Effect of AlB2–Mg interaction on the mechanical properties of Al-based composites
Introduction
Aluminum-based composites are used in structural applications when the strength/weight ratio is a requisite, as in airplane structures and automobiles chassis. These materials have attractive mechanical properties with an almost isotropic behavior when the reinforcing particles are uniformly distributed.
A new series of cast aluminum matrix composites (AMCs) reinforced with diboride particles were fabricated with a matrix containing Mg and Cu (0–5 wt.%). The presence of Cu in such range allows these composites to precipitation hardened while Mg dissolved in the matrix increases its hardness by solid solution strengthening.
The present work seeks to characterize the effect of boron on the impact resistance and hardness of those diboride-reinforced composites constituted with an Al matrix containing 2.5 wt.%Cu and 1 wt.%Mg. Boron levels of 0, 1, 2, 3 and 4 wt.% B were selected to be studied. The boron level is constrained by the boron present in the master alloy used (Al–5 wt.%B) where this element is present in the form of aluminum boride particles Prior studies of Al–Cu–B composites showed that AlB2 particles embedded in the matrix contributed to effectively increase the material stiffness [1], [2]. The same result was obtained when diboride flakes were used as reinforcements in a similar system [3]. Nevertheless, no prior studies addressed the presence of Mg in the matrix (1 wt.%Mg) and its potential interaction with the dispersed phase present in these composites. In effect, the literature reports a large solubility between AlB2 and MgB2, which hints about an interaction between the diborides and the Mg present in the matrix [4], [5]. This phenomenon was studied for the first time in the present work.
Section snippets
Experimental procedure
The composite specimens were prepared by melting carefully measured weights of Al–Cu, Al–Mg and Al–B master alloys using a mass balance calculation, where pure aluminum (99.99%) accounts for the remaining mass. The composition of the master alloys were: Al–5 wt.%B (with boron forming AlB2), Al–5 wt.%B (with boron forming AlB12 and AlB2), Al–33.2 wt.%Cu and Al–10 wt.%Mg.
The diborides remained solid in the liquid matrix at the processing temperatures (max. 750 °C) and are known to be stable phase at
Analysis of results
Fig. 1 presents the absorbed impact energy of the first and second group of composites as a function of boron percent. In this figure, while in samples containing AlB12 + AlB2 the absorbed impact energy increased with the amount of boron, in samples with only AlB2 dispersoids there was no significant variations in that measured energy. However, when the level of boron fell below 2 wt.% there was no significant difference between samples of these two groups.
At this point, it is important to
Conclusions
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Interaction between AlB2 and Mg occurs at casting temperatures as low as 750 °C and a process of substitutional diffusion could be the responsible for such interaction.
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The casting process of these composites containing Mg caused substitutional diffusion of Mg into the AlB2 dispersoids.
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Three different approaches permitted estimating the fraction of Mg (x value) in the formula of doped diborides (Al1−xMgxB2) as ranging between 0.08 and 0.15 for composites containing only AlB2 particles and around
Acknowledgments
The authors would like to thank Lilia Olaya and Boris Renteria for their collaboration in the use of X-ray diffractometry. The authors would also like to express their gratitude to Alexis Torres, Victor Estrella, Nayomi Plaza, Giovanni Sandoval, Edwin Olán, Carlos Cosme and Carla Príncipe, all UPRM undergraduate students, for their help in the sample preparation. The Al–B master alloys were kindly donated by KB alloys Inc. Robesonia, PA, USA. This material is based upon work supported by the
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