rotating on a hardened steel disc which had a hardness of 69 Rc. In this work, a load of 50 N was applied on a pin diameter of 10 mm to obtain a pressure of

0.7 MPa, which was considered to be safe stress acting on the joint during the loading conditions [9]. The above process parameters, including the fast sliding speed, were selected as they were considered to be the conditions for the implants to be working in the actual operating conditions [9,10]. Wear rate was calculated on the basis of volume of material removed from pin while covering a sliding distance of 500 m expressed in m3/m. Hardness was measured by using a Vickers microhardness testing machine under the application of constant load of 5 N. The indentation dwell time was 10 s. Surface roughness of the wear track was measured by using a Mitutoyo SJ 210 surface roughness tester. Surface layer characterization and particle size analysis was conducted by atomic force microscope (AFM). Images were recorded by a multimode Scanning Probe (Ntegra Aura, NTMDT Co, Russia) at ambient condition (25 ± 2 °C) using single crystal silicon N type probes (NSG 03-A) having radius of curvature of 10 nm. The cantilever with long tips (aspect ratio 3:1), with force constants of 0.35–6.06 N/m and resonance frequencies of 47–150 kHz, was used to image the surface morphology. The shotpeening operation was performed according to the SAE AMS2430S [11] standard. The various shotpeening parameters are: type of shot S230, material of shots steel, angle of projection 90°, diameter of shots 0.6 mm, duration of peening 60 s, coverage area 100%. Pressure blast system of shotpeening is primarily used in this work for obtaining good control of the operating parameters, most importantly an appropriate peening intensity for obtaining necessary surface hardening. Various operating pressures from 3.5 to 5.5 bar were used to conduct the peening operation on the titanium alloys. A set of tensile specimens were prepared according to the ASTM: E-8 procedure, as shown in Fig. 2. Tensile specimens were cut from a plate 3-mm in thickness, with a gauge length of 25 mm and gauge width of 6 mm. The cut specimen was fixed in the Almen strip holder between two screws. The cut tensile specimens were first shotpeened when they were rotated with the gripper with the impact of steel shots supplied from a pipe at the required operating pressure and angle of projection. Only one side of the tensile specimens was peened until it was ensured that the entire side was covered with peening action. AFM analysis was conducted on the same, peened side. These specimens were then tested by using a Dak Ultimate tensile testing machine of 50 kN capacity at a speed of 20 mm/min to plot the stress–strain curve for both the shotpeened alloys.

3. Results and discussion 3.1. Wear and microhardness Table 2 shows wear properties of the shotpeened Ti–6Al–4V (Ti64) and Ti–6Al–7Nb (Ti7Nb) alloys at two different pressures. An increase in the wear resistance of the alloys was obtained together with an improvement in the hardness. With increasing pressures, from 3.5 to 4.5 and to 5.5 bar, with 20 s peening duration, the Almen peening intensities are 0.21, 0.32 and 0.42 A, respectively. It should be noted that as the peening pressure was increased, the Almen intensity also increased. The increase in the

Table 1 Chemical compositions, in wt.%, of Ti–6Al–4V and Ti–6Al–7Nb alloys. Element

Ti–6Al–4V

Ti–6Al–7Nb

Ti Al V Nb Fe C

89.6 6.29 3.95 – 0.09 0.029

87.6 5.8 – 6.5 0.037 0.017

Fig. 2. Tensile specimen according to ASTM: E-8 specification.

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B.K.C. Ganesh et al. / Materials and Design 56 (2014) 480–486 Table 2 Properties of shotpeened Ti64 and Ti7Nb alloys under different operating pressures. Alloy

Condition

Surface roughness of wear track (Ra) (lm)

Microhardness HV(0.5)

Wear rate (10

Ti–6Al–4V

No peening Peening 3.5 bar Peening 4.5 bar

0.016 ± 0.002 1.67 ± 0.06 2.27 ± 0.12

326 ± 1 412 ± 47 439 ± 66

2.016 ± 0.110 0.976 ± 0.012 0.907 ± 0.079

Ti–6Al–7Nb

No peening Peening 3.5 bar Peening 4.5 bar

0.609 ± 0.008 1.73 ± 0.10 1.96 ± 0.03

319 ± 1 377 ± 20 352 ± 16

0.974 ± 0.132 0.807 ± 0.012 0.786 ± 0.004

11

m3/m)

