Solidification Microstructures And Magnetic Properties Of Undercooled Fe-Ni Alloys

In the present paper, the cyclic superheating and glass slag purification technologies were used to make the bulk Fe-Ni system melts acquire higher undercooling.The evolution mechanism of solidification microstructure in Fe-Ni alloys, the effect of undercooling on its grain sizes as well as its magnetic properties were studied systemtically at a larger undercooling range. The structure evolution mechanisms were achieved by analysis of thermodynamics and dynamics during the dendritic growth process. The phase structure and soft magnetic properties were studied by using XRD and VSM, respectively. The main research work and conclusions are presented as follows.1. The maximum undercooling of 251K, 390K, 217K and 340K could be obtained in Fe-22at.%Ni,Fe-40at.%Ni,Fe-50at.%Ni and Fe-80at.%Ni melts by using glass slag purification (consisting of 50wt.%B₂O₃, 20wt.%Na₂B₄O₇.10H₂O and 30wt.%Na₂SiO₃ ) and cyclic superheating, respectively.2. The solidification structures of coarser dendrites, the first granular grains, finer dendrites and the second granular grains were appeared orderly in Fe-Ni alloys. The granulations of solidification microstructure would take place twice at a large undercooling range, where the first is that the coarser dendrite turned into the first granular grain under the smaller undercoolings and the second is that the finer dendrite turned into the second granular grain under the higher undercoolings. The coarsening process of the second granular grain occurred in Fe-80at.%Ni alloy under the higher undercooling.3. The analysis of microstructure evolution in Fe-Ni alloys showed that the first granulation mechanism is mainly controlled by dendrite remelting for the smaller undering, and the second by dendrite breaking-up and recrystallizing for the larger undercooling. The coarsening of grains Fe-80at%Ni alloy undercooling can be attributed to the decrease ofinterface energy which causes the secod granular grains to merge into coarse grains.4. The analysis of XRD showed that eutectoid reaction was inhibited under the condition of high undercooling and rapid solidification. Onlyγ-(Fe,Ni) phase was formed in Fe-Ni alloys. Theγ- (Fe,Ni) phase in Fe-22at.%Ni,Fe-40at.%Ni and Fe-80at.%Ni alloy was inclined to preferred orientation of lattice plane, but (200) lattice plane in Fe-50at.%Ni.5. The measures of soft magnetic properties of Fe-Ni alloy showed that it is propitious to for high undercooling and rapid solidification to enhance saturation induction density Bs and reduce residual magnetic flux density Br. The coercive force Hc would decrease with increase of the grain size D. Both of the Bs and Hc of Fe-Ni alloy would decrease with an increase of the Ni content.