Composition Design And Study On High-temperature Oxidation Behavior Of New Fe-Ni-Cr-Al Austenitic Heat-resisting Alloys

Ultra（super）critical thermal power and advanced nuclear power is currently an important direction of energy development in the world,and a major basic industry involving the lifeline of national economic development.With the development of power generation units to large capacity and high parameters,the performance requirements of key equipment materials for power equipment are becoming increasingly higher.Consequently,the development of new heat-resistant materials is urgently needed to meet the requirements of ultra-supercritical thermal/nuclear power equipment tubes.The excellent high-temperature mechanical properties and oxidation resistance of the new Fe-Ni-Cr-Al（Alumina-Forming Austenitic,AFA）heat-resistant alloy make it a potential candidate material for advanced power equipment.The key to improving the service temperature of new alloys is the formation ofα-Al₂O₃ protective layer,and the scientific issues such as the influence of alloying design on the mechanism of oxide film formation,the evolution pattern of alloy oxide film and the antioxidant properties of the material have not been systematically studied yet.In order to investigate these issues,this study was conducted on the basis of Fe-30Ni-20Cr system alloy（Incoloy 800H）,the composition was optimized using JMatPro software,and three materials were produced by vacuum induction melting,with the main compositions of Fe30Ni18Cr2.4Al（B06）,Fe30Ni18Cr3Al（B07）and Fe35Ni20Cr3Al（B08）,The thermal processing properties of the materials were evaluated by combining thermal simulation techniques.Advanced microscopic characterization techniques such as double-beam microscopy（FIB/SEM）,transmission electron microscopy（TEM）,electron backscatter diffraction（EBSD）and transmission kikuchi diffraction（TKD）were used to investigate the oxide film system to understand the influence of alloying design on the oxidation resistance,and to explain the alloy oxide film formation mechanism and oxidation model based on the oxide film evolution pattern.（1）The solid solution state structure of all three alloys is single-phase austenite.The most phases are precipitated after oxidation of the alloy at 800°C,followed by 900°C and least at 700°C.The second phase precipitated in B06 is mainly Ni Al phase,and the distribution is not uniform;the second phase precipitated in B07 and B08 alloys is mainly Ni Al phase andσphase,and the activity of Al element in B07 alloy is the highest,followed by B08 alloy and the lowest in B06 alloy.（2）During the thermal deformation of the new alloys,the discontinuous dynamic recrystallization is dominant,and the thermal deformation destabilization zone is mostly concentrated in the high strain rate region(≥1s^-1);while in the safe processing interval,the power dissipation efficiency shows two peaks,which are located near 1000°C-0.01s^-1 and 1100°C-0.01s^-1,which can be identified as the optimal processing window for the new series of alloys.（3）The oxidation weight gain curves at 700°C,800°C and 900°C were all consistent with a parabolic rule.The rate of oxidation is similar for alloys B07 and B08,with a higher rate for B06.The parabolic rate constants of alloy B06 are both a magnitude higher than those of alloys B07 and B08,alloy oxidation products are mainlyα-Al₂O₃,Cr₂O₃ and spinel oxides.The apparent activation energies during oxidation for the three alloys are 255.53 k J/mol,243.83 k J/mol and 251.7 k J/mol respectively.（4）The alloy oxide film is divided into two layers:the inward-growingα-Al₂O₃ layer and the outward-growing mixed layer composed ofα-Al₂O₃,Cr₂O₃ or spinel oxide. α-Al₂O₃ layer is tightly bonded with the metal interface,while"impurities"such as fine Cr₂O₃ particles and Cr-rich carbides exist between the inner and outer oxide layers.These"impurities"are not tightly bonded with Al₂O₃ grains and produce defects at the interface,which will increase the concentration of oxygen in the oxide film and increase the oxidation rate.（5）When the alloy was oxidized at 800℃,with the increase of oxidation time,the oxide film grew inward/outward at the same time,and the growth rate of both inner and outer oxide layers was close to each other;when the alloy was oxidized at 900℃,with the increase of oxidation time,the inner and outer oxide layers of B06 alloy became thicker,and the oxide film grew inward/outward at the same time, and the thickening rate of the inner layer of B07 and B08 alloy oxide film was about twice as fast as the thickening rate of the outer layer,and theα-Al₂O₃ grains gradually change to columnar crystals.（6）The three alloys were oxidized at 1000～1200℃for 4h,and B07 alloy could form a dense Al₂O₃ oxide layer at 1000～1200℃,while B06 alloy and B08 alloy started to oxidize internally at 1100℃and 1200℃,respectively,forming discontinuous Al₂O₃.The lower the Al activity,the more unfavorable to the formation of Al₂O₃ protective layer at the early stage of oxidation,and the oxygen can quickly diffuse into the matrix for internal oxidation.