Claims:The scope of the invention is defined by the following claims:

Claim:
1. A metal matrix composite comprising:
a) A magnesium matrix; and
b) A reinforcement includes Zirconium, Garnet and Graphite dispersed in the magnesium matrix.
c) The reinforcement has an average particle size ranging from 10 µm - 20 µm.
2. The composite recited in claim 1, wherein the amount of Zirconium 1 wt %; Garnet and Graphite ranges from 0.3 wt % - 0.9 wt % based on net weight of the metal matrix composite material.
3. The composite of claim 1, wherein particulate reinforcement was uniformly distributed within the metal matrix pool which augments the bonding at metal matrix-reinforcement interface
4. The composite recited in claim 1, wherein dissolved Zr element precipitates uniformly forms evenly distributed α-Zr particles in the melt.
5. The composite of claim 1, whereas agglomeration of garnet particle was observed to be minimum for 0.3 wt% of graphite and maximum for 0.6 and 0.9 wt% of graphite
6. The composite recited in claim 1, wherein 0.9wt% of Garnet and Graphite exhibited a higher hardness of 78.14 BHN. , Description:Field of Invention
The present invention pertains to improve the bulk hardness of magnesium matrix composites reinforced with Zirconium, Graphite and Garnet.
Background of the Invention
Magnesium and its alloys as scientific research material gained great attention in commercial application. Magnesium matrix composites have wide range of application in automotive and aerospace industries for reduced fuel consumption and lower cost of production. Modern applications demand energy conservation and refined performance. Because of lower density, and higher specific strength compared with other structural metals, magnesium matrix alloys are considered for aerospace and automobile applications in order to minimize green house emission and fuel consumption [S. Nimityongskul et al., Mater. Sci. Eng. A 527 (2010) 2104, B.L. Mordike et al., Mater. Sci. Eng. A 302 (2004) 37 and M. Habibnejad-Korayem et al., Mater. Sci. Eng. A 519 (2009) 198]. However, lower rate of wear, poor resistance to creep at higher temperatures, lower strength and modulus restricts the usage of these alloys for structural applications [Q.C. Jiang et al., Scripta Mater 48 (2003) 713 and P. Poddar et al., Mater. Sci. Eng. A (2007) 357]. Reinforcing with industrial grade particle reinforcements impart better physical and mechanical properties. Such composites is being considered as a better alternative for Al MMCs as a lightest metal structural material and possess several advantages over monolithic magnesium such as high strength, high elastic modulus, superior creep and wear resistances at elevated temperatures [M.Y. Zheng et al., J. Mater. Sci. 38 (2003) 2647 and V. Viswanathan et al., Mater. Sci. Eng. R 54(2006) 121]. The properties of Mg MMCs can be tailored through judicious selection of reinforcement particles. The reinforcement is selected based on the working temperature and ambience. Some of the commonly used particle reinforcements are Aluminium Oxide (Al2O3), Titanium Carbide (TiC), and Silicon Carbide (SiC). It is reported that addition of carbides as reinforcement improves the ultimate tensile strength, yield strength, hardness, ductility and wear resistance of Mg and its alloys [W.L.E. Wong et al., Comp. Sci. Technol. 67 (2007) 1541]. In our invention, Magnesium is reinforced with particulate Zirconium, Garnet and Graphite and proved its significance in improving the hardness of the composites.
In the area of the matrix, majorly metallic systems were considered as the matrix material including Al, Ag, Be, Co, Fe, Ti, Ni, and Mg. By far, aluminum matrix composites are considered to be the most suitable material for structural applications in aerospace and automobile applications. However, recent studies claimed that magnesium could be considered as better alternative for matrix material. Magnesium has lower density and better mechanical properties compared with aluminum. Magnesium is approximately two thirds lighter than that of aluminum, one fifth of steel and one quarter of zinc [M. Pekguleryuz et al., International Materials Reviews 55 (2010) 197]. Because of this, magnesium and its composites offer higher specific strength. The increase in demand for high performance and lightweight materials increases the need for magnesium matrix composites [M. Yu, Sci. 287 (2000) 63]. Magnesium matrix reinforced with thermally stable reinforcements will make them suitable for high temperature applications [S.C. Sharma at al., Wear 241 (2000) 33]. For magnesium matrix composites, ceramic powders are considered to be the most widely preferred reinforcement. High-temperature stability and structural properties of ceramic materials makes them as a favorable reinforcement candidate. These properties includes lower density and high levels of strength, thermal stability, hardness, and elastic modulus [Q.C. Jiang et al., J Alloys Compd 386 (2005) 177]. However, lower wettability, compatibility and reduced ductility are some of the problems associated with ceramic reinforcements to be used along with magnesium matrix. To overcome the above mentioned problems, most commonly used reinforcements are Aluminium Oxide, Silicon Carbide (SiC), and Titanium Carbide (TiC). The microstructure, damping behavior and mechanical properties of the above mentioned reinforcements were studied by the researchers across the globe [Zhang Xiuqing et al., Composites: Part A 37 (2006) 2011, K.B. Nie et al., Materials Science and Engineering A 528 (2011) 5278 and P.P. Bhingole et al., Composites: Part A 66 (2014) 209]. The most common problem faced by them is decrement in bulk hardness which is rectified in our invention.
Summary of the Invention
In light of the above mentioned drawbacks in the prior art, the present invention aims to improve the bulk hardness of magnesium based composite material by reinforcing with zirconium, garnet and graphite.