Almen intensity could result in the improvement of microhardness up to a specific depth of the peened surface. The high standard deviation values for the results of the peened specimens were due to the errors caused by the rougher surface after peening. However, the increase of hardness values of peened specimens is statistically significant, in all cases. So, it is clear that none of the peened specimens could have the same hardness as unpeened specimens. The same is the case for wear rates, for both alloys, despite the apparent high standard deviation values. Microhardness profile obtained on the cross section of shotpeened specimens failed to reveal obvious and statistically significant trend of variations, but we were only able to measure the hardness in the depth range of 0.1–0.8 mm. It is possible, therefore, that the hardness increase was only significant within a shallow depth, up to 0.1 mm. This does not contradict the surface hardness measurement data (Table 2), because, for 5 N loading, indentation has approximately 70 lm depth. When the Ti64 alloy had been tested at 4.5 bar pressure with increasing time of peening, peening intensities are as shown in Fig. 3. From the figure, it is evident that, at 20 s of peening the alloy with steel shots, a saturation of intensity is reached. There is no improvement of the Almen intensity after 20 s, with no significant change in the Almen intensity reported. The region where hardness is increasing with respect to the depth of the specimen in micrometers could be considered as the region affected by shotpeening and thickness of this layer is proportional to peening pressure. To establish this phenomena, viz., effect of shotpeening up to certain depth of the specimen, a profile of hardness data was collected by measuring the microhardness with respect to the depth of surface layers measured in micrometers. The data collected for shotpeened Ti–6Al–4V specimens at the two operating pressures did not show varying of microhardness values obtained up to a specific depth from the surface. The specimen shotpeened at 4.5 bar had values of 346 ± 8 HV(0.5) up to a depth of 0.8 mm, whereas the specimen shotpeened at 3.5 bar showed hardness values of 347 ± 9 HV(0.5) up to 0.8 mm. While comparing the microhardness values, the alloy Ti7Nb specimens have shown lower microhardness than Ti64 specimens for the same operating conditions, by 2%, 8% and 20% under conditions of no peening, and peened at 3.5 and 4.5 bar, respectively (Table 2). There was no much impact of shotpeening process even with an increase of operating pressure to 4.5 bar. The data presented in Table 2 also clearly indicates that Ti7Nb alloy has responded less strongly to the peening process. There are no

significant changes in the hardness of Ti7Nb up to 0.8 mm from the surface. The specimen shotpeened at 4.5 bar has values of 323 ± 8 HV(0.5) up to a depth of 0.8 mm, whereas the specimen shotpeened at 3.5 bar shows hardness values of 316 ± 16 HV(0.5) up to 0.8 mm. The increase in the surface microhardness of both alloys is instrumental in higher wear resistance of the shotpeened alloy. Doni et al. [12] in their experimental work on wear behavior of cobalt–chromium biomedical alloys have reported that Archard wear rate equation clearly indicates that wear rate is inversely proportional to hardness of the wearing metal. Improvement of wear resistance of the peened specimens shown in Table 2 clearly confirms the effect of surface hardening of the treated alloys due to the application of steel shots at high pressure onto the specimens. Table 2 also shows an improvement of higher than 50% of wear resistance of shotpeened Ti64 alloy as compared to unpeened alloy, whereas smaller improvement of wear resistance is reported for the Ti7Nb shotpeened alloy. Scanning electron microscope (SEM) image of 4.5 bar Ti64 shotpeened specimen in Fig. 4 clearly shows presence of thick serrated coarse wear tracks, which indicates that high hardness of the shotpeened material had developed resistance to abrasive wear when in contact with the steel disc. In relation to wear mechanism, Hovsepian and Münz discussed scanning electron microscopy (SEM) image of pin-on-disk wear tracks, the coarse wear debris generated through the test, and smooth wear track [13]. In contrast, the Ti7Nb alloy peened at the same operating conditions (Fig. 5) shows finer wear tracks as compared to Ti64 alloy, possibly due to its lower microhardness (by 20%, Table 2) values. Previous work found that the wear track has formed as a result of fatigue spallation of individual fine grains [14]. Fatigue spallation contributes to the film wear only in the final stages of testing, with considerable damage.

Fig. 3. Effect of peening time on Almen intensity.

Fig. 4. Wear tracks of 4.5 bar shotpeened Ti64 alloy.

3.2. Tensile behavior The effects of shotpeening on tensile behavior have been studied in the present work. An improvement in the tensile strength is shown. Shotpeening at 3.5 bar has increased the ultimate tensile

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Fig. 5. Wear tracks of 4.5 bar shotpeened Ti7Nb alloy.

