Study On Pack Cementation Process And High Temperature Oxidation Behavior Of Ni-Cr-Fe Alloy Foams

With the research background that served as the foundation for aerospacestructural materials, the reticulated open-cell Ni foam was used as a substrate andwas alloyed with Cr and Fe by pack-cementation, resulting in open-cell Ni–Fe–Cralloy foams with opening ratio of more than90%. Simultaneously, in order tofurther improve the oxidation resistance of the Ni-Cr-Fe alloy foam, the dual-layerAl/Cr（Ce） coating was deposited onto the Ni-Cr-Fe alloy foam by a two-step packcementation process. Scanning electron microscopy （SEM）, the X-ray diffractiontechnique （XRD）, energy dispersive spectrum （EDS） analysis, the electronic tensiletesting machine and micro hardness test were used to investigate the microstructuresand the performance of the Ni–Fe–Cr alloy foams and Al/Cr（Ce） coated alloy foam.The oxidation kinetics and the oxidation mechanism of Ni-Cr-Fe alloy foam wereexplored.Solid powder embedding method was used to make Cr-Fe penetrations to theopen-cell Ni foam. The microstructure, phase and components of the Cr-Fepermeation layer were also researched. The results show that the degree ofdensification of Cr-Fe permeation layer and the bonding strength of the matrix andpermeation layer are affected by different permeation temperature. The bondingstrength of the matrix and permeation layer increases with permeation temperatureincreases, however, the agglomeration of the surface of Cr-Fe coating becomes moreobvious. When the temperature is1050℃, the outside surface of the strut is coatedwith a uniform Cr-Fe deposition layer and the substrate on the bonding strength ofthe coatings is strong. Simultaneously, when the holding time is increased at1050℃,the thickening of the outer layer and the diffusion layer of the permeation layer isapproximate to follow linear function.As the theoretical basis of the Ni-Cr-Fe ternary phase diagram, the compositionof Inconel690high temperature alloy was emulated. The experiment results showthat the stable microstructure, the better the oxidation resistance and thermal fatigueresistance of a single austenitic γ phase can be obtained by high temperature solidphase diffusion of the Ni-Cr-Fe foam. At the same time, the Cr, Fe and Ni elementconcentration gradient of the strut is significantly reduced with the extension of the homogenization time in the high-temperature diffusion process. When thehomogenization time is extended to48h at1200℃, the Cr and Fe elements cancompletely diffuse into the inner strut and ensures a homogeneous alloycomposition. The surface layer of the Ni-Cr-Fe alloy foam is mainly composed ofγ-（Fe, Ni） and γ-（Ni, Cr, Fe） solid solution. Moreover, the Ni-Cr-Fe alloy foam stillretains the three-dimensional network structure and hollow struts of open-cell Nifoam.The room temperature and high temperature mechanical properties testing wereapplied to the Ni-Cr-Fe alloy foam, and the relationship between the organizationand the performance of the different components of Ni-Cr-Fe alloy foam wasrevealed. The results show that the quasi-static stress–strain behaviors of Ni-Cr-Fealloy foams at room temperature show the characteristic of typical ductile metallicfoam. High temperature diffusion homogenization process can effectively improvethe compression strength of open-cell Ni-Cr-Fe alloy foam at room temperature andincrease the length of the plastic yielding platform. At the same time, the energyabsorbed per unit volume of the Ni–Fe–Cr foams exhibits a significant increase withincreasing the Cr and Fe content of the foams. Compared with the Gibson-Ashbyexperience formula, the yield strength and specific strength of the Ni-Cr-Fe alloyfoam after homogenization are significantly improved than ideal open-cell Ni foam,which is mainly solid solution strengthening effect of Cr and Fe elements. Inparticular, Ni-36Cr-24Fe alloy foam exhibits the highest yield strength and unitvolume energy absorption.The high temperature oxidation resistance of Ni-Cr-Fe alloy foam was studiedat800-1000℃. The high temperature oxidation kinetics, thermodynamics andoxidation mechanism were revealed. The results show that the Ni-Cr-Fe alloy foamshave a higher oxidation resistance than Ni foam substrate and Ni-25Cr alloy foam,whose oxidation kinetics curves obey the parabolic law. The oxidation resistance ofopen-cell Ni-Cr-Fe alloy foam increases with increasing the Cr and Fe content. Atthe same time, when the oxidation temperature is increased from800℃to1000℃,the oxide layer of Ni-Cr-Fe alloy foam shows the change of the mixed oxide ofCr₂O₃, NiFe₂O₄and NiCr2O4into a dense single-phase Cr₂O₃layer, which caneffectively enhance the oxidation resistance of the Ni-Cr-Fe alloy foam.The dual-layer Al/Cr（Ce） coatings were deposited onto the Ni-Cr-Fe alloy foam by a two-step pack cementation process. The impact of Al/Cr（Ce） coating onthe oxidation resistance and mechanical properties of open-cell Ni-Cr-Fe alloy foamwas revealed. The oxidation mechanism and deformation behavior after theoxidation were given. The results show that the dual-layer Al/Cr（Ce） coatings arecontinuous and compact. The Ce added can effectively restrain the interdiffusionbetween the Ce–Cr coating and Al coating during the oxidation process. Theoxidation kinetics curve of Al/Cr（Ce） coated foam follows a parabolic law. A denseAl₂O₃layer is formed on the surface of Al/Cr（Ce） coating, and a small amount of Cecan effectively improve the adhesion of the oxide layer and the substrate.Simultaneously, compared to the Ce–Cr and Al/Cr coated foams, the Al/Cr（Ce）coated foam still exhibits higher yield strength, which can further enhance theoxidation resistance and mechanical properties of Ni-Cr-Fe alloy foam.