Study On The Formation Mechanism,Microstructure And Properties Of High Porosity TiAl₃ Intermetallic Compound

TiAl-based intermetallic compound has been considered as a new type of light-weight and high-temperature structural materials because of the advantages of low density,high strength,corrosion resistance,wear resistance,and excellent oxidation resistance at high temperature.Porous Ti-Al intermetallic compound can be used as the potential filter,separation,and purification material since it is easy to form interconnected pore channels with uniform pore distribution during the sintering process and the fabricated Ti-Al materials exhibit excellent oxidation and acid/alkali corrosion resistance.In this work,Ti/TiH₂ and Al powders are used as the raw materials to prepare porous TiAl₃ intermetallic compound by reaction sintering.The combustion behavior and macroscopic morphology of the compact are analyzed by DSC-TG,temperature-time curve,and visual images during the entire sintering process.XRD,SEM,EPMA,TEM,and other equipment are used to investigate the phase composition and pore structures of the sintered compact.The diffusion reaction coefficient and diffusion activation energy of TiAl₃at low-temperature sintering are calculated.The diffusion reaction mechanism at low temperature and interface reaction mechanism at high temperature are also analyzed,and the reaction formation mechanism of the compounds and pore evolution mechanism are further explored.Finally,the factors that affect the compressive strength and high-temperature oxidation resistance of porous sintered compacts are studied.Ti-75 at.%Al powder compact experience the mixing and compacting process,and then porous TiAl₃ sintered compacts are synthesized by thermal explosion reaction and homogenization treatment at high temperature.The disappearance of the silver wires and the melting of Na Cl particles that pressed into the surface of the sample indicate an obvious thermal explosion reaction in Ti-75 at.%Al powder compact during the sintering process,and a large amount of heat is released at the same time,which makes the sample temperature increase significantly in a short time.The entire sintering stage can be divided into unreacted zone,pre-combustion reaction zone,platform zone,thermal explosion zone,and rapid cooling zone.The compact is instantly ignited to complete the entire combustion reaction,and then quickly cools to the furnace temperature.After thermal explosion reaction,the volume expansion ratio and open porosity are 77.1%and 56.3%,respectively,and the pores are evenly distributed;the product particles grow up and form obvious sintering necks after homogenization sintering at high temperature,and the porosity drops to 51.0%,which ensures the rich pore structure and certain mechanical properties of the porous TiAl₃ material.The effect of sintering atmosphere and heating rate on the combustion behavior,macro morphology,volume expansion,and pore structures of porous TiAl₃intermetallic compound are studied.The results show that severe cracking and deformation occur when the sample is sintered in an argon atmosphere.The oxygen content of the sintered compact decreases from 5.83 wt.%to 2.43 wt.%as the heating rate increases from 1?/min to 15?/min.When the sample is sintered in a vacuum atmosphere,it can maintain a complete shape and the highest oxygen content is only1.12 wt.%.It is known that the sample undergoes a multi-step diffusion reaction at a heating rate of 1?/min from the thermal explosion curve,visual images,and thermodynamic analysis.The product particles grow into a continuous skeleton structure with the largest volume expansion rate(129.7%)and porosity(67.2%).When the heating rate increases from 2?/min to 15?/min,the content of liquid Al that involved in the thermal explosion reaction increases,so the heat release is higher,and the combustion temperature of the sample increases from 698? to 1169?;the heat release increases from 533 J·g^-1 to 1303 J·g^-1;the open porosity increases from 49.8%to 56.6%;the air permeability improves from 84 m³/h·k Pa·m² to 182 m³/h·k Pa·m²,and the average size of small pores is between 19?30?m.The combination of thermal explosion and the addition of space holder particles can effectively improve the open porosity of the final sintered compact.The results show that the combustion temperature of the powder compact decreases from 1139? to 997? when the volume content of urea particles increases from 0 vol.%to 60 vol.%,and the phase composition is still pure TiAl₃ phase.Meanwhile,the TiH₂particles in TiH₂-3Al powder compact undergoes a decomposition reaction during the insulation period at 550? when TiH₂ powder is used instead of Ti powder,resulting in the appearance of more active Ti atoms.Thus,the exothermic reaction becomes more obvious,and the maximum combustion temperature and open porosity reach 1193? and 81.4 vol.%.The final pore structures consists of large pores that left by the space holder particles and small pores that formed between the product particles.In addition,porous TiAl₃ materials with three-layer and five-layer gradient symmetrical pore structures are successfully prepared,and the formed layers are tightly bonded.The corresponding porosities are all above 70%,and the large pores as well as the small pores are evenly distributed and interconnected to each other.The effects of sintering temperature and insulation time on the reaction mechanism are explored.The reaction process of Ti-75 at.%Al powder compact can be consisted as diffusion reaction,diffusion-controlled gentle thermal explosion reaction and liquid-induced violent thermal explosion reaction when the sample are kept at 600?,650? and 670? for a certain time.The thickness of the diffusion layer that formed by the diffusion reaction at low temperature increases with the increase of the sintering temperature and the extension of the insulation time.The growth activation energy that controlled by the diffusion reaction is calculated to be 161.5 KJ/mol.When the sample temperature reaches the melting point of Al,the appearance of liquid Al induces the solid-liquid interface reaction,causing that the thickness of the reaction layer grows in a straight line.The pore evolution process of Ti-75 at.%Al powder compact during continuous sintering is concluded as the interstitial pores exist between the powder particles in the green compact;Kirkendall pores are formed at the position of solid Al particles owing to the diffusion reaction between the solid Ti and Al particles when the sample temperature is below the melting point of Al;once the sample temperature reaches the melting point of Al,the liquid Al quickly spreads around the solid Ti particles,leaving a large number of pores in the original liquid Al position and the formation of large amounts of small pores between the product particles after the thermal explosion reaction.In addition,the large in-situ pores left by the space holder particles can greatly increase the open porosity.When the sintering temperature continues to increase(>1000?),part of the pores that existed in the sample will close at the high-temperature homogenization sintering,and the open pores decrease.The compressive strength and high temperature oxidation resistance of porous TiAl₃ materials are studied.The results show that the compressive strength of porous TiAl₃ sintered compacts decreases with the increase of porosity,showing the characteristics of brittle fracture.Similarly,the high-temperature oxidation resistance of porous samples also decreases with the increase the porosity.The weight gain in the initial 12 h is relatively high,exhibiting a linear growth law,and then tends to be stable in the subsequent oxidation process.As the oxidation time is extended to 120 h,the oxidation degree keeps stable,which follows the parabolic growth law.The high-temperature oxidation resistance of porous TiAl₃ material is significantly better than that of commonly used porous metal Ni,porous Ti and porous stainless steel materials.Al-based porous intermetallic compounds that synthesized by the thermal explosion method have uniform pore size distribution,controllable porosity and pore morphology,and it is easy to form a dense Al₂O₃ layer around the TiAl₃ skeletons during the oxidation process.Therefore,it has a wide application prospect in the fields of solid-gas and solid-liquid filtration and separation,therefore has the potential feasibility for large-scale production and use in the future industrialization.