Microstructure And Compressive Properties Of Porous TiAl Matrix Composites Prepared By Pressureless Reactive Infiltration

Among inorganic porous materials,metal porous materials have poor high-temperature oxidation resistance and corrosion resistance,and ceramic porous materials are brittle and difficult to be processed,while porous TiAl matrix composites have broad application prospects due to their excellent performance.In this study,a new type of porous material with cellular connected pore structure and TiAl matrix composites as the skeleton were fabricated by pressureless reaction infiltration with simple process and relatively low cost using Al-12 Si alloys and spherical,irregular loose Ti powder as raw materials.The influences of reaction temperature and holding time on pore structure of porous materials were studied,and the optimum pressureless reaction infiltration technology was determined.Then the influences of titanium powder shape,the volume fraction of Si element and the mass fraction of TiH₂ foaming agent on porosity,pore diameter distribution and microstructure of porous materials were analyzed.The effects of the volume fraction of Ti5Si3 reinforcing phase and the shape of titanium powder on the compressive properties of porous TiAl matrix composites were studied and the pore formation mechanism of porous TiAl matrix composites during sintering were discussed.The higher the reaction infiltration temperature is,the more complete the reaction is,and the higher the porosity is.With the extension of the holding time,the number of pores of the material increases gradually.After the heat preservation time exceeds 2h,the change of porosity is not obvious,and the reaction infiltration is complete.Therefore,the heat preservation time of 800? for 3h is chosen as the best pressureless reaction infiltration technology.The phase composition of the porous materials prepared by spherical and irregular titanium powder after lamination treatment was TiAl,Ti3 Al and Ti5Si3.Compared with the porous material of porous TiAl matrix composites prepared by spherical Ti powder,the porosity of the material prepared by irregular Ti powder is higher,with the total porosity reaching 72.26% and the spherical Ti powder only 54.67%.The percentage of open porosity in both of them is above 70%.After lamination,the irregular Ti powder is completely lamellar,while the spherical Ti powder is partially lamellar.When using TiH₂ as the foaming agent,the porosity of the porous materials prepared without TiH₂ is the highest.With the increase of mass fraction of TiH₂,the porosity of the materials first decreases and then increases.The porosity of porous materials with volume fraction of Ti5Si3 reinforcement phase of 1.2vol.%,3.5vol.% and 6.9vol.%,respectively,showed little difference with different Si content,indicating that the porosity of porous TiAl matrix composites was independent of the content of Ti5Si3.The pore formation mechanism of porous TiAl matrix composites is as follows: With original Ti particles as the precast body,Al-Si alloy melted and infiltrated into porous titanium precast body.A loose and thin TiAl3 layer was formed on the surface of the titanium particles after Al reacted with Ti.At the same time,the Al-Si alloy solution infiltrated into the loose Ti A3 layer under the action of capillary force and destroyed it to form honeycomb TiAl3,and the aluminum solution continued to diffuse into the titanium powder to form dense TiAl3 until the aluminum solution was completely consumed,at which time there were unreacted titanium particles in the center.Pores were formed at the vanishing point of Al liquid,mostly were open pores.After the depletion of the liquid aluminum,only Ti and TiAl3 were left,which continued to react to produce the final product,with Ti3 Al at the heart,followed by the dense TiAl and honeycomb TiAl at the surface,and the Ti5Si3 particles were precipitated at the edge of the dense TiAl.The Ti5Si3/TiAl skeleton was formed at the original titanium powder skeleton,that is,the preparation of the TiAl matrix composites was performed simultaneously with the pore formation.The final material is composed of composite material as the skeleton,and the pores are connected pores and cellular pores.In the process of quasi-static compression,the compressive stress-strain curve of porous TiAl matrix composites is divided into three regions: elastic region,platform region and densification region.With the increase of the volume fraction of Ti5Si3,the yield strength of porous materials first increased and then decreased.When the volume fraction of Ti5Si3 is 3.5vol.%,the maximum compressive strength of porous TiAl matrix composites is 110 Mpa.The stress-strain curve in the platform area is not smooth,but shows jagged fluctuation,and the fluctuation is the largest when Ti5Si3 volume fraction is 3.5vol.%,which is typical of brittle fracture.The porosity of porous TiAl matrix composites prepared by irregular Ti powder of the same quality is higher than that of spherical Ti powder,and the compressive strength is much higher than that of spherical Ti powder.