Study On The Synthesis And Properties Of Porous FeAl-based Intermetallics

Fe-Al intermetallics provide excellent mechanical properties,outstanding oxidation resistance and excellent sulphidation resistance at elevated temperatures.However,the brittleness at room temperature and low high temperature strength at high temperature of Fe-Al intermetallics hinder its application as a structural material.By combining the excellent properties of Fe-Al with the functional characteristics of porous materials,the porous materials can be prepared to solve the disadvantages of poor oxidation resistance and sulfidation resistance for the traditional porous metal,providing the options for the extreme high temperature flue gas filtration in the coal gasification industry.In this paper,FeAl-based intermetallics were fabricated by thermal explosion?TE?from Fe-50Al at.%powders and porous bodies are obtained.At the same time,this preparation method has the characteristics of simple and controllable process and low cost,which is of great significance for the development and application of porous intermetallic compounds.The mechanism of thermal explosion is analyzed,the mechanism of Fe and Al phase transformation and pore formation is discussed,and the relevant physical model is established.The results show that thermal explosion of Fe-Al is ignited at 636?by the solid diffusion reaction between Fe and Al and the TE is performed at the solid-liquid interface.FeAl is evolved as dominant phase in sintered materials at1000?.The porous materials are composed of interconnected skeleton,large pores among skeleton and small pores in the interior of skeleton.The interstitial pores in green powder compacts are the most important source of big pores of porous Fe-Al intermetallics,and the in-situ pores from the melting and flowing of aluminum powders is also significant to the formation of large pores.Small pores are from the precipitation of Fe-Al intermetallics particles.Porous FeAl material possesses the average pore diameter for 23?m and the porosity is up to 60%.The pore formation process of porous Fe-Al intermetallic compounds prepared by thermal explosion is investigated.Pore formation process mainly consists of four stages.First,the interstitial pores are formed between the powder particles of green compact by press process.Second,before the thermal explosion reaction,Al diffuses into Fe,forming the Kirkendall pore at the original Al particles.When TE reaction occurs,Al melting with good wettability infiltrates into particle boundary by the capillary force,which diffuses rapidly and reacts with solid Fe and Fe₂Al₅continuously.The movement and consumption of Al result in lots of larger in-situ pores in the original Al particle sites.In the high temperature homogenization process,the pore morphology is microscopically adjusted to form a three-dimensional porous structure with high porosity.The effects of dynamic factors on TE reaction kinetics and pore structure properties of Fe-Al system are revealed.Results show Al content not only affects TE reaction behavior,but also affects the composition of phase.In a certain range,as the content of Al increases,the ignition temperature of the thermal explosion decreases and the combustion temperature increases.In addition to the FeAl phase,FeAl₂ phase is generated when the content of Al increases.Both porosity and average pore size increase.Heating rate has a certain influence on TE reaction behavior.In a certain range,with the increase of heating rate from 1?/min to 15?/min,the ignition temperature decrease from 655?to 632?,the combustion temperature increases from 856?to1121?.And the thermal explosion reaction is more intense,the heat released by the reaction is more,and the porosity and average pore size of the product increase.However,the heating rate has no effect on the phase of matter.The powder particle size has certain influence on the hot explosion behavior,microstructure and pore structure of Fe-Al system.For Fe,as the powder particle size decreases from 100?m to 5?m,the ignition temperature of Fe-Al system decreases,the combustion temperature increases and the porosity and average pore size decrease significantly.For Al,as the powder particle size decreases,the ignition temperature of the system decreases,the combustion temperature increases,and the porosity and average pore size decrease.Compared with Al powder,Fe powder particle size has more significant effects on combustion temperature,phase composition and porosity.Chromium?Cr?alloying is carried out on the basis of Fe-40 at.%.The results show that the ignition temperature of the thermal detonation reaction increases and the combustion temperature decreases with the increase of Cr content.When Cr content is up to 20 wt.%,FeAl phase is the dominated phase.With the increase of Cr content,the pore distribution becomes more uniform,the pore size becomes larger and the degree of pore connectivity increases,and the porosity increases up to 6%.Porous FeAl/Al₂O₃ composites are synthesized in situ using Fe,Al and Fe₂O₃powder.The results show that the ignition temperature and combustion temperature of FeAl-Fe₂O₃ system change significantly with the addition of Fe₂O₃.The high exothermic rate of aluminum thermal reaction increases the combustion temperature.After sintering at 1100?,the phase of the product is FeAl phase and a small amount of Al₂O₃ phase,and the content of Al₂O₃ phase increases with the increase of the amount of Fe₂O₃ added.The porosity of porous materials is between 52%and 61%,with an average pore diameter of 27-32?m.The mechanical properties,oxidation resistance and sulphidation resistance of porous FeAl materials are investigated.The results show that the relation between compressive strength and porosity of porous FeAl materials conforms to Knudsen's empirical formula,that is,compressive strength decreases monotonously with the increasing porosity.The effect of Al content on the compressive strength of porous FeAl is not monotonically increasing.When the Al content exceeds 50 at.%,the compressive strength decreases with the increase of Al content.The oxidation behavior of porous FeAl material at high temperature follows parabola law.As the porous FeAl materials produced a compact Al₂O₃ film during the oxidation process,their xidation resistance were significantly superior to the commonly used metal porous materials 316L stainless steel and Ni.With the increase of Al content,the xidation resistance of the material is improved.The high temperature oxidation resistance of porous FeAl materials is affected by the FeAl-based phase structure and pore structure characteristics of porous materials.The sulphidation behavior of porous FeAl materials in SO₂ atmosphere of 500?obeys the parabola law.The sulphidation performance is better than that of metal porous 316L stainless steel and Ni.With the increase of Al content,the sulphidation property of the material is improved.The porous FeAl material has good sulphidation resistance mainly due to the formation of a compact Al₂O₃ film in the process of sulphidation resistance,thus preventing further sulphidation of the matrix.In addition,the oxidation and sulphidation resistance of FeAl with Cr alloyed materials are studied.The results show that the oxidation and sulphidation resistance of FeAl with Cr alloyed materials are improved.