Research On Structure Evolution And Formation Of Fe-6.5Si Mono Phase Powders By Gas Atomization

High quality metals and alloys powder play important role in powder metallurgy technique, and their applications are also widened with the development of latter. Most researches focused on the processing to produce finer particles with narrow size range distribution. The solidification of the droplets and its microstructure evolution is therefore attracting considerable attentions. In this work, Fe-6.5Si powder was prepared to study the solidification behavior and its microstructure evolution, which are great significant to control the size and microstructure of powders to improve properties of soft magnetic materials.Firstly, the heat transfer during the solidification of Fe-6.5Si gas atomized droplet was analyzed: liquid undercooling & nucleation, recalescence as well as post-recalescence solidification. The undercooling prior to nucleation was calculated and evaluated, 302, 291, 255,160 and 39 K for 20, 30, 50, 80 and 120μm droplet respectively. During recalescence process, the temperature of 20, 30 μm droplets go up to 1678 and 1690 K, lower than the liquidus temperature, 1700 K. For 50, 80 and 120μm droplets, the temperature at the end of recalescence can reach liquidus of alloy. The smaller droplet experienced faster recalescence because of higher undercooling. For example, the recalescence rate of 50 and 120μm droplet is about 6 110 Ks- and5 110 Ks-, respectively.The microstructural evolution of Fe-6.5Si alloy powders prepared by high-pressure gas atomization was characterized. Dendritic, Equiaxed dendritic and seaweed-like morphologies were observed on the surface of the particles above 20μm. The dendrite arm spacings 2l satisfy the power law relation with particle size D,0.482l =0.31×D. When droplet size is larger than 80 μm, the dendrite breakup time（）buDt DT becomes shorter than the plateau time（）plDt DT during solidification which is deemed to be the reason causing the fine microstructures in larger droplets. when the droplets sized decreased below 20μm, the solidification patterns are varied significantly from dendritic to cellular. As the droplet size reach 10 μm, the smooth surface with no growth patterns was observed.The influence of purity of the droplet melt on the microstructure is also studied. Results showed that, grain sized of larger particles is significantly refined, however, it has less effect on smaller ones. Also the purity of the droplet melt has little influence on dendrite arm spacing.The grain size of crystal growth d has the following relation with the particles size:d=D₀ +a·（D-D₀）^b（D>D₀）d=D（D≤D₀） where, the D0, is the particle size critical size for single cristal growth, a and b is parameters related to nucleation effect and thermal properties of the alloy, for droplets obtained in the nitrogen and in the air, D0 is 8 and 5μm, a is 0.4722 and 0.2754, b is 0.7301,respectively.The observation of the particles below critical sized found that, no micron celluar microstructure exists, confirming the planar growth pattern in the particles below the critical size. Both theoretical and experiment results showed the solid liquid interface velocity is far below the absolute stability velocity for droplets of several microns. Spherical linear stability analysis was conducted to predict the critical size for planar growth in droplet undercooled to 300 K, the predicted result, 1.8μm, lies in the same order of magnitude with the observed one. The high speed of moving interface can stabilized the planar growth pattern which is induced by the Gibbs-Thomas effect at initial growth of nuclei to several microns.