Mg. Ti 2 AlC. Mg Nanograins that do Not Coarsen Even Upon Melting Michel W. Barsoum, Drexel University, DMR 0736218.
Ti2AlCMg Nanograins that do Not Coarsen Even Upon MeltingMichel W. Barsoum, Drexel University, DMR 0736218
In this work, a unique and novel class of Mg matrix composites reinforced with Ti2AlC was fabricated, for the first time, by spontaneous melt infiltration. Surprisingly, the Mg-matrix grains formed upon solidification were ~ 35nm (Fig. 1a) These nano-grains were also exceptionally stable: repeatedly heating of the composite to 700 °C – 50 °C above the melting point of Mg – remarkably did not lead to coarsening (Fig. 1b). These results are extraordinary because to date, not only is it difficult to make low melting point metals with grains in the nanometer scale – as far as we are aware this is a first for Mg – but such microstructures, when made, are typically thermally quite unstable. Here the Mg grains are apparently jacketed in a very thin, oxide skin that is mechanically robust enough to withstand the solidification and melting stresses and that prevents coarsening! This bodes well for many high temperature applications. The nanograins also suggest that these composites could be excellent H-storage materials.
Fig. 1: a) TEM image of nanocrystalline Mg matrix in the Mg-Ti2AlC composite samples; b) 3 successive DSC cycles. Repeated melting of the Mg did NOT result in coarsening as evidenced by the constancy of melting point depression due to presence of nanograins.
Mg alloys have always been attractive to designers due to their low density, only 2/3 that of Al. Mg is the third most commonly used structural metal, following steel and Al. Mg is widely used for manufacturing of mobile phones, laptop computers, cameras, and aerospace and automotive components. One of our goals is to further enhance the uses of Mg by making Mg-composites with unique and extreme properties.
At 380±20 MPa, 700±10 MPa and 1.5±0.5 GPa, the ultimate tensile and compressive strengths and Vickers hardness values, respectively, of the composites we fabricate are significantly greater than other pure Mg-composites reported in the literature; a result that is attributable to the nano-crystalline nature of the Mg-matrix.
Our composites also exhibit ultrahigh damping capacity, due to fully reversible kinking in the Ti2AlC phase. As shown in figure above the composites we make also have a combination of damping capability (y-axis) and stiffness (x-axis) that to date has not been possible as shown by intersection of 2 dashed lines!