Gd及其合金是良好的磁性材料，向Gd-Ti合金中加入过渡族元素Co可形成热稳定磁性化合物，Gd-Co-Ti合金在磁性原位复合材料研发方面具有较好的前景。但Gd-Co-Ti是偏晶合金，常规凝固条件下易形成偏析组织，其凝固特性研究极为困难。本工作在落管微重力条件下对Gd-Co-Ti合金开展了凝固实验，获得了富TiCo相以近球形颗粒形式均匀分布于Gd基体中的球形合金样品。建立了Gd-Co-Ti合金的凝固理论模型，模拟分析了落管微重力条件下Gd-Co-Ti合金凝固组织演变过程，研究了样品尺寸(冷却速率)对凝固组织的影响。结果表明，富TiCo相液滴仅在液-液相区间内形核，在液-液-固三相区间内未发生富TiCo相液滴的形核现象；除表面区域外，富TiCo相液滴的Ostwald熟化作用较弱；合金样品尺寸越大，冷却速率越小，凝固组织中富TiCo相粒子的数密度越低，平均尺寸越大，富TiCo相液滴的最大形核率(I_DMax)和凝固后富TiCo相粒子的数密度(N_D)与形核阶段熔体冷却速率({\dot{T}}_{\mathrm{n}\mathrm{u}\mathrm{c}})分别满足指数关系I_{\mathrm{D}\mathrm{M}\mathrm{a}\mathrm{x}} = 7.202 × 10^-5{\left({\dot{T}}_{\mathrm{n}\mathrm{u}\mathrm{c}}\right)}^{\mathrm{2.2}}和N_{\mathrm{D}} = 3.385 × 10^-4{\left({\dot{T}}_{\mathrm{n}\mathrm{u}\mathrm{c}}\right)}^{\mathrm{1.3}}。

Gd and its alloys are good magnetic materials, and the addition of the transition metal Co to Gd-Ti alloy can facilitate the formation of thermally stable magnetic compounds. The Gd-Co-Ti alloy exhibits significant potential for the development of magnetic in situ composite materials. However, because of the large positive mixing enthalpy between Gd and Ti, the Gd-Co-Ti alloy is a typical monotectic alloy, exhibiting a miscibility gap in the liquid state. Under the ground gravity conditions, the alloy tends to form a phase-segregated solidification microstructure resulting from liquid-liquid phase transformation. Strong convection in the melt during solidification aggravates this process, rendering it difficult for various influencing factors to interact. However, research on solidification theory for these alloys is limited. Microgravity environments can effectively weaken or even eliminate natural convection in alloy melts, which is beneficial for studying the solidification process and microstructure formation in monotectic alloys. Previous studies have focused on the phase structures, material properties, and thermodynamic behavior of Gd-Co-Ti ternary monotectic alloys. However, research on their solidification process is sparse. In this study, rapid and sub-rapid solidification experiments under drop-tube microgravity conditions were performed using Gd-Co-Ti ternary monotectic alloys. The effects of cooling rates on the solidification microstructure of the alloy were investigated. The resulting samples exhibited a composite microstructure comprising homogeneously dispersed subspherical TiCo-rich particles in the Gd matrix. These particles include: (i) TiCo-rich phase particles formed via liquid-liquid phase transformation, and (ii) TiCo-rich nanoparticles formed through desolventizing precipitation during the cooling process after solidification. To elucidate the microstructure evolution in Gd-Co-Ti alloys solidified under drop-tube conditions, a population dynamics model was established. The model comprehensively considers the thermal and mass transfer characteristics during solidification, as well as the nucleation, growth, and spatial motions of TiCo-rich phase droplets. The algorithm for solving the controlling equations in this model was developed based on the finite volume method. The microstructure formation was simulated, and the results were consistent with the experimental data, thus validating the accuracy of the model. The numerical results demonstrated that the nucleation of the TiCo-rich phase droplets occurred during the liquid-liquid phase transformation under drop-tube microgravity conditions. The number density of these TiCo-rich phase droplets remained unchanged after nucleation, indicating that the Ostwald coarsening of the TiCo-rich droplets was weak during the cooling of the alloy melt. Thus, nucleation and diffusion growth were the primary factors influencing the size of TiCo-rich phase droplets formed during the liquid-liquid phase transformation. With the increase in the sample sizes, the cooling rate of the alloy melt and the number density of the TiCo-rich particles decreased; thus, the average radius of the TiCo-rich particles in the solidification microstructure increased. Furthermore, the maximum nucleation rate (I_DMax) and the number density (N_D) of the TiCo-rich phase droplets/particles exhibited an exponential dependence on the cooling rate ({\dot{T}}_{\mathrm{n}\mathrm{u}\mathrm{c}}) during the nucleation period as per the following expression: I_{\mathrm{D}\mathrm{M}\mathrm{a}\mathrm{x}} = 7.202 × 10^-5{\left({\dot{T}}_{\mathrm{n}\mathrm{u}\mathrm{c}}\right)}^{\mathrm{2.2}} and N_{\mathrm{D}} = 3.385 × 10^-4{\left({\dot{T}}_{\mathrm{n}\mathrm{u}\mathrm{c}}\right)}^{\mathrm{1.3}}.