| Current inorganic porous materials including ceramics and metals show some insurmountable disadvantages such as severe brittleness and unworkability for ceramics and poor corrosion and high temperature oxidation resistivities for metals,and for most inorganic porous material their complex fabrication procedures and high production cost restrict their further developments and applications.Ti-Al alloys have excellent material properties;however,due to the forming difficulty and poor strength at elevated temperature,the current dense Ti-Al alloys can only apply in automobile industry in a limited scale.It is the first time in this paper to systematically present that porous Ti-Al intermetallic compound material can be prepared through the Kirkendall effect using the diffusion rate discrepancy between Ti and Al atoms.The fabricated porous material is a novel substitute for current inorganic porous material.The mentioned fabrication method has the special characteristics of simplicity,controllability,and low cost,which shows greatly propitious to the development of other systems of porous intermetallics and the corresponding industry applications.Three typical porous structures including porous Ti-Al alloy body,asymmetric porous Ti-Al alloy membrane and porous Ti-Al alloy paper membrane were fabricated through the reactive synthesis of Ti/Al elements.The pore structure properties,mechanical properties,and environment corrosion resistivities were further investigated by various advanced testing methods.Meanwhile, the pore formation mechanism was discussed in detail by structural model building and mathematical analysis.Finally,the porous Ti-Al alloy material was successfully applied in the solid-liquid separation of suspended TiCl4 raw liquid which has strong toxicity and corrosivity in industry production procedure,More details were given as follows.Firstly,the three stage sintering process with characteristics of solid reaction and faster diffusion of one component,which was firstly presented and further discussed in the paper,was used to fabricate sound porous Ti-Al alloys kept in good shape condition,which show three typical phases:α2-Ti3Al,γ-TiAl and TiAl3 structures.Bases on the process,the confined sintering procedure was presented to prepare fine-pored and high-accuracy alloys through strictly limiting the radial expansion of compacts.A novel fabrication method based on the reactive synthesis between powders and films, or films and films was presented to prepare homogeneous asymmetric porous Ti-Al alloy membrane or porous paper membrane,respectively.The synthesized porous Ti-Al alloys have the characteristics of the combination of most interconnected pores generated between the resultant particles or along the grain boundaries and minor equiaxial closed pores generated in the resultant particles.Next,the pore structure properties of Ti-Al alloy were investigated thoroughly and detailedly,and the relation equations between the fabrication parameters and pore structure properties were established.Al content was found to be one of the main factors for the porosity,and strictly linear increment rule between the porosity and Al content was confirmed in Ti-Al alloy with the Al content ranging from 20wt%to 60wt%.The Kirkendall porosityθK and Al content CAlwere proved to satisfy the proportion equation ofθK=Kc·cAl.Powder size was the one of the main factors determining the maximum pore size.The maximum aperture dm and powder size dp were investigated to obey the equation dm=Kp·dp strictly.Pressing pressure did not affect the pore structures directly;however,the pore structure properties could be changed through the deformation of powders and pores in green compacts in the pressing procedure.The permeability K,the open porosityθand the maximum aperture dm were verified to meet the Hagen-Poiseuille equation. The universal equation K=A·dm2·θwith A=(2.26±0.05)×107m-1Pa-1s-1was established to reflect the relation between the permeability and pore structure parameters for porous Ti-Al alloy.The mechanical properties and the oxidation and corrosion resistivities were investigated quantitatively in porousγ-TiAl alloy.The tensile strengthσb and the porosityθof porous TiAl alloy obey theБалъшинequationσb=σ0·(1-θ)m strictly.The kinetic equation for the cyclical oxidation of porous Ti-Al alloy at 600℃was determined to beΔm2=1.08×10-5·t.After total oxidation time of 140 hours,porous TiAl alloy shows high pore structure stability and the gain in weight of Ti-Al alloy is only 0.042g·m-2,10.6%of that of 316L stainless steel.The kinetic equation for the cyclical corrosion of porous TiAl alloy at 90℃with PH=2 was determined to beΔm2=5.41×10-5·t-2.08×10-4.After total corrosion time of 50 hours,the lost in weight of TiAl alloy is only 0.049g·m-2,which is 14.8%and 5.57%of porous Ti and stainless steel,respectively.The kinetic equation when PH=3 was determined to beΔm2=2.63×10-6·t-3.72×10-6.The denial criterion to depress the Self-propagation High-temperature Synthesis(SHS)for Ti-Al alloy was established from the pre-reaction model and the thermodynamics condition of SHS reaction.The criterion equation reflecting the relation between the solid-reaction layer thickness correlation parameter and the ignition temperature was deduced.The ignition temperature would be raised rapidly with increasing the correlation parameter,indicating the high efficiency of the three stage sintering process to avoid the possible SHS reaction which would harm the final compact shape.Based on the model and the phase and microstructure analyses,the formation procedure and characteristics of Kirkendall pores in Ti-Al alloy were investigated in details.The asymmetric diffusion couple model for Ti/Al elemental powders was built,and thus the structure characteristic of the powder system was investigated.Based on the model,the kinetic equation system was established to reflect the relationship of Kirkendall porosity,atom diffusion rate,sphere coordinate parameters and diffusion time.As a result,the relation equation reflecting Kirkendall porosity,material properties and alloy composition was built,which shows that the theoretical porosity of Ti-Al alloy depends on the density of Ti/Al components and the alloy composition,and that the final pore size depends strongly on the size of faster diffusion component in compacts.Finally,the fabricated porous Ti-Al alloy material was successfully applied to the solid-liquid separation of suspended TiCl4 raw liquid in industry procedure.The sealed,continuous and long-term filtration procedure was realized using the filtration-backwash technology and porous Ti-Al alloy tubes as filtration elements,which can alleviate dramatically or even avoid raw material loss and severe environment pollution.The alloy tubes exhibited long-term,stable and high flux and accuracy,and the pore structure parameters and the flux were investigated to meet the Hagen-Poiseuille equation.The filtration structure model for porous Ti-Al alloy filters was built. Based on the model,the Rush equation and the experimental data,the filtration equation for porous Ti-Al alloy material in the solid-liquid separation was ultimately built,and the medium resistance Rm for porous Ti-Al alloy was then determined to be 6.12×1010m-1.
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