Understanding the reaction mechanism of in-situ synthesized (TiC–TiB2)/AZ91 magnesium matrix composites

Abstract Magnesium matrix composites reinforced with a network of TiC and TiB 2 compounds have been successfully synthesized via an in-situ reactive infiltration technique. In this process, the ceramic reinforcing phases, TiC and TiB 2 , were synthesized in-situ from the starting powders of Ti and B 4 C without any addition of a third metal powder such as Al. The molten AZ91 magnesium alloy infiltrates the preform of 3Ti–B 4 C by capillary forces. Furthermore, adding different weight percentages of MgH 2 powder to the 3Ti–B 4 C preforms was used in an attempt to increase the Mg content in the fabricated composites. The results reveal a relatively uniform distribution of the reinforcing phases in the magnesium matrix with very small amounts of residual Ti, boron carbide and intermediate phases when they are fabricated at 900 °C for 1.5 h using a 3Ti–B 4 C preform with 70% relative density. On the other hand, after adding MgH 2 to the 3Ti–B 4 C preform, TiC x and TiB 2 formed completely without any residual intermediate phases with the formation of the ternary compound (Ti 2 AlC) at the expense of TiC. The percentage of reinforcing phases can be tailored by controlling the weight percentages of MgH 2 powder added to the 3Ti–B 4 C preform. The results of the in-situ reaction mechanism investigation of the Ti–B 4 C and Mg–B 4 C systems show that the molten magnesium not only infiltrates through the 3Ti–B 4 C preform and thus densifies the fabricated composite as a matrix metal, but also acts as an intermediary making the reaction possible at a lower temperature than that required for solid-state reaction between Ti and B 4 C and accelerates the reaction rate. The investigation of the in-situ reaction mechanism with or without the addition of MgH 2 powder to the 3Ti–B 4 C preforms reveals similar mechanisms. However, the presence of the MgH 2 in the preform accelerates the reaction resulting in a shorter processing time for the same temperatures.

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