Investigation On The Porous FeAl Intermetallics

In this paper systematically present that porous Fe-Al intermetallics can be prepared through the reactive synthsis of Fe and Al elemental powders.The fabricated porous material is a novel substitute for current inorganic porous materials.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.Firstly,the three stage sintering process with characteristics of solid diffusion reaction and liquid Al reaction was presented and further discussed in the paper.The there stage sintering process was used to fabricate near net shape porous Fe-Al intermetallics.The synthesized porous Fe-Al intermetallics have the characteristic of the combination of most interconnected pores generated between the resultant particles or along the grain boundaries.The pore structure properties of Fe-Al intermetallics were investigated thoroughly and detailedly.Based on the reactive synthsis procedure and the pore structure evolution,pore formation mechanisms of porous Fe-Al have been explored.Pores of porous Fe-Al are formed in four different steps in our reactive synthesis of porous Fe-Al intermetallics.They are(1) inter-particle pores formed during the pressing procedure,(2) Kirkendall pores formed during the sintering below the melting point of Al,(3) growth of the pores due to the rapid liquid Al reaction when the sintering temperature passes the melting point of Al and(4) the further pore growth due to the phase transformation.Tube diffusion couple model for Fe/Al elemental powders was built,and thus the structure characteristic of the powder system was investigated.The relation equations between the fabrication parameters and pore structure properties were established.Powder size was the one of the main factors determining the maximum pore size.The maximum pore size d_m and powder size d_p were investigated to obey the equation d_m =0.4·d_p strictly.The pore structure properties could be changed through the deformation of powders and pores in green compacts in the pressing procedure.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 Fe-Al intermetallics with the Al content ranging from 20wt%to 45wt%.The permeability K,the open porosityθand the maximum aperture d_m were verified to meet the Hagen-Poiseuille equation.The universal equation K=Ad_m²Î¸with A=(2.32±0.05)×10¹¹m^-1kPa^-1h^-1 was established to reflect the relation between the permeability and pore structure parameters for porous Fe-Al intermetallics.The pore structure of porous Fe-Al intermetallics can be adjusted in pressing procedure and reactive synthsis procedure.In pressing procedure, the pore structure of porous FeAl intermetallics can be adjusted through changing the mechanical composition,powder size and pressing pressure. The porosity was resulted with the Al content;the maximum pore size was decided by raw Al powder size.In reactive synthsis procedure,the pore structure of porous Fe-Al can be adjusted by changing the solid diffusion reactive process parameters including temperature and holding time.In our case,the open porosityθand holding time t at 600℃has the relationship ofθ=44.4-0.075t(60≤t≤240).The application of adjusting the pore structure of porous FeAl in pressing procedure and reactive synthsis procedure at the same time can adjust the pore structure parameters in a large range.The mechanical properties including room temperature and high temperature were investigated quantitatively in porous Fe-Al intermetallics.The tensile strengthσ_b and the porosityθof porous Fe-Al intermetallics obey theБалъшинequationσ_b=σ₀(1-θ)^m strictly. Porous Fe-Al featured with excellent high temperature mechanical property.The tensile strength of porous FeAl at 600℃can be improved～36.8%of tensile strength of room temperature.The tensile strength of porous FeAl at 800℃is～10MPa。Homogenous and heterogeneous brazing of porous Fe-Al intermetallics have been successfully brazed using Cu-10wt%Sn as-pressed compact as brazing filler.Cu-Sn intermetallic phases were formed in the brazing line for homogenous brazing,while as(Cu,Sn) solid solution was the main phase formed for heterogeneous brazing. Maximum tensile strength for homogenous and heterogeneous brazing is 75.0MPa and 83.9Mpa which is about 81.5%and 91.2%of that of porous Fe-Al intermetallics(～92.0Mpa),respectively.The interface structure of the stainless steel/porous Fe-Al joint brazed is S-S+(Cu,Sn)/(Cu,Sn)/ Cu₉Al₄+(Cu,Fe)+(Cu,Sn)/ AlFe₃+ Al₄Cu₉ +(Cu,Sn).The weakness surface of heterogeneous brazing is filler/porous Fe-Al.In the porousFeAl/porous FeAl joint brazed with Cu-Sn filler at 940℃,Cu-Sn intermetallics and(Cu,Sn) solid solution were the main phases in the joint line.The weakness surface of homogenous brazing is the center of filler. Homogenous and heterogeneous sound joining of porous FeAl can be obtain by laser welding.The tensile strength of homogenous welding is the same with porous FeAl.For heterogeneous welding,since there are gas pores and flaw in the joining line occasionally,the tensile strength is 86.3MPa,which is almost 93.2%of that of porous FeAl intermetallics.