THERMODYNAMIC ANALYSIS OF INTERFACIAL REACTION SiN SiC / Mg MATRIX COMPOSITES

Gibbs free energy of chemical reactions between SiC particles and the Mg matrix at the different temperature has been calculated based on the Gibbs-Helmholtz equation and thermodynamic equilibrium of chemical reactions. The thermodynamic stability of Al4C3 and MgAl2O4 in the interface was investigated. The results showed that when the activity of Si on interface is more than a critical value of ASi 0 . A stable Al3C4 cannot formed when ASi is more than ASi 0 , which is not a constant, increasing with the temperature. The mass fraction and distribution of SiO2in the interface have directly effects on the reactions of SiO2 with Mg and Al. There is a critical value of AMg 0 , AAl 0 and AAl 1 , which increases with the temperature. When the mass fraction of Si in the interface is greater than a critical value, there is no interfacial reaction at a certain temperature. The interface reaction models have been proposed.


Introduction
Particle reinforced magnesium matrix composites have a high specific strength and modulus of elasticity, good damping properties and low coefficient of thermal expansion, which have a great potential in aerospace, automotive and military fields.SiC has many excellent properties such as good thermal stability, low expansion coefficient and high hardness, making it widely used as a reinforcing phase in series of magnesium alloy [1][2][3][4].More attentions have been paid on phase structure, mechanical properties and strengthening mechanisms in SiC particle reinforced magnesium matrix composites.The effect of the distribution of SiC particle size on magnesium matrix composites has illustrated that the reinforcing effect is optimal due to the particles completely distributed in grains [5].It is found that the pulsed magnetic field has a significant influence on the solidification structure of SiCp / AZ91D Composites [6].In the SiCp/AZ91D composites fabricated by vacuum pressure infiltration process, the SiC particles were homogenously distributed, the interface bonding of the particles and the matrix were favourable [7].Reaction of Mg and Al with SiO2 on the surfaces of oxidized SiC, leads to the formation of MgO and MgAl2O4 crystals, which act as diffusion barriers between Al and SiC at the interfaces [8].ASiO2 layer is naturally present or artificially introduced at the SiC surface, which is presumed to modify the wettability and interfacial bonding between the reinforcement and the Al-Mg alloys DOI 10.12776/ams.v22i4.828p-ISSN 1335-1532 e-ISSN 1338-1156 [9][10][11].A particle pull-out fracture will be the predominant mode if the particle strength is higher, while a tensile loading-induced SiC particles fracture will take place prior to interface failure if the interface strength is higher [12].However, a thermodynamic analysis of interfacial reactions in SiC/Mg matrix composites has not been reported.
In the present work, the thermodynamic conditions of interface reactions in the SiC/Mg matrix composite, thermodynamic stability of Al4C3 and MgAl2O4 and the process of interfacial reactions have been investigated.The interface reaction models have been proposed.

Interfacial chemical reactions
There are many chemical elements such as silicon (Si), aluminium (Al), zinc (Zn), copper (Cu), oxygen (O) and carbon (C)in magnesium alloy.During the solidification processing of SiC/Mg composites, the possible reactions are shown in Table 1.The calculation formula for the constant pressure and heat capacity is defined as [13]: Equation ( 1) is integrated to obtain a reaction process that function relationship is between∆H θ and temperature, The equation ( 1) is divided by T, integrated, and multiplied by the temperature T, the obtained formula is subtracted by Equation (2).∆G θ is a function of temperature during the reaction.Here, ∆H 0 is the integral constant.The thermodynamic parameters a, b, c, d can be obtained from inorganic thermodynamic data sheet.The temperature of solidification processing SiC/Mg composites is generally between 700 -1000K.Therefore, thermodynamics parameters a, b, c, d arechosen in the range of 700 -1000K.

Thermodynamics of interfacial products 3.1 Thermodynamic stability analysis of interfacial MgAl2O4
Reaction equations ( 5)-( 8) show the possible way to generateMgAl2O4on the interfaces.Equation ( 1) is satisfied the reaction equilibrium: Equation ( 5) is satisfied the reaction equilibrium: content [15].When the mass fraction Mg is below8%, the interfacial reaction is controlled by the Equation⑤.When the mass fraction Mg is more than8%, the interfacial reaction is controlled by the Equation ① [16].It is consistent with the present theoretical thermodynamic analysis.Equation ( 4) is satisfied the reaction equilibrium: ) In the Equation ( 9), A Mg and A Al meets the following equilibrium: ) The A MgAl 2 O 4 , A MgO , A SiO 2 and A Al 2 O 3 are considered as 1 [14],and taking the Equation ( 10) into the Equation( 5).It can be obtained

Thermodynamic Stability Analysis of interface Al4C3
The Al4C3 is generated in the Equation③ in the  The activity of A SiC and A Al is considered as 1, and then: ) The main factors affecting the generation of Al4C3 are temperature T and the activity of A Si .The enhancing of Si activity can obviously suppress the formation of harmful interface reaction product Al4C3 [17].At a certain temperature, ln A Al 4 C 3 and ln A Si expressed as the linear relationship in double commonly logarithmic coordinates [14].
In the decomposition reaction ⑩ of SiC, it is satisfied by the equilibrium:  When A Al is greater than a certain value ofA  [18].The interfacial reaction at the SiC/Al interface involved with the elements Mg, Al and O can be identified as MgAl2O4 spinel [11,19].The case of oxidized SiC particles covered with a layer of SiO2 is of advantage to control the formation of A14C3 [20].In the solidification processing of SiC/Al composites, SiO2generated by oxidative treatment of SiC surface will reacted into the spinel, Si, oxidation of Aland Mg, which can not only improve interfacial wettability of the SiC/Mg composite, but also the formation of Si can suppress the DOI  3) When the mass fraction of Si in the interface is greater than a critical value, there is no interfacial reaction at a certain temperature.

