Formation Mechanism,Microstructure And Oxidation Resistance Of Porous Al-Containing Intermetallic Compound

Inorganic porous materials are widely used in separation of mineral,chemical engineering,environmental protection and high-temperature gas purification due to their low density,large specific surface area,sound insulation,heat insulation and good permeability.Porous ceramic materials have the disadvantages of high brittleness and poor thermal shock resistance,so it is difficult to ensure the stability of the parts.Meanwhile,porous pure metal materials have poor corrosion resistance and high temperature resistance,which limits their application.Porous Al-containing intermetallic compounds have the common advantages of ceramic and metal materials because they contain both covalent bonds and metal bonds.In view of the shortcomings of the existing preparation methods of porous intermetallics,such as long production cycle and high energy consumption.This work uses the heat released by the system itself in the process of thermal explosion reaction,combined with the highly porous structure characteristics of combustion synthesis products,to rapidly,energy-efficient and simply prepare porous Al-containing intermetallics with high porosity and adjustable pore structure.And explores the effect of thermal explosion reaction parameters on the phase composition,microstructure and oxidation resistance.Revealing the diffusion mechanism of elements during thermal explosion reaction,and clarifies the source and formation mechanism of pores in porous products.By mixing and pressing Cu powder and Al powder,heating the powder compacts to 550℃to ignite the thermal explosion reaction,and the porous Cu-Al intermetallics can be prepared within 1 h,which proves that the thermal explosion reaction is a time-saving and energy-saving synthesis process.With Al content increases from 33 at.%to 66 at.%,the ignition temperature of thermal explosion reaction remained unchanged at 555℃,while the maximum combustion temperature decreased from 968℃to 593℃.The porosity of porous Cu₉Al₄,CuAl and CuAl₂ is 45%,41%and 23%respectively.The oxidation behavior of porous Cu-Al can be divided into early rapid oxidation,intermediate slow oxidation and final stable stage.Compared with commercial porous 316L stainless steel,Cu foam,Ni foam and Al foam,the porous Cu-Al intermetallic synthesized in this thesis has excellent high-temperature oxidation resistance and can be used as a potential alternative material in high-temperature environment.In order to solve the problem of insufficient porosity caused by shrinkage of products after thermal explosion reaction of Cu-Al system,porous Cu-Al intermetallic with high porosity were prepared by thermal explosion reaction and traditional sintering process using NaCl particles as space holder.The results show that the temperature of the sample rises sharply from 560℃to 775℃within 3 s due to the heat released by the self-chemical reaction between Cu andAl.Porous Cu-Al has a bimodal pore size distribution by using a space holder and dissolving and eliminating it before the reaction,wherein the macropore size ranges from 200 to 300μm,showing the characteristic morphology of the original NaCl particles,the pores with size of 5～10μm are distributed on the wall of the large pore,which is derived from inter particle pores generated by thermal explosion reactionFe-Al intermetallic foam was prepared by a simple and rapid thermal explosion process.The effects of Al content and sintering temperature on the phase composition,microstructure and oxidation resistance of Fe-Al intermetallic foam was studied.The Fe-Fe₂Al₅ core-shell structure skeleton was formed by Fe and Fe₂Al₅,which were not completely involved in the transformation after the reaction,and a single Fe-Al phase porous product could be obtained through subsequent high temperature homogenization sintering.WithAl content decreases from 50 at.%to 40 at.%,The ignition temperature gradually increased from 623℃to 636℃,while the maximum combustion temperature decreased from 1059℃to 981℃.Fe-Al based intermetallic foam has a high porosity of 59-61%,and the three-dimensional connected pores provide an open foam structure for the product.In order to solve the problem that the highest combustion temperature（1644℃）of Ni-Al system exceeds the melting point（1638℃）of NiAl phase,which leads to the melting of products,highly porous NiAl intermetallic compounds were successfully prepared by combining low-temperature heat treatment with thermal explosion reaction.The effects of preheating process and Al content on exothermic behavior,phase composition and microstructure were studied.The results showed that NiAl₃ and Ni₂Al₃ intermediate phase were formed at 550℃for 0.5 h,the maximum combustion temperatures of Ni-25 at.%Al,Ni-50 at.%Al and Ni-75 at.%Al powder compacts are1315℃,1530℃and 850℃respectively,which are lower than the melting point of the corresponding product phase.WithAl content increases from 25 at.%to 75 at.%,the phase composition of thermal explosion products is changed from Ni₃Al+NiAl+Ni mixed phase to single NiAl phase and NiAl₃ phase.The open porosity of porous Ni₃Al,NiAl and NiAl₃ is 34%,52%and 63%respectively.The pore structure of porous Ni₃Al and NiAl₃ consists of the space occupying pores left by Al in the process of thermal explosion reaction and the interstitial pores between small skeleton particles;Porous NiAl is only composed of a single integral NiAl skeleton and occupied pores ofAl.After oxidation at 900℃for 120 h,the mass gain of porous Ni₃Al,NiAl and NiAl₃ is 12.18%,2.9%and 16.9%respectively.The application of thermal explosion reaction in the preparation of refractory Al compounds has been further expanded,and porous NbAl₃ intermetallic have been prepared simply and quickly.The thermal explosion reaction of Nb and Al was ignited at 791℃and completed by liquid-solid reaction mode.The prepared porous product consists of single-phase NbAl₃,and Nb and Al elements are uniformly distributed in the sample.The obtained porous NbAl₃ intermetallic compound has a high open porosity of 65.7%,a density as low as 1.56 g/cm³,and a sponge like and highly porous skeleton structure.When the porous NbAl₃ sample is oxidized at 400℃,the protectiveAl₂O₃ oxide film is formed on the skeleton surface at the early stage of oxidation.The porous NbAl₃ intermetallic follows the parabolic oxidation rate law in the stable oxidation stage,and the mass gain is only 0.19%after 144 h of oxidation.In addition,porous NbAl₃/TiAl₃ intermetallic composites with controllable pore morphology and porosity were prepared by using NaCl as space holder.The addition of TiH₂ powder into Nb-Al system can not only be used as the stabilizer of Al,but also as the diluent of thermal explosion reaction to prevent the cracking and deformation of the products.The ignition temperature and the maximum combustion temperature decreased from 876.7℃and 1368.5℃to 780℃and 818.1℃respectively.The density of porous NbAl₃/TiAl₃obtained from powder compacts containing different volume fractions of NaCl particles ranges from 1.1 g/cm³ to 3.1 g/cm³.The source and forming mechanism of pores of porous Al-base intermetallic are clarified:interstitial pores between raw material particles in powder compacts;Kirkendall pores formed in the process of low temperature solid-state diffusion before thermal explosion reaction;In the process of thermal explosion reaction,Al melts and spreads on the surface of another high melting point component or enters into its interior under the action of capillarity,leaving a large number of space occupying pores in the original position of Al;After the thermal explosion reaction,there are a lot of interconnected small pores between the precipitated product particles;In the process of homogenization at high temperature,a part of pores are lost due to sintering densification,and the porosity of porous products is reduced;NaCl space holder acts as a temporary template.A large number of foreign pores are generated in the powder compacts before the thermal explosion reaction.These pores remain after the reaction,and their size,shape and distribution are the same as that of NaCl particles.