| As new light-weight high-temperature structural materials, titanium aluminides have attracted much attention due to their low density, high melting point, high specific strength and specific modulus, low creep rate and excellent high-temperature oxidation resistance. Therefore, titanium aluminides have become front-runners in replacing Ti alloys and Ni-based superalloys, and have the potential to enable high temperature automobile and aerospace applications. A foremost application under consideration for titanium aluminides is high performance gas turbine engines. However, the bottleneck problems including low ductility, poor high-temperature stength and oxidation resistance above 800 °C limit their practical applications.The composite technology is an effective approach to improve the properties of Ti-Al intermetallics, mainly through the methods of composition optimization and microstructure control. The selection of reinforcements is very important. Recently, layered ternary compounds, Mn+1AXn(where M is an early transition metal, A is an A group element, X is C or N, and n = 1-3) have been identified as compatible and thermochemically stable reinforcing phases for Ti-Al intermetallics, due to their ternary layered structure and their properties with both metallic and ceramic characteristics.In the present work, high purity Ti3 Al C2 and in situ MAX(Ti2Al C/Ti3 Al C2/Ti3 Al C2-Ti2 Al C) reinforced Ti-Al based intermetallics were fabricated by in situ hot-pressing process. The effect law of different kinds, contents and combination ways of in situ ceramic particles(Ti2Al C/Ti3 Al C2/Ti3 Al C2-Ti2 Al C) on the microstructure and mechanical properties of Ti-Al based intermetallics were investigated in detail. Meanwhile, we established the relationship between microstructure and properties, and explored the strengthening and toughening mechanisms. Our researches are to provide a new preparation technology of in situ ceramic particles reinforced Ti-Al based intermetallics. The results are listed as follows:High-purity Ti3 Al C2 was prepared by in situ reaction process of Ti-1.2Al-2Ti C, 2Ti C-Ti-1.2Al-0.1Sn, 2Ti C-Ti-1.2Al-0.1Si, 2Ti C-Ti-1.2Al-0.05Sn-0.1Si and 2Ti C-Ti-1.2Al-0.1Sn-0.1Si systems. The Ti3 Al C2 purity of the product corresponding to 2Ti C-Ti-1.2Al system is 90.62 wt.%, which is higher than the purity of the product corresponding to 2Ti C-Ti-Al system. Al-rich can offset the volatility of Al at high temperature. By means of doping Si or Sn into Ti C-Ti-Al system, the synthesis purity of Ti3 Al C2 can be improved observably. Especially doping Si and Sn into Ti C-Ti-Al system simultaneously, the purity of Ti3 Al C2 corresponding to 2Ti C-Ti-1.2Al-0.05Sn-0.1Si system can reach to 98.94 wt.%. The properties such as Vickers hardness, flexural strength and fracture toughness of the sample corresponding to 2Ti C-Ti-1.2Al-0.05Sn-0.1Si system are the best, which are 3.68 GPa, 530.23 MPa and 7.06 MPa·m1/2, respectively.A new Ti3 Al C2-Ti2 Al C/Ti Al composite was successfully fabricated by in-situ reaction hot-pressing process in the Ti-Al-Ti3 Al C2 system at 900 °C for 2 h. During the process, Ti3 Al C2 partially decomposed into Ti2 Al C and Ti C, and then Ti C reacted with Ti Al intermetallic to form Ti2 Al C further. The reinforcements mainly distribute in the grain boundaries. Ti3 Al C2 additive significantly restrains the growth of Ti Al grain. The dispersion of Ti3 Al C2 phase becomes worse with the increasing Ti3 Al C2 content. The composite with 5 wt.% Ti3 Al C2 additive presents better properties, the Vickers hardness, three-point bending strength and fracture toughness are 2.7 GPa, 316 MPa and 7.3 MPa·m1/2, respectively. The toughening of the composite is primarily attributed to zigzag crack deflection, transgranular and/or translamellar cracking, pull-out of the reinforcements and cleavage phenomenon, especially, suitable interface bonding strength, and the combination toughening of Ti3 Al C2 and Ti2 Al C.Ti2Al C/Ti Al in situ composites were successfully fabricated by reaction hot-pressing process using Ti3 Al C2, Ti, and Al powders as initial materials. The products are mainly composed of Ti2 Al C, γ-Ti Al and α2-Ti3 Al as major phases. In situ Ti2 Al C reinforcements are mainly distributed in the grain boundaries resulting in an obvious γ+α2 grains refinement. With increasing the Ti3 Al C2 content from 0 to 15 wt.