The proposed particulate reinforcement was uniformly distributed within the metal matrix pool. All samples exhibit good bonding at the metal matrix-reinforcement interface. The dissolved Zr element precipitates uniformly across the melt and got evenly distributed as α-Zr phase in the melt. The agglomeration of garnet particle was observed to be minimal if the graphite weight percentage was limited to 0.3. However, agglomeration was visible for composites containing 0.6 and 0.9 wt% of graphite. Mg MMCs reinforced with 0.9wt% of Garnet and Graphite exhibited a higher hardness of 78.14 BHN. The composite reinforced with 0.9wt% of garnet and graphite exhibited increased hardness of 70.2 HV0.1 compared to the 41 HV0.1 hardness exhibited by composite reinforced with 0.3wt% of reinforcements.
Brief Description of Drawings
The invention will be described in detail with reference to the exemplary embodiments shown in the figures wherein:
Figure 1 Weight percentage of particle reinforcement.
Figure 2 Micrograph of Mg metal matrix composites reinforced with various percentages of reinforcement particulates.
Figure 3 SEM image of as-cast sample.
Figure 4 Brinell hardness and Vickers’ hardness of reinforced composites.
Detailed description of the drawing
Figure 1 gives the varying weight percentage of reinforcement particles used to prepare the composites. The weights of the particles are varied in 9 different levels to find the optimized combination.
Figure 2 (a-e) exhibits the microstructure of metal matrix composites fabricated by stir casting process. Distribution of particles plays a significant role in imparting superior mechanical properties to the MMC.
Figure 3 gives the SEM image of evenly distributed α-Zr precipitated uniformly across the magnesium matrix.
Figure 4 illustrates the hardness taken across the cross-section using Brinell hardness and Vickers hardness testing equipment. From the test results it was observed that the hardness increased with particulate addition. For every composite, hardness was measured in three different locations and the average value was reported. A noticeable variation was not observed for the hardness taken across three different locations. This was attributed through uniform distribution of reinforced particles.
Detailed Description of the Invention
The Magnesium MMCs is reinforced with Zirconium, Graphite and Garnet particles having a particle size of approximately 10-20μm in diameter. The microstructure investigation shows the presence of reinforcement particles in magnesium composites. Figure 2 (a-e) exhibits the microstructure of metal matrix composites fabricated by stir casting process. Distribution of particles plays a significant role in imparting superior mechanical properties to the MMC. It was observed that graphite and garnet particles are uniformly distributed throughout the magnesium matrix phase. The lack of cracks observed from the micrographs attribute to the quality of castings. The volume fraction of the reinforcement is varied in different composition. As observed from the optical microscope images, the microstructure of the composite consists of three phases: magnesium matrix, Graphite particles, and Garnet particles. It was observed that the Magnesium MMCs and the reinforcement particles are well bonded and few precipitates exist in the matrix or at the interface. The precipitate observed is formed from the elemental addition of Zirconium.
From Figure 2 a-c, it was observed that Garnet and Graphite was uniformly distributed within the metal matrix pool. All castings exhibit good bonding along the Mg matrix-reinforcement interface. The stirring parameters were optimized to avoid segregation and agglomeration of the reinforcement particles. The wettability of reinforcement has significant influence in bulk porosity. Inclusion of moisture along the periphery of the reinforcement particles entraps oxygen inside the molten metal matrix pool. This creates voids and acts as stress-nucleation sites. In order to avoid this, the reinforcement particles were preheated to remove the moisture. The dispersed graphite particles were observed to have a dendritic structure. The density of these dendritic structures increased with higher percentage of graphite addition.
The variation in density of reinforcement is observed clearly with increase in percentage of reinforcement. It is observed that there is an increase in density with an increase in Garnet and Graphite reinforcements. It is also understood from figures that the porosity of the composites decreased with the increase in weight percentage of reinforcements. The porosity content of composite is minimized through uniform rotating speed of stirrer in stir casting and also size of reinforcements. The agglomeration of garnet particle was observed to be minimal for MGG 1 where the percentage was limited to 0.3 wt%. Whereas, agglomeration was visible for composites containing 0.6 and 0.9 wt% of graphite. This was attributed through higher binding force between the graphite particles. Optimized percentage of reinforcement is supposed to impart enhanced bonding strength induced through better metal-matrix packing. Higher interface bonding strength improves the mechanical properties of the material under the induced shear stress. A significant amount of Zr is dissolved in Mg melt during smelting of Zr-containing magnesium alloys, with respect to the melting temperature of above 7500C. When the melt is poured into casting mold, it solidifies at a relatively high cooling rate. When the temperature of the melt decreases to 6500C, the dissolved Zr element precipitates uniformly forms evenly distributed α-Zr particles in the melt.
Mg composite reinforced with 0.3wt% of Garnet and Graphite exhibited a hardness of 37.32BHN. From the test results it was observed that the hardness increased with particulate addition. The Mg MMCs reinforced with 0.9wt% of Garnet and Graphite exhibited a higher hardness of 78.14BHN. For every composite, hardness was measured in three different locations and the average value was reported. A noticeable variation was not observed for the hardness taken across three different locations. This was attributed through uniform distribution of reinforced particles. The Mg Matrix composites exhibited an observable increase in the micro-hardness value. The composite reinforced with 0.9wt% of garnet and graphite exhibited increased hardness of 70.2 HV0.1 compared to the 41 HV0.1 hardness exhibited by composite reinforced with 0.3wt% of reinforcements. The hardness of composites increased with the increase in particulate reinforcement. The increase in hardness was attributed to the increased volume fraction of the refined dendritic structure.