Study On High-temperature Oxidation Behavior Of Ternary Multiphase Fe-Co-Cr/Ag-Cu-Cr Alloys

The critical Cr content required to form the continuous external Cr₂O₃ scale on the alloy surface is very high due to the equilibrium between the phases and lack of the diffusion driving force in two-phase or multi-phase alloys.In fact,the high Cr content in the alloy will affect alloy mechanical properties.The nanocrystalline materials contain a large number of grain boundaries,which can increase the diffusion driving force.This can provide some conditions for decreasing the critical Cr content required to form the continuous external Cr₂O₃ scale.Thus,it is of great theoretical and practical significance to study the high-temperature oxidation properties of the nanocrystalline alloys in order to form the continuous external Cr₂O₃ scale under the condition of the lower Cr content.In this paper,the Fe-Co-20Cr and Ag-20Cu-30Cr alloyed powders were prepared by powder metallurgy(PM)and mechanical alloying(MA)methods,and the corresponding bulk alloys were also prepared by vacuum hot pressing technique.The microstructures of the bulk alloys were characterized by XRD and SEM,and their high-temperature oxidation behavior and the effect of the grain refinement on them were also studied.The main results are as follows:The densities of the conventional size and nanocrystalline Fe-Co-20Cr and Ag-20Cu-30Cr alloys are very high and exceed 98%.The grain sizes of MA Fe-Co-20Cr and MA Ag-20Cu-30Cr bulk alloys are in the nanometer range.The Fe-Co-20Cr and Ag-20Cu-30Cr alloys are composed of the three phases except the MA Fe-60Co-20Cr alloy containing two phases.The alloy substrates of the Fe-Co-20Cr alloys are composed of the Fe-rich?phase,while the Co-rich?and Cr-rich?phases are distributed within the alloy substrates.The alloy substrates of Ag-20Cu-30Cr alloy are composed of the Ag-rich?phase,while the Cu-rich?and Cr-rich?phases are distributed within the alloy substrates.The high-temperature oxidation kinetics of the Fe-Co-20Cr alloys deviates from the parabolic rate law,and is not composed of a single parabolic stage but several parabolic stages with different rate constants.Both PM and MA Fe-20Co-20Cr alloys form the external oxide scale containing Fe?Co and Cr oxides,but cannot form the continuous external Cr₂O₃ scale finally.In addition,there is a peeling layer between the oxide scale and alloy substrate for the PM Fe-20Co-20Cr alloy.The oxide scale formed on the MA Fe-20Co-20Cr surface is well combined with the substrate,indicating that the nanocrystallization can improve the adhesion of the Fe-20Co-30Cr alloy.The MA Fe-40Co-20Cr and Fe-60Co-20Cr alloys can form the continuous external Cr₂O₃ scale at 800?.Thus,the oxidation rates of MA Fe-40Co-20Cr alloy and MA Fe-60Co-20Cr alloy were lower than those of the corresponding conventional size PM Fe-40Co-20Cr and PM Fe-60Co-20Cr alloys.The PM Ag-20Cu-30Cr alloy at 700?forms the discontinuous external scale containing metallic Ag,the middle region is composed of the continuous Cu O layer,while the internal mixed internal oxidation layer is composed of alloys and oxides.An irregular continuous thin Cr₂O₃layer can form at the base of oxide scale.At 800?,the outer region consists of Cu O layer,the middle region consists of a metal Ag layer plus a mixed layer of Cu,Ag and Cr oxides or their double oxides,while the inner region consists of the continuous thin Cr₂O₃ layer with some protrusions into alloy.The MA Ag-20Cu-30Cr alloy forms an outer oxide layer composed of the Cu oxides and double oxides of the metallic Ag,Cu and Cr.The inner region is composed of the regular,continuous Cr₂O₃ layer.Thus,the nanocrystallization can complete the transition from an internal to external oxidation of the reactive component Cr,and enables the Ag-20Cu-30Cr alloy to form the regular and continuous Cr₂O₃ layer finally.