Effect of Cooling Rate on Microstructural and Microhardness Properties of Al-(Mg2Si + Al3Ni) Matrix Composite

Among the high-tech industries like automotive, aerospace, electronics, etc., aluminum matrix cast composites (AMCCs) are widely applied for the fabrication of accountable and especially acute pieces. During the present study, hybrid aluminum base composites containing Mg2Si and Al3Ni particles were fabricated successfully in casting moods and their structural characteristics were evaluated under different solidification conditions. A variety of microstructural measurements were performed on the composite microstructure in this study, including X-ray diffraction (XRD) and optical microscope (OM). Furthermore, a hardness test was conducted to evaluate the mechanical properties of the material. Results indicate that increasing the cooling rate during solidification reduces the average size of the Mg2Si initial phases, improves their distribution uniformity and increases their final amount whereas the average size of the Al3Ni particles decreases greatly but their content remains the same. In comparison to base alloys, hybrid composite with Mg2Si and Al3Ni particles shows the highest hardness.


INTRODUCTION
One of the most significant issues in materials science and engineering is the manufacture of materials with predicted properties. Currently, standard alloys cannot compete adequately with advanced structural and functional materials in terms of their mechanical and useful properties [1,2]. It is possible to achieve such goals by using aluminum (Al) matrix composites (AMCs) containing particles of oxides, carbides, silicides, borides, and other refractory materials [3]. This characteristics, and volume or weight percent of reinforcing phases. Depending on the requirements, it can produce structural, heat-resistant, antifriction, electrotechnical, and other useful materials with bold properties.
In addition to the low degree of realization of the physicomechanical properties of the second phase in the matrix, the technological challenges that limit the wide application of AMCs are the most significant factors in limiting the wide application of AMCs in the industry. This problem is primarily caused by poor wettability of the reinforcing particles by the matrix melt [3]. AMCs have been achieved through several technological routes [4]. Considering quality and economic standards, as well as the feasibility of metallurgical processing during the fabrication, liquid-state processes such as infiltration of porous powder preforms with matrix melts [5]; mechanical stirring of disintegrated particles into metallic melts [6]; chemical reactions at high-temperature that produce in-situ reinforcing compounds [7] and others are preferred.
Stir casting [8] is the most widely used method for making cast composites by mechanically mixing the melt with reinforcing particles. The process, in spite of its numerous advantages such as being simple and most economical method, has several rigid disadvantages: oxidation and gas glut of the matrix alloy during active stirring (leading to elevated porosities in castings [9]), poor bonding between the matrix and reinforcement, agglomeration of reinforcement particles.
This process produces composites that are not at equilibrium, and the reinforcing materials and the matrix alloy can react severely, resulting in damage to the reinforcing materials and formation of unwanted products [10]. As a result, stir casting methods are difficult to achieve continuous and full contact between second phases and matrix, resulting in unstable mechanical and functional properties.
As an alternative to stir casting, liquid-state reactionary synthesis (in-situ process) produces novel endogenous reinforcing components through controlled exothermic reactions between the constituents of Al matrix composites prior to processing [7]. Those composite materials achieved by in-situ methods have improved thermodynamic stability and reinforcement dispensing, plus enhanced adhesion bonds along the interface between the matrix and reinforcing phases, resulting in better mechanical and operational properties.
By selecting the technology of mixing the phases imported in the in-situ reactions [11], the distribution of the new compounds can be adopted. There is no need for a special rig for most routes of endogenous reinforcing. Therefore, making endogenous ceramic compounds directly in the matrix melt is more economical than making exogenously-reinforced composites with ready-made ceramic powders.
Among the many reinforcing phases, the particular attention of researchers is paid to the Mg2Si intermetallic compound, as it can be easily created in-situ via ingot metallurgy at high volume fraction [7]. The possibility for utilizing Mg2Si as a reinforcing agent is related to the set of high physical and mechanical properties of this phase, such as low coefficient of thermal expansion (7.5×10-6 K-1), high melting point (1358 K), high hardness (4.5×109 N.m-2), low density (1.88 g/cm 3 ) and high elastic modulus (120 GPa) [7]. Although, Al/Mg2Si composites have not yet acquired a vast industrial application, due to the obtained degree of mechanical properties is relatively low due to the structural characteristics of these compounds [12]. By adding more than one reinforcing agent to a composite material, the mechanical properties can be improved. A promising choice for use along with Mg2Si for the reinforcing of an Al matrix is the intermetallic compound Al3Ni, which have a low density (~4.03 g/cm 3 ), high melting point (~1127 K), considerable high-temperature mechanical stability up to 773 K, high Young's modulus, attractive chemical stability, low coefficient of thermal expansion, high bulk modulus (~113 GPa), and is capable to be as heterogeneous nucleation sites for the α-Al grains leading to more increase of mechanical properties of the composites [13]. In addition, the structure of endogenously-reinforced composites could be controlled by changing the cooling rate during crystallization and finally achieving a determined degree of properties [14].
It is the objective of this study to develop hybrid aluminum matrix composites (HAMC) reinforced with in-situ formed    Intermetallic compound Al3Ni crystallizes mostly in the form of dense and block phases, and their content is almost the same for both of molds. When using the copper mold, the average size of Al3Ni particles reduced to 5 μm. In addition, the size distribution of Mg2Si and Al3Ni particles is more uniform and nearby the Gaussian distribution, that is seen from the effects of distribution histograms (Fig. 2, c,d).   On the other hand, the space among the reinforcing particles reduces with the aid of lowering their length. This issue become schematically proven in Fig. 5 and can be defined with the aid of using Equation (1), because the reinforcement particle size decreases, the space among the particles may even decreases (λ2 < λ1) [16].

RESULTS AND DISCUSSION
Where in λ is the space among the reinforcement particles, f is the particle extent fraction and r is the particle radius, assuming them spherical. In different words, in line with Equation (2) lowering the space among the Mg2Si particles will boom the specified stress for dislocations motion among them, ensuring in a boom withinside the composite strength.
The shear stress needed to overcome the obstacle is: where G is shear modulus, b is burger's vector and λ is space between obstacles [16]. The XRD pattern proves that the structural phases of the gained in-situ composites are α-Al, Mg2Si and Al3Ni.
An increase in the cooling rate of solidification, by the use of a copper mold alternated steel mold, results in decreasing the mean size of the primary Mg2Si phases from 12.5 to 8.5 μm and enhancement of the distribution homogeneity; synchronously, the mean size of Al3Ni particles reduces from 5.7 to 5 μm but their amount is almost the same for both molds.