| The ternary layered compound Ti2AlC combines the merits of bothmetals and ceramics. For example, low density, high melting point, highmodulus, high electrical conductivity and thermal conductivity, readilymachinable, excellent thermal shock resistance and high temperatureoxidation resistance. Such unique properties make it possible to useTi2AlC in structural components for high-temperature applications, i.e.,high temperature components, protective coating on alloy surfaces, andelectrical contacts. However, compared with traditional binary carbidesceramics, Ti2AlC has low hardness and strength, which limits its wideapplications in structural components. A combination of solid solutionand incorporation a second reinforcement is an effective way to modifythe microstructure and mechanical properties of Ti2AlC.In this work, monolithic Ti2AlC,(Ti1-x, Mox)2Al solid solution, and(Ti, Mo)2AlC/Al2O3composites were successfully synthesized at1350℃by in-situ reactive hot-pressing method of Ti-Al-TiC,Ti-Al-TiC-Mo and Ti-Al-TiC-MoO3systems, respectively. The reactionprocedure of various systems, and the different additives on themicrostructure, grain size and mechanical properties of the sampleswere studied by X-ray diffraction analysis (XRD), scanning electronmicroscopy (SEM), energy dispersive analysis (EDS) andThermogravimetry (TG) experiments. The microstructure of oxidescales formed on (Ti1-x, Mox)2AlC surface after oxidation at900-1400°C was also investigated.X-ray diffraction results of the (Ti1-x, Mox)2Al solid solution indicated that the as-synthesized (Ti1-x, Mox)2Al solid solution preparedwith various Mo contents consisted of Ti2AlC and Mo containingphase. The reaction path of solid solution showed that the reactionbetween Ti and Al produced Ti-Al intermetallics, and the reactionbetween Al and Mo formed Mo-Al intermetallics. And then the solidsolution was formed by the reaction of Ti-Al, Mo-Al intermetallics, andTiC. The similar microstructure characteristics from monolithic Ti2AlCand (Ti1-x, Mox)2Al solid solution indicated that the effect of the solidsolution in Ti2AlC on the grain size refinement was slight. Withincrement of Mo in (Ti1-x, Mox)2AlC solid solutions, the latticeparameter c decreased dramatically, while the lattice parameter a almostunchanged. The Vickers hardness, flexural strength and fracturetoughness were5.48GPa,363.60MPa and5.78MPa·m1/2, which wereimproved by44%,34%and136%for (Ti0.80, Mo0.20)2AlC, respectively,compared with the single-phase Ti2AlC.It was found that the production of the (Ti, Mo)2A1C/Al2O3composite was composed of a series of intermediate reactions: first, thereaction between Ti and Al form Ti-Al intermetallics, and the thermitereaction between Al and MoO3produced Al2O3and Mo. And then theintermetallics and the residual Ti and Al transformed to TiAlequilibrium phase. Finally, the (Ti, Mo)2A1C/Al2O3composite wasformed by the reaction of TiAl TiC, and Mo. The introduction of in situAl2O3resulted in a submicron grain size and a homogeneouslydistributed matrix phase of (Ti, Mo)2AlC. The matrix grain size wassignificantly refined with increasing the Al2O3content. Compared withmonolithic Ti2AlC, the addition of13.81wt%MoO3evidently enhancedthe hardness, flexural strength and fracture toughness by25%,69%and146%, respectively.The results revealed Al atoms diffused out and formed a protectiveinner α-Al2O3layer, Ti atoms diffused out and formed an outer TiO2(rutile) layer during oxidizing at900-1300°C. TiO2grains began todissolve, and it existed planar defects at1300°C. After elevating at1400°C, an outer layer of mixed TiO2and Al2TiO5phases and an inner α-Al2O3layer were formed, along with cavities and cracks werepresented on surface and cross section of the oxide scale, which wascaused by the volume changes and the thermal expansion mismatch ofthe phases in the oxide scale. A small amount of Mo diffused into theoxide scale without formation of molybdenum oxide in all thetemperature ranges. At1400°C, the loose oxide layer structureincluding cavities and cracks provided channels for rapid ingress ofoxygen to the substrate, resulting in severe oxidation. |
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