| Herein we report on the fabrication and mechanical properties of Mg composites fabricated by pressureless melt infiltration of Mg and Mg alloys into porous preforms of TiC and Ti2AlC. The latter is a member of the MAX phases---viz. layered machinable ternary carbides and nitrides---some of which are relatively light and stiff. In this study, pure Mg and three, commercially available, aluminum-containing Mg alloys---AZ31, AZ61 and AZ91-were used as matrices at a loading of ~ 50 vol.%. For the most part, increasing the Al content enhanced the elastic moduli, Vickers hardness values and yield and ultimate compressive strengths. Reducing the particle sizes of the TiC and Ti 2AlC particulate reinforcements also had a large impact on the mechanical properties. At 1028+/-5 MPa, the ultimate compressive strength of a TiC-AZ61 composite, in which the TiC particle size distribution is Lorentzian and centered at, dc = 0.41+/-0.01 microm, was ~ 40% higher than that of the same composite with coarser TiC particles with bimodal size distributions centered around dc=1.6+/-0.1 microm, and 5.8+/-0.3 microm. In addition, the elastic modulus and Vickers hardness of the former composite were measured to be 174+/-5 GPa and 3.4+/-0.3 GPa, respectively. For the Ti2AlC reinforced composites, the best properties were obtained when AZ61 was reinforced with Ti2AlC particles with dc = 0.51+/-0.01 microm. The enhancements in elastic and mechanical properties are attributed to finer grained Mg-matrices, the presence of Al in the matrices which enhances the wetting of TiC and Ti2AlC by Mg to create a strong interface and finer reinforcement particle sizes. The latter two attributes, in turnlead to better mechanical interlocking.;For the composites studied herein better elastic and mechanical properties, were obtained at the expense of damping. The TiC-reinforced Mg matrix composites despite their high mechanical properties, have very small energy dissipation capabilities. However, by using Ti2AlC, which inherently dissipates mechanical energy, it is possible to achieve higher damping while simultaneously enhancing the mechanical properties almost to the same levels as for the TiC reinforced composites. Using Mg alloys instead of pure Mg and reducing the reinforcement particle sizes also reduced the damping capabilities of these composites.;There is a threshold stress below which the damping capacities of the Ti2AlC reinforced composites are comparable to those of their TiC reinforced counterparts. This was ascribed to the negligible damping of Ti2AlC below the threshold stress ( ~ 200 MPa).;The Ti2AlC composites are slightly lighter and can be fabricated at lower temperature than comparable TiC composites; the former are also readily machinable but more expensive. |
