Investigation Of Grain Growth And Thermal Stabilities In FeZr Nanocrystalline Alloys

Fe-based alloys are kinds of important engineering materials. Nanostructurization of the Fe-based alloys is proved to be an efficient way to improve their properities. However, due to the high density of grain boundaries, nanocrystalline materials are in a high energy state. Thermal stability of these materials is normally rather low. The materials have a high tendency to grain coarsening. Recently, it was shown that by alloying elements showing high grain boundary segregation tendency, the thermal stability of the nanocrystalline materials can be improved significantly. In this work, high grain boundary segregation Fe-Zr system was chosen as research object. A series of nanocrystalline Fe-Zr alloys with different Zr concentrations were prepared by ball milling method. The as-prepared nanocrystalline alloys were subjected to annealing treaments at different temperatures for different time. Combining the XRD2 and TEM techniques, microstructures of the nanocrystalline Fe-Zr alloys were characterized. On this basis, the grain growth behaviors and stabilization mechanisms of the nanocrystalline Fe-Zr alloys were analyzed and discussed in details. The main conclusions were summarized as follows.?1? The addtions of Zr exerts strong stabilization effects on the nanocrystalline Fe-Zr alloys. With increasing Zr concentration, grain size of the as-prepared alloys decreases continuously. Upon the annealing treatment at temperatures lower than 600 ?, the alloys consist of single ferrite phase. While the annealing temperatures are higher than 600 ?, Fe₃Zr phase precipitates. With the rise of annealing temperature, the grain size of the nanocrystalline alloys increases, whereas, the grain size after annealing treatment is remarkably smaller than that of nanocrystalline pure Fe. For example, the grain size of nanocrystalline Fe-5 at.% Zr alloy can maintain at around 100 nm after annealing treatment at 900 ?for 1 h.?2? The quantitative analyses indicate that when the distribution of Zr is equilibrium in the nanostructure, a metastable equilibrium state corresponding to zero grain boundary energy will appear. In this case, the metastable equilibrium grain size will decrease continuously with increasing Zr concentration. In the as-prepared state, when the Zr concentration is higher than 7 at.%, the distribution of Zr has already reached equilbirum, the stabilization of grain size of the alloys is totally controlled by thermodynamic stabilization mechanism, i.e. the reduction of grain boundary energy; while Zr concentration is lower than 7 at.%, the reduction of grain boundary energy and solute drag effect of Zr on migration of grain boundary are both responsible for stabilization of the nanostructures.?3? Upon annealing treatments, the thermal stability of nanocrystalline Fe-Zr alloys are controlled by complex mechanisms. In the range of temperature where the precipitation of Fe₃Zr phase does not happen, the reduction of grain boundary energy and the solute drag effect arisen from the segeregation of Zr in grain boundaries are responbile for stabilization of the nanoscale grain size. In the range of temperature where Fe₃Zr phase precipitates, because of the significant decrease of Zr concentration in grain boundaries, the above thermodynamic and kinetic stabilization effects reduced remarkably, the Zener pinning effect of Fe₃Zr precipitates turns to be the major mechanism for stabilization of the nanostructures.