Tribology International 66 (2013) 182–186

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Tribology International

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Microstructure–wear behavior correlation on a directionally solidifiedAl–In monotectic alloy

Emmanuelle S. Freitas a, José E. Spinelli b,n, Luiz C. Casteletti c, Amauri Garcia a

a Department of Materials Engineering, University of Campinas, UNICAMP, 13083-970 Campinas, SP, Brazilb Department of Materials Engineering, Federal University of São Carlos, UFSCar, 13565-905 São Carlos, SP, Brazilc Department of Materials, Aeronautical and Automotive Engineering, University of São Paulo, USP, 13566-590 São Carlos, SP, Brazil

a r t i c l e i n f o

Article history:Received 21 March 2013Received in revised form10 May 2013Accepted 15 May 2013Available online 21 May 2013

Keywords:Al–In alloySolidificationMicrostructureWear

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esponding author. Tel.: +55 16 3351 8512; faxail address: spinelli@ufscar.br (J.E. Spinelli).

a b s t r a c t

Al–In monotectic alloys are potential alternatives for application in the manufacture of wear-resistantautomotive components, such as cylinder liners and journal bearings. The comprehension regarding thedevelopment of distinct microstructures of monotectic alloys and their interrelation with wear behaviorare challenges of prime importance. The present study aims to contribute to a better understanding ofthe relationship between the scale of the minority phase of the monotectic microstructure and thecorresponding wear behavior. Transient directional solidification experiments were carried out with anAl–5.5 wt% In alloy with a view to provide samples with significant differences in the microstructuralscale along the casting length. The results of wear tests permitted an experimental quantitativeexpression correlating the wear volume (V) with both the interphase spacing between indium droplets(λ) and time of wear tests (t) to be proposed. The increase in λ is shown to improve the wear resistance.The effect of λ on V becomes more significant as the sliding distance (or time) is increased.

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1. Introduction

Aluminum base immiscible monotectic alloys such as Al–Pb,Al–Bi and Al–In having the minority phase typified by fibrous and/or droplet-like shapes have significant potential for practicalapplications. These include self-lubricated bearings, electricalcontact materials and the fabrication of porous materials [1–5]. Alowmodulus of elasticity (E) is one of the requirements for bearingalloys, and from these soft metals (Pb, Bi and In), which are alloyedwith Al, In has the lowest E [6].

Indium when combined with aluminum forms an immisciblealloy system characterized by a monotectic reaction (L14αAl+L2)for a composition of 17.3 wt% In at a temperature of about 910 K.The resulting microstructure is formed by an Al matrix with adispersion of In embedded particles [7–9]. The magnitude anddistribution of these particles will depend on the rate of displace-ment of the solid/liquid interface during solidification and themovement of such particles, which can eventually be entrapped bythe growth front. Silva et al. [7] analyzed the microstructuralevolution of a hypomonotectic Al–In alloy and reported that forgrowth rates (v) higher than 0.95 mm/s, the Al matrix had acellular morphology with In particles of different sizes remainingin the intercellular regions. On the other hand for vo0.95 mm/s

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the microstructure was characterized by In droplets disseminatedinto the Al matrix. Ozawa et al. examined the microstructure ofmonotectic (Al–17.3 wt% In) and hypermonotectic (Al–20 and25 wt% In) Al–In alloys solidified in a continuous casting setup[8]. These authors reported a very homogeneous structure of Inspheroids in the Al matrix, which increased with the increase in Inalloying. Liu et al. [9] studied the microstructure development ofan Al–In alloy of monotectic composition (Al 17.5 wt% In) duringrapid solidification in a melt spinning apparatus. These authorsreported that the as-solidified microstructures were characterizedby a homogeneous distribution of nanometer sized In particlesembedded in the Al matrix.

Recent studies pointed out the effect of the grain size and of thescale of microstructure parameters of metallic alloys, such as thecellular, dendritic and interphase spacings, on the resultingmechanical, corrosion and wear resistances [10–16]. Hall–Petchtype correlations have also been recently proposed describing thedependence of microhardness on the cellular and primary den-dritic arm spacings [17,18]. The effects of microstructural defectssuch as porosity [19] and of mechanical and heat treatments ofalloys on the resulting wear behavior have been investigated inrecent studies [20,21]. Despite the potential for the use of Al–Inalloys in tribological applications, detailed studies on the interac-tion between wear and microstructure of these alloys cannot befound in the literature.

