Thermodynamics and Kinetics to Alloying Addition on In-Situ AlN/Mg Composites Synthesis via Displacement Reactions in Li
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INTRODUCTION
METAL matrix composites (MMCs) show great potential for automotive industry and aerospace industry due to their high specific modulus, strength, and thermal stability.[1,2] Many feasible techniques, such as powder metallurgy, reactive gas injection, self-propagating high-temperature synthesis, and mechanical alloying,[3,4] have been used to synthesize MMCs. Compared with the traditional MMCs produced by ex-situ techniques, the in-situ MMCs have received more attention since they exhibit the finer reinforcement and cleaner matrix-reinforcement interfaces. The in-situ methods have been used for the synthesis of Al matrix composites. For example, AlN particulates were in-situ formed successfully in Al alloy melts by bubbling nitrogen-containing gases.[5–8] Furthermore, the mechanism of in-situ formation of AlN in Al alloy melt and the dependence of alloying element on the formation of AlN in Al alloy have been investigated extensively. Kudela et al.[8] reported that the minimum nitridation temperature in a molten Al-Mg alloy greatly depended on the Mg content. Under a N2 pressure of 1 MPa, the minimum nitridation temperatures were around 1300 C, 1180 C, and 1090 C for alloys with magnesium contents of 1.96, 3.98, and 4.85 pct, respectively. Scholz et al.[7] reported that depending on the Mg-Si content in the Al alloys, three forms of AlN can be in situ formed, including a dense layer, dispersed particles, and pure AlN ceramics. As compared to in-situ AlN/Al composites, the in-situ AlN/Mg composites have drawn more interest as lightweight engineering materials in recent years. Tyagi TONGXIANG FAN, Professor, CONGFA ZHANG, Doctor, JIANQI CHEN, Postgraduate Student, and DI ZHANG, Professor, are with the State Key Lab of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai, 200240, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted August 5, 2008. Article published online August 28, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
et al.[9] and Ye et al.[10] attempted to in situ synthesize AlN in Mg-Al alloys by liquid nitridation. Although the experimental results indicated the method is feasible, the microstructure of the reinforcements formed by liquid nitridation is difficult to be controlled; moreover, these composites contain a lot of pores and thus secondary processing must be carried out to decrease the pore content. Therefore, much research began to investigate in-situ synthesis of AlN by adding a solid nitride. Ye et al.[11] investigated in-situ synthesis of AlN in Mg-Al alloy by Mg3N2 addition. The results showed that AlN can be formed at 973 K through the reaction 2½AlMg þ Mg3 N2 ¼ 2AlN þ 3½Mg
½1
In Reaction [1], the liquid-state reactant and product are involved. Therefore, alloying element addition can be an important factor influencing the formation of AlN, since the additional alloying elements can change the activities and the diffusion rate of the component species. However, little research emphasizes the influence of alloying element addition.
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