Study On Solidification Behavior And Volume Variation With Temperature Of Fe-Ni-B Alloy During Deep Undercooling

Currently,there are many unsolved problems in the eutectic transformation mechanism of multi-component eutectic alloys.Especially,the research on the eutectic transformation of alloys with off-eutectic composition is very rare,which not only limits the understanding on the transformation mechanism of multi-component alloys,but also affects the ideas of designing new alloys.Therefore,in this paper,the eutectic transformation theory was studied.And then,the rapid undercooled solidification of Fe-Ni-B alloy were investigated by Matlab image processing,high-speed photography and other technologies.The relationships between recalescence degree and recalescence rate,interface morphology,growth rate,phase fraction,volume change and microstructure morphology of Fe-Ni-B alloy during solidification were systematically studied.The effect of heat treatment on the microstructure and properties of Fe38Ni38B24 alloy was analyzed.The main conclusions are as follows:The theoretical model of eutectic transformation involving intermetallic compounds was derived,based on the Jackson-Hunt model of the classical eutectic transformation theory.The model clarified the application method of the theory in the growth of intermetallic compound phase.Model calculation shows that with the increase of growth rate,the fluctuation number of the liquid interface's concentration increases,and the eutectic lamellar spacing decreases.The eutectic growth rate increases with the increase of solid solubility and solute distribution coefficient,while the growth rate increases with the decrease of liquidus slope and the increase of diffusion coefficient.The relationships between the recalescence degree(undercooling),composition and the recalescence rate,growth rate were studied.From the temperature curve and the high-speed photography of Fe-Ni-B alloy solidification,it can be found that with increase of recalescence degree(undercooling),both the recalescence rate and the growth rate of hypoeutectic alloy increase.With the increase of B content under the same recalescence degree,the recalescence rate and the growth rate of primary phase transformation decrease,while the recalescence rate and the growth rate of eutectic phase transformation increase.For hypereutectic alloys,both the recalescence rate and the growth rate increase with the increase of recalescence degree and B content,while the ratio of the growth rate to the recalescence rate decreases with the increase of recalescence degree.The relationship between phase fraction and volume change of Fe-Ni-B alloy during solidification was obtained.The cooling curve baseline method was applied to calculate the phase fractions of Fe42Ni42B16 and Fe44Ni44B12 alloys during solidification under different recalescence degree.It can be found that the primary phase fraction continuously increases with the increase of the primary phase transition recalescence degree(undercooling).Moreover,the accuracy of the cooling curve baseline analysis in calculating the phase fraction was verified by the box counting method.Meanwhile,the change of sample volume from Fe-Ni-B alloy during solidification with temperature was also studied,finding that the volume decreases with the decrease of temperature.The effects of composition,recalescence degree and heat treatment on the structure and properties of Fe-Ni-B alloy were discussed.With the increase of recalescence degree,the y-(Fe,Ni)microsturcture changes from coarse dendrite to fine grain with certain directivity,and the eutectic microsturcture changes from regular lamellar to irregular anomalous eutectic.The hardness test revealed that the hardness increases with the increase of recalescence degree at the same composition;at the same recalescence degree,the hardness increases with the increase of B content.The density test of hypereutectic alloy showed that the density decreases gradually with the increase of recalescence degree;at the same recalescence degree,the density decreases with the increase of B content.When the microstructure of Fe38Ni38B24 alloy was heat treated at 950?for 10 hours,the regular lamellar structure began to change into granular structure with the increase of recalescence degree.At the recalescence degree of 230K,the microstructure changes from dendrite to honeycomb and finally to granular with the increase of heat treatment time,and the microhardness decreases with the increase of heat treatment time.