Fig. 1
Fig. 1 Variations of the ln A Mg value vs. the temperature as the maximum and the minimum A Si in the interface ) = ∆G 5 θ /RT − ∆G 9 θ /3RT + 2 ln A Si(12.)In Fig.2the line 3 and 4 represent the value of A Al in the equilibrium when A Si =1 corresponding to the Equation (11) and (12) at the different temperatures, respectively.Line 3'and 4' represent the value of A Al when A Si is the minimum value according to the Equation (11) and (12).It is shown that the interfacial reaction is controlled by the Equation ④when the activity of Al is below A Mg 0 which is related to the line 4' in the interface.The interfacial reaction is controlled by the Equation⑤ when the activity of Al is more than A Mg 0 which is related to the line 4'in the interface.When the value A Al is in the range of A Al 0 and A Al 1 , the Equation④ and ⑤ occurs simultaneously.Both of A Al 0 and A Al 1 are the function of temperature.

Fig. 2
Fig. 2 Variations of theln  Al value vs. the temperatures as the maximum and the minimum A Si in the interface varies with temperatures RT(ln A Al 4 C 3 + 3 ln A Si − 3 ln A SiC − 4 ln A Al )(13.)

.)Fig. 3
Fig.3is the magnitude of A C × A Si at the different temperatures, which is corresponding to the value of A Si asA C = 1, the value of A C asA Si = 1, or the minimum A Si or A C in SiC at different temperatures.In the Equation⑪, the equilibrium formula of A Al 4 C 3 and A C can be expressed as:3 ln A C + 4 ln A Al − ln A Al 4 C 3 = −∆G 11 θ /RT (16.)When A Al is regarded as 1, A Al 4 C 3 and A C can be expressed as the linear relationship in the double commonly logarithmic coordinates.When the silicon is rich in the interface, the minimum of A C obtained by the Equation (15) issubstituted into the Equation (16), and then it can get the minimum activity of a stable Al 4 C 3 in the interface.In Fig.4 it is shown that the lines AA' is represented by the linear relationship of A Al 4 C 3 and A Si determined by the Equation (6) in the double commonly logarithmic coordinates as the temperature is ranged from 700K to 1000K.The zero point is represented by the minimum A Al 4 C 3 when a stable Al 4 C 3 forms in the corresponding temperature determined by the Equation (16).Similarly, it represents the maximum value A Si 0 for A Al 4 C 3 0 and A Si .

Fig. 3 Fig. 4
Fig. 3 The magnitude of A C × A Si with different temperatures

Table 1
Chemical reactions during the solidification processing of SiC/Mg composites

Table 1 .
At a certain temperature, it meets the equilibrium: DOI 10.12776/ams.v22i4.828p-ISSN 1335-1532 e-ISSN 1338-1156 Al 1 and A Mg is less than a certain valueofA Mg 0 , Mg and Al are diffused to the interface of SiO2 and the liquid.Mg and Al are formed into MgAl2O4, and Si will be desorbed from the interface.III.When A Al is greater than a certain value ofA Al 1 and A Mg is less than a given valueofA Mg 1 , Mg and Al are diffused to the interface of SiO2 and the liquid.Mg is adsorbed and reacted into MgO and Si in the interface, and then MgO and Al are reacted into MgAl2O4.IV.When A Al is less than a certain value ofA Al 0 and A Mg is more than a given valueofA Mg 0 , Mg and Al are diffused to the interface of SiO2 and the liquid.Al is adsorbed and reacted into Al2O3and Si in the interface, thenAl2O3 and Mg are reacted into MgAl2O4 and it will be desorbed from the interface.V.When the value of A Al is between A Al 0 and A Al 1 , A Mg is greater than a certain value ofA Mg 0 , Mg and Al are diffused to the interface of SiO2 and the liquid.Mg and Al are adsorbed and reacted into MgO, Al2O3and Si in the interface, and then MgO and Al will reacted intoMgAl2O4.VI.When the value ofA Al is between A Al 0 and A Al 1 ,A Mg is less than a certain value A Mg 0 ,Mg and Al are diffused to the interface of SiO2 and the liquid.Mg and Al are reacted into MgAl2O4 and Si in the interface and will desorb from the interface.At the same time, Al is adsorbed and reacted into MgO and Si.The MgAl2O4 spinel phase can improve the bonding strength of the interface characteristics of Al-Mg-Si/SiC composites 10.12776/ams.v22i4.828p-ISSN 1335-1532 e-ISSN 1338-1156 generation of Al4C3.Because the percentage of Si increases with the formation of Mg, Al oxide and spinel, it is difficult for the related react of Mg and Al in the interface.When a stable Al3C4 forms in the interface, which is significantly dependent on  .There is a critical value of    .A stable Al3C4 cannot formed when   is more than    .   is not a constant, it increases with the temperature.2) The mass fraction and distribution of SiO2 in the interface have directly effects on the reactions of SiO2 with Mg and Al.The reactions process and the ratio of products are greater affected by the interface activity of Mg and Al.There is a critical value of    ,   and    , which increases with the temperature.