The present study aims to contribute to a better understandingof the effect of the size and distribution of the minority phase of an

E.S. Freitas et al. / Tribology International 66 (2013) 182–186 183

Al–In monotectic alloy, represented by the microstructural inter-phase spacing (λ) and the corresponding wear behavior. Transientdirectional solidification experiments were carried out with a viewto provide a range of interphase spacings for micro-abrasive weartests. A correlation between the wear volume and the timecorresponding to the sliding distance of wear tests and λ is aimed.

2. Experimental procedure

The vertical water-cooled directional solidification (DS)apparatus used in the experiment allows unsteady-state heatflow conditions to be attained. This experimental setup has beendetailed in a previous article [22].The DS experiment wascarried out with an Al 5.5 wt% In alloy and samples wereextracted along the casting length at different positions from

Fig. 1. Schematic diagram of the sliding wear tester.

P=50

P=70

Fig. 2. Microstructures of the Al 5.5 wt% In alloy at different positions along the castingcorroded Al matrix, and (b) optical micrographs.

the cooled bottom corresponding to the range of growth rates(vo0.95 mm/s) that is conducive to microstructures formed byIn droplets disseminated in the Al matrix [6]. Longitudinal andtransversal samples were extracted along the casting length.The longitudinal samples were electropolished and etched witha solution of 0.5% HF in water to reveal the microstructure.Image processing systems were used to measure the diameter(d) of the In droplets and the interphase spacing (λ), which wasdetermined by averaging the horizontal distance between thecenter of adjacent In particles (about 50 readings for eachexamined position in casting). The microstructure was alsocharacterized by scanning electron microscopy.

The transverse samples were used in the micro-abrasive (ballcrater) wear tests in order to analyze the effect of λ on the wearvolume (V). A schematic presentation of the used wear tester isshown in Fig. 1. During the tests, a hard spherical bearing steel ball(AISI 52100, diameter of 25.4 mm and hardness of 818 HV) wasrotated against the sample, producing a wear crater. The ball isdriven directly by clamping the ball in a split drive shaft. Thesample is pressed into the rotating ball from the side by test loadsplaced on the weight hanger. As the test duration (number ofrotations or sliding distance) increases the size of the craterincreases. The used ball sliding speed was 0.33 m s−1 (260 RPM)and the applied normal contact load was 0.2 N. The wear volume Vwas calculated as follows, where d is the crater diameter, and R theball radius [15]:

V ¼ πd4

64Rð1Þ

The diameter was measured at least four times for each wearcrater. The tests were carried out under dry sliding conditions toprevent any interfacial element from causing influences on thefeedback of the microstructure [23,24].

mm

mm

length: (a) SEM images of samples evidencing non-corroded In particles and the

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3. Results and discussion

Fig. 2 shows typical microstructures of Al 5.5 wt% In samples atdifferent positions (P) from the cooled bottom of the casting. It canbe seen that these microstructures are characterized by In dropletsembedded in the Al matrix, as shown by the optical images on theright side of Fig. 2. The left side exhibits images of samples aftercorroded by an acid solution, highlighting the non-corroded Inparticles.

Fig. 3 depicts the experimental values of wear volume, V, as afunction of the sliding distance during the wear tests for samplescollected at three different positions (P) from the casting cooledsurface (P¼50, 60 and 70 mm). It can be seen that the wearvolume tends to decrease with the increase in P. The significantdifferences in the microstructures at such positions are associatedwith the scale of microstructural parameters, which can becharacterized by the diameter of the In droplets, d, and theinterphase spacing, λ.

Fig. 4 relates λ and d with the position, P, along the castinglength from the cooled bottom. A trend of increase in λ and d with

200 300 400 500 600 700 800 900

0.020.040.060.080.100.120.140.160.180.200.220.24

Wea

r V

olum

e (m

m3 )

Sliding Distance [m]

P = 50mm P = 60mm P = 70mm

Fig. 3. Wear volume as a function of the sliding distance during wear tests of Al5.5 wt% In samples collected in the casting at different positions (P) from thecasting cooled surface.

Fig. 4. Evolution of λ (a) and d (b) with P along the casting l

P can be observed, i.e., the In particles tend to coarsen. As reportedin the literature, the motion of particles during the growth ofmonotectic alloys can cause a quick separation of the two liquidphases and a coarsening effect due to collisions and coagulationsbetween droplets [25].

The sliding behavior of bearing alloys depends on the amountof the soft phase in the microstructure, which confers appropriateantiscoring and antifrictional characteristics. The soft phase acts asa solid lubricant as it spreads on the surface of a harder material,since metallic contact between the moving surfaces can occureven in the presence of a lubricant [26,27].

