落管中Ni-Fe-Ti合金的快速凝固机理及其磁学性能

采用落管自由落体方法实现了Ni_(45)Fe_(40)Ti_(15)合金在微重力无容器条件下的快速凝固,获得了直径介于160—1050μm的合金液滴.理论计算表明冷却速率及过冷度随液滴直径减小而增大,并呈指数函数关系,实验获得的最大过冷度为210 K(0.14 T_L).随着过冷度增大,凝固组织中粗大的γ-(Fe,Ni)枝晶逐渐细化,二次枝晶间距减小,溶质Ti在γ-(Fe,Ni)相中的固溶度显著扩展.对不同直径合金液滴的凝固样品进行磁学性能分析,结果表明随着凝固合金液滴直径减小,其饱和磁化强度增大,矫顽力减小,矩形比下降,软磁性能明显提高.

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

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 (0.14T_L) 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.