strength (UTS) to 1100 MPa, as compared to the UTS of unpeened Ti64 specimen which was 1000 MPa. When the specimens were shotpeened at 4.5 bar pressure, further improvement in UTS was observed (1150 MPa). From this, it can be understood that with increasing peening intensities such as at 4.5 bar pressure, it is possible to improve the UTS. Tensile and percentage of elongation have improved for the shotpeened specimen compared to unpeened specimen. Zhan et al. [15] have reported an improvement of proof stress for a newly developed austenitic steel, S30432, from 240 MPa to 830 MPa and 940 MPa by the application of conventional and dual shotpeening, respectively. In terms of microstructure, they stated that shotpeening can refine the domain size and increase the micro-strain. They further stated that the domain size of 300 nm for untreated austenitic steel was reduced to 90 nm for both shotpeened and dual shotpeened specimens. According to the Hall–Petch equation, the yield strength of the materials is related with the domain size. Smaller domain size gives larger material yield strength [15]. It is also well known that the interaction of high density dislocations can also improve the yield strength of the material. During shotpeening, a large amount of small balls repeatedly impact on the material surface. Cho et al. [16], with their experimental work on AA2024 aluminum alloy, have stated that an improvement of Vickers hardness was noted after conducting the peening action by zinc shots. Their experimental results confirm that both the aluminum and zinc underwent severe plastic deformation followed by phase transformation between the two materials which promoted grain refinement. Such nanocrystallization is instrumental in improving the hardness from 65 to 140 HV. Their experimental work establishes the fact that surface nanocrystallization is a key factor for improving mechanical properties. Feng et al. probed the size and density of silicon nanocrystals in nanocrystal memory device applications, using contact-mode atomic force microscopy (AFM), because they found it hard to detect Si nanocrystals with electron microscopy [17]. Bachand et al. used atomic force microscopy (AFM) to further examine the assembly and transport of nanocrystal CdSe quantum dot nanocomposites using microtubules and kinesin motor proteins [18]. The AFM images of shotpeened tensile specimens at various peening pressures are shown in Fig. 6. These specimens were tested at gauge length of the treated specimens for a maximum scan size of 10 lm and maximum height of 600 nm. A smooth surface was observed for the unpeened specimen with surface roughness of 26.4 nm, whereas shotpeened specimens at 3.5 bar and 4.5 bar pressures have reported surface roughness values of 38.2 nm and 53.2 nm, respectively.

Fig. 6. 3D AFM images of (a) unpeened specimen and specimens shotpeened at (b) 3.5 bar and (c) 4.5 bar.

This increase in the UTS can also be possible when the titanium alloy is solution treated above the beta transus temperature followed by water, air or furnace cooling. Venkatesh et al. [19] have clearly stated that the type of cooling results in the formation of various bi-modal and lamellar microstructures, due to which an improvement in low cycle fatigue strength as well as tensile strength can be obtained. Fig. 7 shows the various stress strain curves obtained for base and solution treated specimens followed by water quenching (WQ) and aging and air cooling and aging. The figure clearly explains that tensile strength of the water quenched specimen is higher, by 25%, as compared to untreated specimen. However, ductility has reduced significantly with the improvement of tensile strength. While comparing the tensile behavior of STA specimens with shotpeened specimens, improvement of UTS can also be undertaken by selecting suitable shotpeening operating parameters. Further, the improvement in tensile strength by shotpeening is accompanied by an improvement in the ductility up to certain

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ment and aging. However, this increase in strength is at the expense of ductility and elastic modulus. In their work on heat treatment of beta titanium alloy such as Ti–29Nb–13Ta–4.6Zr, it was reported that yield strengths as high as 1100 MPa have been attained after long aging treatments, at 450 °C for 48 h. However, this increase occurs at the cost of elongation which has decreased to lower than 3% with an increase in elastic modulus to 85 GPa. Again, the longer aging at higher temperatures results in the formation of omega phase. Therefore, it can be concluded that, compared to the problems encountered in the development of tensile strength and hardness by heat treatment, shotpeening can be the best alternative for enhancing the above properties. 4. Conclusions The wear and tensile behavior of the Ti–6Al–4V and Ti–6Al–7Nb titanium alloys, after shotpeening, has been studied. The main findings are: Fig. 7. Stress strain curves of various heat treated specimens [19].

