Effect of AlB₂–Mg interaction on the mechanical properties of Al-based composites

Abstract

A series of Al-based composites with a matrix containing 2.5 wt.%Cu and 1 wt.%Mg, and reinforced with Al diborides was manufactured and characterized via microscopy and mechanical testing. The impact resistance of the composites revealed interaction between AlB₂ dispersoids and Mg originally present in the Al matrix. An unexpected increase in the absorbed impact energy of composites with diborides prompted complementary experiments leading to this finding. Hardness tests were correlated to those results and provided further evidence of the interaction between Mg and the dispersoids. Additional energy dispersive spectroscopy (EDS) analysis and X-ray diffraction tests supported with crystal modeling demonstrated that Mg effectively diffused into the AlB₂ crystal producing a doped structure with an Al_1−xMg_xB₂ formula where x lies between 0.08 and 0.15.

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.