Processing and Characterization of Mg Matrix Composites Reinforced with TiC and TiB2 Phases using an In-situ Reactive Infiltration Technique (2011)

Magnesium matrix composites are attractive for different applications especially in automotive and aerospace industries due to their superior specific properties. The main purpose of this work is to produce a new magnesium matrix composite reinforced with a network of TiC and TiB2 compounds via an in-situ reactive infiltration technique. In this process, the ceramic reinforcement phases, TiC and TiB2, were synthesized in-situ from the starting powders of Ti and B4C without any addition of a third metal powder such as Al. The molten magnesium infiltrates the preform of 3Ti-B4C by capillary forces. Furthermore, adding Mg or MgH2 powder with different weight percentages to the 3Ti-B4C preforms was used in an attempt to increase the Mg content in the fabricated composites. The results of the in-situ reaction mechanism investigation of the Ti-B4C and Mg-B4C systems show that the infiltrated magnesium not only infiltrates through the 3Ti-B4C 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 B4C and accelerates the reaction rate. The investigation of the in-situ reaction mechanism after adding Mg or MgH2 powder to the 3Ti-B4C preforms show that the reaction mechanisms are similar. However, the presence of the Mg or MgH2 in the preform accelerates the reaction rate making the reaction take place and finish in a shorter time. \nAlso, the results of the parametric study show that the processing conditions such as temperature, holding time and green compact relative density have a significant influence on the reaction mechanism and the fabrication of the composite. Based on this work, it is recommended to fabricate the composite samples at 900ºC for 1.5 h and using a green compact of 70% relative density. The required equilibrium phases, TiCx and TiB2, formed in the composites with very small amounts of the residual Ti, boron carbide and intermediate phases such as TiB, Ti3B4 and MgB2. The fabrication of composites at these processing conditions avoids significant oxidation of Mg and formation of the ternary compound (Ti2AlC) in the cases of AZ91D or AM60B alloys composites, which can adversely affect the mechanical properties of the composites. \nFurthermore, the results reveal that the percentage of reinforcing phases, when the optimal processing parameters are used, can be tailored by controlling either the green compact relative density or the weight percentages of Mg or MgH2 powder added to the 3Ti-B4C preform. \nMicrostructural characterization reveals a relatively uniform distribution of the reinforcing phases TiCx and TiB2 in the magnesium matrix. Mechanical properties of these composites such as elastic modulus, flexural and compressive strengths are greatly improved compared with those of the unreinforced Mg or Mg alloys. In contrast, the ductility of TiCx-TiB2/Mg composites is lower than that of the unreinforced Mg or Mg alloys. However, this lower ductility was improved by the addition of Mg or MgH2 powder in the preform. Secondary scanning electron microscopy was used to investigate the fracture surfaces after the flexural strength test. The composites show signs of mixed fracture; cleavage regions and some dimpling. In addition, microcracks observed in the matrix show that the failure might have initiated in the matrix rather than from the reinforcing particulates. Also, the results show that the hardness and the wear resistance of the composites are improved, compared with those of the unreinforced Mg alloy.