Rapid solidification mechanism and magnetic properties of Ni-Fe-Ti alloy prepared in drop tube

Abstract

Ni-Fe-Ti ternary alloys, as a type of structural and magnetic material, have received more attention in the industrial fields in recent decades. For the purpose of providing necessary experimental data and theoretical basis for industrial appliance of these alloys, the researches of rapid solidification mechanism and relevant application performances of Ni45Fe40Ti15 ternary alloy are carried out in this paper. Rapid solidification of undercooled Ni45Fe40Ti15 ternary alloy is realized in a 3 m drop tube under the condition of containerless and microgravity state. In an experiment, the sample with a mass of 2 g is placed in a φ16 mm×150 mm quartz tube with a 0.3-mm-diameter nozzle at its bottom. The quartz tube is then installed in the induction coil on the top of the drop tube. The tube body is evacuated to a pressure of 2×10-5 Pa and backfilled with the mixture gas of Ar and He gases to about 1×105 Pa. After that the sample is melted by induction heating and superheated to about 200 K above its liquidus temperature. Under such a condition, the melt is ejected through the nozzle by a flow of Ar gas and dispersed into fine liquid droplets. These liquid droplets solidify rapidly during free fall, and the droplets with the diameters ranging from 160 to 1050 μm are achieved. As droplet diameter decreases, both cooling rate and undercooling of the alloy droplet increase exponentially, i.e., from 1.10×103 to 3.87×104 K·-1 and from 42 to 210 K (0.14TL) respectively. The microstructure consists of γ -(Fe, Ni) solid solution and interdendritic Fe2Ti intermetallic compound. As undercooling increases, the coarse γ -(Fe, Ni) dendrites become refined, the secondary dendrite arm spacing linearly decreases. Compared with the result in the glass fluxing experiment, the dendrites are much refined by drop tube processing due to the higher cooling rate obtained. The amounts of solute Ni and Ti content in the γ -(Fe, Ni) phase enlarge evidently with the increase of undercooling, suggesting the occurrence of solute trapping. The magnetic properties of thealloy droplets sre also analyzed. When droplet diameter decreases from 1100 to 300 μm, the saturation magnetization increases from 22.47 to 41.82 Am2·kg-1, the coercive force decreases from 3.33 to 0.80 KAm-1, and the squareness ratio decreases approximately by four times. This indicates that the soft magnetic properties of the alloy are improved remarkably by drop tube processing. Furthermore, the mechanism for substantial effect of undercooling on magnetic parameter such as coercive force needs to be further investigated.