%, the content of Ti2 Al C increases from 0 to 20.77 wt.% and the γ+α2 grain size decreases from 55 to 19 μm. With increasing Ti3 Al C2 content up to 5 wt.%, the Vickers hardness, flexural strength and fracture toughness of the as-sintered composite reach to the maximum values of 3.7 GPa, 651.5 MPa, and 10.89 MPa·m1/2, respectively. Analysis of fracture surface and crack propagation paths indicates that the grain refinement, crack deflection, crack bridging, in situ precipitated Ti2 Al C phases obtained by decomposing of Ti3 Al C2 are the main reasons responsible for the toughening of the composites.Ti Al/Ti2 Al C composites are successfully fabricated by in-situ reactive hot pressing method in the TixAly-Ti3 Al C2 system at 1150 °C. With increasing Ti3 Al C2 content, the grain size decreases gradually. With increasing Ti3 Al C2 content up to 5 wt.%, the Vickers hardness, flexural strength and fracture toughness of the as-sintered composite reach to the maximum values of 5.14 GPa, 921.8 MPa and 7.2 MPa·m1/2, respectively, which is 232.6% and 23.6% higher than Ti Al alloy. The action mechanism of in situ Ti2 Al C/Ti Al matrix composites is mainly attributed to the grain refinement, the uniform distribution of Ti2 Al C, the second-phase strengthening, grain refinement, transgranular cracking, crack deflection, crack bridging and pullout of Ti2 Al C. On account of the above reasons, the crack propagation needs more energy, which will hinder gradually. As a result, the toughness is improved.Ti Al/Ti2 Al C composites are successfully fabricated by in-situ reactive hot pressing method in the Ti-Al-Ti C system at 1300 °C. The size of the in situ Ti2 Al C particles is 5-10 μm. Ti2 Al C are mainly distributed in the Ti Al grain boundaries. With increasing Ti2 Al C content, Ti2 Al C are distributed over cluster status. Ti Al/15 wt.% Ti2 Al C composite possesses the highest bending strength of 486 MPa and the highest fracture toughness of 7.78 MPa·m1/2, due to the uniformly distributed Ti2 Al C grains with the moderate content. With increasing Ti2 Al C content, the ceramic particles distribute more densely, which lead to the decrease of mechanical properties and the increase of density. The action mechanism of in situ Ti2 Al C/Ti Al matrix composite is mainly attributed to the second phase strengthening, grain refinement and mixture crack modes such as transgranular cracking, interlaminar tearing, layered stripping, crack bridging, crack branching and crack deflection.Dense Ti3 Al C2/Ti Al3 composite has been successfully synthesized by in situ hot pressing in the Ti-Al-Ti C-CNTs system at 1350 °C for 2 h. As reinforcing agent, Ti3 Al C2 are mainly distributed in the Ti Al3 grain boundaries, and exhibits significant strengthening and toughening effect to the Ti Al3 matrix. The bonding between Ti3 Al C2 and Ti Al3 is very tight. There are two different morphologies of reinforcements, particle-like and needle-like precipitates. The flexural strength and fracture toughness of the Ti3 Al C2/Ti Al3 composite can reach to 487.2 MPa and 5.5 MPa·m1/2, respectively, which is higher than the flexural strength(162 MPa) and fracture toughness(2 MPa·m1/2) of Ti Al3. Toughening is mainly attributed to crack deflection, crack bridging, crack branching and pull-out of Ti3 Al C2 particles as well as transgranular cracking. The strong interface combination strength and the dispersed Ti3 Al C2 particulates are responsible for the enhancement of the strength.Ti2Al C/Ti Al3 composites are successfully fabricated by in situ hot pressing in the Ti-Al-Ti C-CNTs system. Overlap joint lamellas can be obtained. The flexural strength and fracture toughness of the Ti2 Al C/Ti Al3 composites reach to 343.21 MPa and 6.5 MPa·m1/2, respectively, which is 225% and 11.86% higher than Ti Al3 alloys. |
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