Preparation And Properties Of Micro-nano Porous Structure Iron Alloys

Porous iron alloy,a novel metallic material with unique structural and functional properties developed in recent decades,shows promising application prospects in anti-collision devices and damping components used in modern transportation,due to its lightweight,high vibration and energy absorption capability.Compared with porous aluminum,titanium,nickel based alloys,porous iron based alloys have a much wider application prospects and more excellent mechanical properties.Nevertheless,the inability to simplify preparation process and reduce preparation cost has limited their further developments and applications,causing a main issue in the future development of porous iron based alloys that how to optimize preparation process and greatly improve mechanical properties.However,the low mechanical properties and complicated preparation process are the obvious drawbacks of porous metallic materials limiting their further applications.Therefore,experimental investigation focusing on simplifying the preparation process and greatly improving the mechanical properties remains an issue of great concern,which contributes to the future development of porous iron alloy.For porous metallic materials at a similar porosity level,ultra-fine pores size is a guarantee of much higher specific surface area and interfacial areas,and simultaneously contributes to refine the microstructure and improve the mechanical properties.Therefore,based on our previous researches,this paper aims at further improving the mechanical properties of porous iron alloy and eliminating the influence of foaming agent(or pore-forming agent)on the environment during the preparation process.In this paper,the micro-nano porous structure Fe–N particles were used to fabricate micro-nano porous structure iron alloy with high porosity and good mechanical properties via sintering technology.The Fe–N powder utilized in the present study was synthesized using ammonia reduction and nitridation of iron oxide powders at 600?.The Fe–N powder obtained in this work has an average particle size of 360 nm,with its composition consisting of ?-Fe2 N and ?-Fe3 N.It has hollow porous structure and the average pore size is about 89.8 nm.The thermal stability of Fe-N powders were tested by differential thermal analysis(DTA)and vacuum annealing.The results show that the submicron hollow Fe-N alloy powder is metastable.The more nitrogen content of the Fe-N alloy has,the worse its thermal stability is.During heating in the temperature ranging from 240 ? to 780 ?,the Fe-N alloy with a high nitrogen content will continuously transform into Fe-N alloy with a lower nitrogen content as the temperature increases.Finally,almost all Fe-N alloys were transformed into high purity ?-Fe.The submicron porous Fe-N powders were sintered by conventional sintering and SPS technology,whereas the sintering process parameter varies.The porosity,microstructure,and compressive properties of sintered specimens were studied by modern test methods.The neck between powders can rapid form and grow quickly after the temperature reaches up to 950 ? under conventional sintering condition.Accompanying by grain coarsening,massive pores were swallowed in this case.The main component of sintered specimens is ?-Fe,benefiting a higher compressive strength and excellent plasticity.By comparison,bulk Fe-N specimens were successfully fabricated at lower sintering temperature by using SPS technique under axial pressure of 50 MPa.It should be noted that the nitrogen concentration in the sintered specimens is gradually decreases with increasing sintering temperature,but the compressive properties and the soft magnetic properties are improved.When the sintering temperature is 700 ?,the compressive strength is significant improved due to the formation of hybrid phase of ?'-Fe4 N and ?-Fe.It indicates that nitrogen content in a low level can cause an obvious strengthening effect.When the sintering temperature reach up to 750 ?,almost all Fe-N alloys are transformed into ?-Fe,whilst the grains are coarsened as well.There are some pores formed in the sintered specimens,which is mainly because of denitrification reaction during Fe-N phase transformation.Moreover,high porosity(>40 vol %)iron alloy specimens with micro-and nanoscale isotropic pores were successfully fabricated by carrying out free pressureless spark plasma sintering(FPSPS)at 750?.The porous structure in Fe-N powder and denitrification reaction during sintering process have contributed to the formation of ultrafine porous structure,resulting excellent compressive properties and energy absorption behavior.The pore characteristics are adjustable by controlling precompression molding pressure.The dispersion of these pores effectively slows down the grain coarsening and the matrix structure is significantly refined and strengthened.In order to further improve the properties of micro-nano porous iron alloys,Ni-P alloying and surface carburizing treatment were carried out respectively.The Fe-N-Ni-P composite powder has been prepared by applying electroless Ni-P plating on the surface of submicron hollow Fe-N powder.This composite powder was to be used in preparation of higher porosity(41%)Fe-Ni-P alloy via liquid phase sintering at 900?.The sintered porous Fe-Ni-P alloy shows a optimal combination of higher hardness,excellent compressive strength,and good corrosion resistance.This can be ascribed to the solid solution strengthening of phosphorus and second phase strengthening of precipitated phosphide.In addition,the micro-nano porous iron alloys prepared by FPSPS were carburized in solid medium at 950 ?,with followed quenching and tempering processes.To be delighted,the surface hardness and compressive properties of micro-nano porous iron alloys were enhanced significantly by surface martensite strengthening.Ultimately,the sintering process and influencing factors of the submicron porous Fe-N alloy powder were analyzed by utilizing sintering theory.Based on the experimental results of the FPSPS,the formation process and strengthening mechanism of micro-nano porous structure were analyzed.More crucially,the relationship between pressure,sintering temperature,and porosity was investigated and deduced carefully,with a detailed discussion on the strengthening effect and mechanism of Ni-P alloying and surface carburization.