boundaries, which is a significant development. This improvement is very significant which clearly suggests the potential application of shotpeening for load bearing implants which could improve the fatigue life of the peened specimen but also results in the improvement of the tensile strength and ductility of the specimen. So, it has been observed that there is an increase in the tensile strength. Structuring the grain size to ultra-fine grain size (grain sizes 0.1–0.5 lm) would result in increased yield strength and ductility [20–22]. Koch [20] in his experimental work on copper and various other materials stated that reducing grain size to ultrafine grain size can obtain better mechanical properties such as higher yield stress with good ductility. Meyers et al. [22] reported that grain size structuring was possible by certain processes such as equal channel angular processing (ECAP), where the material is passed between two channels of a constricted die, through which reduction of the billet diameter is expected to develop plastic hardening and grain size remodeling to ultra-fine grain size, by passes through the dies of decreasing diameter. They also suggested that techniques like cryo milling and high pressure torsion (HPT) can also be considered for grain size remodeling. The application of the experimental work can also be important for some of the beta titanium alloys such as Ti–35Nb–7Zr–5Ta (TNZT) alloy which have low modules of elasticity of 55 GPa to compensate stress shielding effect of implant materials, and also have a low tensile strength of 596 MPa. Another problem with metastable beta titanium alloys is that if the amount of beta stabilizers is high in the beta alloys it reduces the martensitic start temperature to below the room temperature, due to which nucleation and growth of alpha phase is restricted and hence metastable beta is retained at room temperature under rapid cooling. During this stage depending upon the composition and heat treatment parameters, precipitation of omega (x) phase is possible. The presence of this phase in titanium alloys causes embrittlement [23]. In general, for limited beta stabilizer content only and depending upon cooling conditions, titanium alloys show only alpha and beta phases. However, if the thermodynamic equilibrium is not reached, metastable phases may be retained at room temperature, mainly, martensitic and x phases. The presently available beta alloys are required to be solution treated for long hours and complex heat treatment cycles to obtain the desired mechanical properties. Rack and Qazi [24], based on their experimental work on beta alloys, have reported that strength of the beta titanium alloys increases with solution treat-

(1) Shotpeening has improved the surface hardness and ultimate tensile strength of titanium implant alloys. Surface hardening in turn greatly enhances their wear resistance. Ti–6Al–4V has responded better to shotpeening as compared to Ti–6Al–7Nb. (2) Effect of shotpeening is more pronounced while considering the tensile behavior of the shotpeened specimens of both alloys. Higher, by 15%, tensile strengths have been obtained, as compared to same composition unpeened specimens of both alloys. (3) Shotpeening improves surface roughness. (4) Shotpeening, as compared to other surface modification techniques for the improvement of various mechanical properties, can be considered as cost effective, time saving with superior advantages such as enhanced wear resistance and strength. References [1] Petit Renaud F. Optimization of shot peening parameters. In: Wagner L, editor. 8th ICSP Proceedings. Wiley-VCH; 2003. [2] Fathallah R, Inlebert G, Castex L. Correrlation of Almen arc height with residual stress in shotpeening process. Mater Sci Technol 1998;14:631–9. [3] Tsuji N, Tanaka S, Takasugi T. Effects of combined plasma-carburizing and shotpeening on fatigue and wear properties of Ti–6Al–4V alloy. Surf Coat Technol 2009;203:1400–5. [4] Azar V, Hashemi B, Yazdi Mahbooteh Rezaee. The effect of shotpeening on fatigue and corrosion behavior of 316L stainless steel in Ringer’s solution. Surf Coat Technol 2009;204:3546–51. [5] Cvijovic´-Alagic´ I, Cvijovic´ Z, Mitrovic´ S, Panic´ V, Rakin M. Wear and corrosion behaviour of Ti–13Nb–13Zr and Ti–6Al–4V alloys in simulated physiological solution. Corrosion 2011;53:796–808. [6] Chlebus E, Kuz´nicka B, Kurzynowski T, Dybała B. Microstructure and mechanical behaviour of Ti–6Al–7Nb alloy produced by selective laser melting. Mater Charact 2011;62:488–95. [7] Geetha M, Singh AK, Asokamani R, Gogia AK. Ti based biomaterials, the ultimate choice for orthopaedic implants – a review. Progr Mater Sci 2009; 54:397–425. [8] Faria ACL, Rodrigues RCS, Claro APRA, da Gloria Chiarello de Mattos M, Ribeiro RF. Wear resistance of experimental titanium alloys for dental applications. J Mech Behav Biomed Mater 2011;4:1873–9. [9] Mujumdar P, Singh SB, Chakraborty M. Wear response of heat treated Ti–13Zr– 13Nb alloy in dry condition and simulated body fluid. Wear 2008;264: 1015–25. [10] Ganesh BKC, Ramanaih N, ChandraSekhar Rao PV. Dry sliding wear behavior of Ti–6Al–4V alloy subjected to various surface treatments. Trans Indian Inst Met 2012;65:425–34. [11] SAE AMS2430S. Shot Peening. Automatic. SAE International. 2012-07-23. [12] Doni Z, Alves AC, Toptan F, Gomes JR, Romalho A, Buciumenu M, et al. Dry sliding and tribocorrosion behavior of hot pressed CoCrMo biomedical alloy as compared with the cast CoCrMo and Ti6Al4V alloys. Mater Des 2013;52: 47–57. [13] Hovsepian PEh, Münz W-D. Synthesis, structure, and applications of nanoscale multilayer/superlattice structured PVD coatings. In: Cavaleiro Albano, De Hosson Jeff Th M, editors. Nanostructured Coatings. Springer; 2006. p. 555–644.

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