| TiAl alloy is a high-temperature alloy that integrates the advantages of high strength,high elastic modulus,high specific strength,good oxidation resistance,creep resistance and corrosion resistance.However,the alloy still has two shortcomings that need to be solved urgently:poor plasticity at room temperature and low strength at high temperature.At present,the optimization method of TiAl alloy has not yet achieved the hope of improving both plasticity and strength.Internationally,the update iteration of TiAl alloys is developing in the direction of changing the microstructure to improve the mechanical properties,but the influence mechanism of the microstructure configuration on the properties has not been clearly revealed,which restricts the TiAl alloy as an excellent superalloy.The development prospect requires in-depth analysis of the microstructure regulation of the main constituent phases of the alloy and the regulation of the reinforcement phase,and from the multiple perspectives of the regulation of different levels of microstructure and configuration to achieve the performance enhancement of TiAl alloy and reveal its regulation mechanism.Therefore,in this paper,the TiAl system,Ti-Al-C-Mn and Ti-Al-B-Ta/Mo system were consolidated by two means of hot pressing integration and electric sintering.The influence of the compressive properties of the system was discussed,and the influence of the two strengthening modes of metal+in-situ ceramic particles and dual-phase in-situ ceramic particles on the microstructure and properties of TiAl alloys was also revealed.,the regulation law of existing state on TiAl alloy,and the strengthening mechanism of TiAl alloy by two strengthening modes was analyzed from the perspective of constructing a new microstructure and fracture.This paper mainly has the following three innovations:1.Revealed the principles and differences of different sintering mechanisms for consolidating Ti-Al system and Ti-Al-C-Mn system:1)The hot-pressing sintering reaction window time is very short and it is not easy to achieve densification;the electro-sintering mechanism was powder flashing.The densification method of the specimen is realized under the combined action of rapid consolidation,rapid atomic diffusion and high-speed plasma migration.Compared with TiAl alloys and TiAl matrix composites produced by traditional hot pressing sintering,the consolidated bodies produced by electrosintering have higher density and the composites have both high strength and high plasticity.2)Two matrix phases ?-TiAl and ?2-Ti3Al and a heterogeneous ceramic phase Ti2AlC are obtained in both sintering methods.Due to the high-energy reaction between the powders during the electric sintering process,the hot-pressing sintering method should be solid-dissolved in the matrix.Under the action of high-intensity direct current,the Mn in it is precipitated between the laths of the matrix as another heterogeneous Mn3AlCceramic phase.2.Revealed the remarkable properties of Ti-Al matrix composites constructed by electrosintered Ti-Al-C-Mn,Ti-Al-B-Ta/Mo system with multi-scale,multi-morphic and multi-type microstructure of ceramic particles The goal of strengthening and toughening the composite material was achieved:1)Trace ceramic particles exist in the TiAl matrix in the form of different types,different sites and different sizes.In the Ti-Al-C-Mn sintered system,Ti2AlC was pinned at the grain boundary while Mn3AlC is pinned in the matrix lath and the particle size of Mn3AlC was much smaller than that of Ti2AlC;in the Ti-Al-B-Ta sintered system The ceramic phase TiB2 was pinned at the grain boundary,and the ceramic phase TaB was interspersed in the whole microstructure in the form of needles;in the Ti-Al-B-Mo sintered system,the ceramic phase TiB2+MoB were alternately arranged and pinned at the grain boundary.The heterogeneous ceramic phases Ti2AlC,Mn3AlC,TiB2,and TaB were semi-coherently combined with the matrix interface,and only MoB is approximately coherently combined with the matrix interface.2)The composite strengthening of different types of ceramic particles will have different effects on the grain size of titanium-aluminum matrix composites.Compared with the average grain size of the electrosintered TiAl alloy(58?m),the average grain size of Ti2AlC+Mn3AlC/TiAl,TiB2+TaB/TiAl,TiB2+MoB/TiAl was refined by 67.2%,74.1%,and 62.1%.3)The regulation of the properties of TiAl matrix composites by the combination of endogenous nano-strengthening phases is as follows:Compared with TiAl alloys,the yield strength(1122 MPa)and fracture strain(22.3%)by 55.8%and 28.3%respectively.The yield strength of TiAl matrix composites strengthened by complex TiB2+TaB increased from 715 MPa to 1090 MPa,the fracture strain increased from 18.8%to 23.1%,and the bending yield increased from 348 MPa to 441 MPa;The yield strength(1156 MPa)and fracture strain(23.5%)of the composite were increased by 61.7%and 25.0%,respectively;the yield strength(1264 MPa),compressive strength(2238 MPa),and fracture strain(23.8%)of the composite were compressed at 500?.)increased by 76.8%,16.1%,and 27.6%,respectively.3.Revealed the strengthening and toughening mechanism of the four composite materials controlled by the heterogeneous strengthening phase:1)Strength improvement mechanism:composite material Mn+Ti2AlC/TiAl:grain boundary strengthening,dislocation strengthening,Orowan strengthening and solid solution strengthening;Ti2AlC+Mn3AlC/TiAl,TiB2+TaB/TiAl,TiB2+MoB/TiAl:grain boundary strengthening,dislocation strengthening,Orowan strengthening,and dislocation strengthening is the main one.During high temperature deformation,the strength enhancement mechanism is mainly grain boundary strengthening.2)Plasticity enhancement mechanism:At room temperature,the strengthened grain boundaries of the composites will coordinate the slip deformation between adjacent grains,making the plastic deformation of the composites more uniform.At high temperature,nanoparticles pin the grain boundary,which prevents the grain boundary from slipping to a certain extent,but it can still assist high-temperature plastic deformation through coordinated deformation between different grains,which is better than room temperature plastic deformation.3)Toughness improvement mechanism:At room temperature,the improvement of the toughening behavior of TiAl-based composites is mainly achieved by nanoparticle-pinned grain boundary scattering crack propagation.At high temperature,serpentine fracture ridges and cliff-like slip morphology appear in the composites,making the crack propagation more slowly. |
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