Processing, microstructural characterization and mechanical properties of a Ti2AlC/nanocrystalline Mg-matrix composite

Herein we report on the processing and microstructural characterization of 50 vol.% Ti2AlC/nanocrystalline (nc) Mg-matrix composites fabricated by pressureless melt infiltration at 750 °C for 1 h. X-ray diffraction and transmission electron microscopy both confirmed that the Mg grain size was ∼35 ± 15 nm. The microstructure was also exceptionally stable; annealing for 6 h at 550 °C did not alter the size of the Mg-grains. Some Mg was dissolved in the Ti2AlC confirming the existence of a (Ti1-xMgx)2AlC solid solution, with x as high as 0.2. A small amount of Ti (3 ± 1 at.%) was also found in the Mg matrix. At 350 ± 40 the ultimate tensile strength is significantly greater than other pure Mg composites reported in the literature. At 700 ± 10 MPa, the ultimate compressive stresses of these composites were ≈ 40% higher than those of a 50 vol.% Ti3SiC2–Mg or a 50 vol.% SiC–Mg, in which the Mg-matrix grains were not at the nanoscale. The Ti2AlC/nc-Mg composites are readily machinable, stiff (≈70 GPa), strong, light (2.9 g/cm3) and exhibited exceptional damping capabilities, that increased as the square of the applied stress to stress levels of the order of ≈ 500 MPa. The energy dissipated per cycle per unit volume at such stress levels is believed to be the highest ever reported for a crystalline solid and to be due to the formation and annihilation of incipient kink bands. The technological implications of having such solids are briefly discussed.