The experimental results of V as a function of λ are depicted inFig. 5a for different times during the wear tests, corresponding tothe sliding distances during the wear tests of Al–5.5 wt% In alloysamples. The points are experimental results and the lines repre-sent empirical fits to the experimental points. It can be seen thatthe wear volume decreases with the increase in λ, and it is alsosignificantly affected by the test time, t. Fig. 5b presents anexperimental law which combines the two variables (λ and t)affecting the wear volume. The lubrication effect of the soft Inareas seems to be improved for coarser microstructures. The slopeof the curves tends to increase with the increase in test time,indicating that the effect of λ on V becomes more significant as thesliding distance (or time) is increased. It seems that indiumdroplets of higher diameter (and hence higher λ) are capable ofproviding a more extensive and continuous film thickness, enhan-cing the lubrication effect. A previous study on the influence of thescale of the microstructure, typified by dendritic spacings, on thewear behavior of Al–Sn and Al–Si alloys, reported a similarbehavior for Al–Sn alloys and an opposite behavior for Al–Si alloys[15]. Si was shown to act as a reinforcement of the Al-rich ductilephase, while Sn favors the interfacial layer during the wear processacting as a lubricant.

The present experimental observations correlating V with theinverse of the square root of λ permitted an experimental equationto be derived, which can be applied to analyze the trend of thewear volume as a function of the interphase spacing and test time

ength, and SEM image for a specific position in casting.

Fig. 5. (a) Wear volume as a function of λ and (b) wear volume as function of λ and time (t), 10, 20 and 30 min corresponding to the sliding distances 205, 410 and 820 m,respectively during the wear tests.

Spectrum O Al Fe In Total

Spectrum 1 7.22 49.37 43.41 100.00

Spectrum 2 39.16 57.17 1.39 2.28 100.00

Max. 39.16 57.17 1.39 43.41

Min. 7.22 49.37 1.39 2.28

Fig. 6. Secondary electron image of the worn surface and composition (EDX)of In-rich (Spectrum 1) and Al-rich (Spectrum 2) areas, evidencing the action of thesolid lubricant (left area) which has been spread by the rotating ball.

E.S. Freitas et al. / Tribology International 66 (2013) 182–186 185

(sliding distance). This can also be done for other importantmonotectic bearing alloys having a soft minority phase, such asAl–Pb and Al–Bi.

Fig. 6 shows the typical deformation of the alloy microstruc-tural arrangement that resulted at the surface of a sample after a30 min wear test in air. The deformed In-rich layers (left side)behave as a solid lubricant on the friction surface. The left side areaof Fig. 6 corresponds to the deformation of In droplets that werespread, resulting in an aligned distribution of this minority phase.The Al ductile phase (right side) is also subjected to distortion. Theresulting damage pattern suggests a behavior typical of adhesivewear. In spite of being insoluble in each other, mutual solubility ofmetallic pairs is not a requirement for adhesion. Landheer et al.showed that mutually insoluble metals can also adhere to eachother strongly [28]. Relative sliding between the contact surfacesof aluminum and indium caused rupture of the formed junctions,since indium has been transferred to the surface of aluminum.Material transfer is representative of surfaces worn by adhesive

wear [2]. Indium has a lower strength than that of aluminum(modulus of elasticity and Brinell hardness of about 11 GPa and8.8 HB; and 70 GPa and 245 HB, respectively [29]) and is predo-minantly ruptured due to its lower strength. Considerable amountof Fe is incorporated into the alloy caused by sliding frictionbetween the bearing steel ball and the alloy surface during thewear test. As a consequence, high Al and Fe oxide areas developed,which can be inferred from the composition data depicted in thetable of Fig. 6—Spectrum 2. On the other hand, the protection ofthe Al-rich matrix by the In minority phase seems to inhibit oxideformation, as shown by the composition data corresponding toSpectrum 1.

4. Conclusions

The present experimental correlation between microstructuralfeatures and wear behavior of samples extracted from a direction-ally solidified hypomonotectic Al–In alloy casting has shown thatthe wear volume decreases with the increase in the interphasespacing and diameter of In droplets, and it is also significantlyaffected by the test time. An experimental law combining twovariables affecting the wear volume, the interphase spacing andthe test time (sliding distance), is proposed. The lubrication effectof the soft indium areas was shown to be improved for coarsermicrostructures.

Acknowledgments

The authors acknowledge the financial support provided byFAPESP (The Scientific Research Foundation of the State of SãoPaulo, Brazil) and CNPq (The Brazilian Research Council).

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