Microstructural and mechanical characterization of Al–15%Mg₂Si composite containing chromium

Abstract

The microstructure and mechanical properties of Al–15%Mg₂Si composite containing different Cr contents (0.5 wt.%–5 wt.%) were studied. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis were utilized to study the microstructures and fracture surfaces of the composite. The results revealed that the addition of Cr contents changes the size and morphology of both primary and eutectic Mg₂Si phases. A new intermetallic compound (Al₁₃Cr₄Si₄) was detected through the microstructural studies at higher Cr levels. Adding Cr also made significant raise in the hardness, ultimate tensile strength (UTS) and elongation values of the composite. A slight reduction in tensile properties was seen at high Cr concentrations (>2 wt.% Cr). The study of fracture surfaces of the Al–15%Mg₂Si–2%Cr composite revealed the presence of less broken particles and more fine dimples.

Introduction

Al- and Mg based composites, reinforced with particulates of Mg₂Si have been lately introduced as a new group of particulate metal matrix composites (PMMCs) that introduce some advantages such as low density, suitable wear resistance and excellent castability and much lower costs of production, which make them appropriate candidates to replace steel and iron in the automotive and airplane industries [1], [2]. In situ preparation of Al–Mg₂Si composites seems to be the best way of fabricating such composite since advantages like an even distribution of reinforcing phase; good particle wetting and low costs of production are usually achieved [3], [4]. There have been reports of Mg–Mg₂Si composites produced by mechanical alloying and hot extrusion methods in which the alloys showed interesting tensile properties, but they are rather expensive and noneconomic methods [1], [4], [5]. The Al–Mg₂Si composites have high potential to be used as automobile brake disk material, due to the fact that Mg₂Si intermetallic particles exhibit a high melting temperature, low density, high hardness, low thermal-expansion coefficient, and equilibrium interface. The coarse and rough morphology of Mg₂Si phases in normal as-cast Al–Mg₂Si composites has been found to be the main reason for low ductility observed in these materials. Therefore, composites with predominantly coarse primary Mg₂Si crystals must be modified to ensure adequate mechanical strength and ductility [3]. Several investigations have been carried out to modify the primary and eutectic Mg₂Si phases in the structure of these composites such as rapid solidification [6], [7], mechanical alloying [4], heat treatment [8] and chemical modifications [9], [10], [11], [12], [13], [14], [15]. Some efforts have been focused on the modification of the composite microstructure with the addition of different alloying elements and materials such as Sr [9], P [14], Ce [10], extra Si [13], sodium salt [11] and K₂TiF₆ [12]. Rare earth elements have also been reported to be capable of modifying the primary and eutectic Mg₂Si phases [15]. The influence of different elements such as Li [16], Cu [17] and pure Na [18] on the microstructural evolution and mechanical properties of Al–Mg₂Si composites have been fully studied. Yet, the finest size of primary Mg₂Si particles reported in the literature is about 6 μm [13].

The influence of microstructural refinement on mechanical properties of PMMCs has been studied and the results confirmed the expectation that the finer the particle size, the higher the strength of the composite [19]. Zhang et al. have investigated the effects of a salt mixture and extra Si on tensile properties of Al–Mg₂Si composites [11], [13]. Although no substantial difference was found in ultimate tensile strength (UTS) by adding salt mixture, tensile strength values were increased with increasing Si content up to 8 wt.% in the composite. Previous studies resulted that Li addition in trace levels has enhanced the tensile strength and elongation values of Al–15%Mg₂Si MMC to some extent [16].

The aim of the present study is to evaluate the hardness and tensile properties of Al–15%Mg₂Si composite and fracture characteristics of the composite by adding different concentrations of Cr.