Elsevier

Materials Characterization

Volume 61, Issue 2, February 2010, Pages 135-140
Materials Characterization

Study of particle–matrix interaction in Al/AlB2 composite via nanoindentation

https://doi.org/10.1016/j.matchar.2009.11.002Get rights and content

Abstract

Aluminum diboride (AlB2) particles enhance wear resistance of functionally-graded aluminum-AlB2 composites. A critical factor governing the wear resistance of these composites is the mechanical interaction between the diboride particles and the aluminum matrix. To study this interaction nanoindentation experiments were performed on 3–10 µm size AlB2 particles embedded in the aluminum matrix of an as-received Al–5 wt.%B alloy and a centrifugally cast one. Under large nanoindentation loads (2–8 mN) diboride particles could be pushed into the matrix. The results show that on a per unit area basis, smaller particles are more difficult to push-in than larger particles. Strain gradient plasticity (SGP) theory was used to explain the size dependence of the push-in force.

Introduction

Aluminum matrix composites (AMCs) have been used by automotive and aeronautic industry for a large number of applications due to their light weight, high strength and stiffness, and enhanced wear resistance. The weight reduction achieved by the use of AMCs results in superior fuel economy and higher engine performance [1] and heavier payloads for aerospace applications.

New advanced AMC's can be manufactured as functionally graded materials (FGMs) where volume fraction of the reinforcement phases varies continuously in a specific direction by design. FGMs are useful in applications where high wear resistance and hardness are required in combination with high bulk toughness. This combination of properties may not be readily obtained in homogeneous AMCs [2].

Researchers have been working on FGM-AMCs produced by centrifugal casting, and reinforced with SiC particles embedded in the aluminum matrix [3]. However, SiC/Al composites have shown inconsistent wear resistance. This behavior has been attributed to the weak interface between SiC and the aluminum matrix [1].

Alloys from the Al-rich side of the Al-B system are potentially better candidates because AlB2 coexists in thermodynamic equilibrium with the surrounding Al matrix. As a consequence of this equilibrium, there is no chemical reaction to weaken the interfaces between constituent phases. Additionally, AlB2 dispersoids can act as nucleation sites for solid Al, which enhances the interface and improves load transfer between particles and matrix [4]. The tribological performance of functionally graded Al/AlB2 composites has demonstrated the suitability of these materials for light weight and high wear resistance components [5]. Hard AlB2 particles embedded in the composites promote wear strength, which is critically dependent on the Al matrix–particle interaction.

A better understanding of factors affecting the wear response such as orientation, size, volume fraction, modulus of elasticity, relative hardness, and brittleness of the reinforcing second phase, as well as the matrix alloy properties [6] has contributed to improved tribological properties of FGM-AMCs. However, particle–matrix interaction during the wear process has not been studied in detail. Although the optimum interfacial properties for wear application are not well known, there is a consensus that strong and ductile interfaces are desirable, leading to reduced particle pull out [1].

At the macroscale, the effect of normal load on wear of an Al–Si–Mg composite containing SiC particles (47 μm) has shown that higher normal loads (5 N) increase the wear loss compared to lower normal loads (2 N) [1]. Reinforcement particles in these composites resist scratching because of their high hardness under low loads. However, under heavy loads hard particles can crack, and the broken particles remain embedded in the matrix during the wear process [6]. At the micro- and nanoscale, the pushing of hard particles into an Al matrix may be a useful way of characterizing the particle–matrix interface in relation to wear. Notably, Mahato et al. [7] have recently studied particle push-in silicon–aluminum AMC's.

Some finite element models have described matrix–particle interaction in two-phase materials during indentation [8], [9], [10]. For instance, Durst et al. [8] performed a finite element study of conical indentations in two-phase elastic plastic materials to investigate the influence of the shape and the aspect ratio of particles embedded in a matrix material on the deformation behavior and hardness during nanoindentation.

In the present research, an experimental study of the mechanical interaction between particles and matrix is performed. Instrumented indentation was used to push micrometer-sized AlB2 particles into the aluminum matrix. In our preliminary experimentation we observed that the smaller the particle, the higher the pressure (load per unit area of particle) required to push the particle in. We attributed this size effect to a higher yield stress in the matrix surrounding the smaller particle consistent with strain gradient plasticity theory. Furthermore, particles in the centrifugally cast alloy require a systematically higher pressure to push-in than particles in the as-cast alloy. The latter trend might be caused by an increase in interfacial strength brought about by casting.

Section snippets

Experimental Procedure

The materials used in this research were: a) an Al–5 wt.%B master alloy from Milward Alloys Inc. and b) an FGM produced by centrifugally casting that master alloy [2], [3]. The specimens contained 3–10 μm AlB2 reinforcement particles embedded in an aluminum matrix, as shown in Fig. 1A. The distribution of particles in the FGM and as-received Al-B composites resulted in clusters of particles and particle-free regions. At the macroscale, the FGM contained different volume fractions of particles

Results

Multi-load indents were performed near the center of each particle as shown in Fig. 3A and B. Fig. 4 depicts the resulting load–depth (L–d) curves obtained from indents in three different AlB2 particles. Usually the loading segment in the load–depth (L–d) curve is parabolic (solid line in Fig. 4). We observed two, general deviations from this behavior. If the particle was thin or if the indent was placed near the edge, then the particle fractures, and the load–depth curve contained discrete

Discussion

A likely explanation for the effect of particle size comes from strain gradient plasticity (SGP) theory [14], [15], [16], [17], [18]. Yielding beneath the particle is restricted to the area of the matrix immediately surrounding that particle, as shown in Fig. 7. Therefore, a gradient in plastic strain exists between the region immediately adjacent to the particle and the undeformed region further away. The gradient gives rise to geometrically necessary dislocations, which in turn contribute to

Conclusions

A study of the interaction between the AlB2 particles and Al matrix in which they are embedded was performed to probe the particle–matrix interaction relevant to the wear behavior of Al/AlB2 composites. Nanoindenter was used to push under large nanoindentation loads (2–8 mN) individual AlB2 particles (3–10 µm diameter) into the aluminum matrix in the Al–5 wt.%B as-received and centrifugally cast alloys. On a per unit area basis, smaller particles were more difficult to push-in than larger

Acknowledgements

This material is based upon a work supported by the National Science Foundation under Grants no. DMR 0351449 (PREM Program) and no. DMR 0520527 (MRSEC Program). Aluminum–boron master alloys were kindly donated by Milward Alloys, Inc., Lockport, New York.

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    1

    The authors contributed equally to the paper.

    2

    University of Wisconsin-Madison, M148 Materials Science and Engineering Bldg, 1509 University Ave, Madison, WI 53706, USA. Tel.: + 1 608 262 0125.

    3

    University of Wisconsin-Madison, 2149 Engineering Centers Building, 1550 Engineering Drive, Madison, WI 53706, USA. Tel.: + 1 608 265 4170.

    4

    University of Puerto Rico-Mayagüez, Dept. of Engineering Science and Materials, P. O. Box 9000, USA. Tel.: + 1787 832 4040x2350; fax: + 1787 265 3816.

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