Wang Qiaoli, Li Rui, Xie Weixin, Yang Fang, Du Beining, Sheng Liyuan
Shenzhen Institute, Peking University, Shenzhen 518057, China.
PKU-HKUST Shenzhen-HongKong Institution, Shenzhen 518057, China.
Materials (Basel). 2023 Apr 25;16(9):3362. doi: 10.3390/ma16093362.
Due to their potential improvement of high-temperature properties, the refractory metal hafnium (Hf) and the rare earth holmium (Ho) have attracted much attention. In the present research, NiAl-Cr(Mo) eutectic alloys with different Ho and Hf additions were fabricated by conventional smelting method and heat-treated to study the synergetic influence of strengthening elements and heat treatment. The samples were characterized using XRD, SEM, and TEM, and the three-point bending test was performed to obtain fracture toughness. The results demonstrate that Hf addition leads to the formation of NiAlHf Heusler phase and that Ho promoted the formation of NiAlHo phase. The microstructure of the alloy is affected by thermal treatment, with the coarsening of eutectic lamellae after heat treatment. The mechanical properties are improved by Hf and Ho additions, with increased fracture toughness. Overall, this study provides insights into the microstructure and properties of NiAl-Cr(Mo) eutectic alloys and highlights the potential of Hf and Ho addition to improve room-temperature properties. Specifically, the as-cast NiAl-Cr(Mo)-Hf eutectic alloy contains a relatively fine NiAl/Cr(Mo) eutectic lamella but coarse eutectic cell with NiAlHf phase embellished along the cell boundary. Minor Ho addition induces the formation of NiAlHo phase, which leads to the coarsening of the intercellular region but contributes to the refining of eutectic cell. In addition, the synergetic effect of Ho and Hf promotes the precipitation of NiAlHo and NiAlHf phases in the intercellular zone and increases the interface dislocations. Heat treatment benefits the solid solution of NiAlHo and NiAlHf phases, which improves their size and distribution by secondary precipitation. The NiAlHf phase in the NiAl-Cr(Mo)-Hf eutectic alloy becomes fine and uniformly distributed, but the NiAl/Cr(Mo) eutectic lamella in the eutectic cell becomes coarse. In comparison, heat treatment mainly optimizes the size and distribution of the NiAlHo and NiAlHf phases in the NiAl-Cr(Mo)-Hf-Ho eutectic alloy. Furthermore, heat treatment helps to eliminate the interface dislocations in the large NiAl precipitates and the NiAl/Cr(Mo) phase interfaces, which also contributes to fracture toughness by decreasing stress concentration. Minor Ho addition decreases the fracture toughness of as-cast NiAl-Cr(Mo)-Hf eutectic alloy from 6.7 to 6.1 MPa·m, which should be ascribed to the coarsened intercellular region including aggregated NiAlHo and NiAlHf phases. However, minor Ho-doped NiAl-Cr(Mo)-Hf eutectic alloy obtained the highest fracture toughness of 8.2 MPa·m after heat treatment. This improved fracture toughness should be mainly attributed to the refined and well-distributed NiAlHo and NiAlHf phases in the heat-treated NiAl-Cr(Mo)-Hf-Ho eutectic alloy.
由于难熔金属铪（Hf）和稀土钬（Ho）具有改善高温性能的潜力，它们备受关注。在本研究中，采用传统熔炼方法制备了添加不同Ho和Hf的NiAl-Cr（Mo）共晶合金，并进行热处理以研究强化元素与热处理的协同影响。使用XRD、SEM和TEM对样品进行表征，并进行三点弯曲试验以获得断裂韧性。结果表明，添加Hf导致形成NiAlHf休斯勒相，而Ho促进了NiAlHo相的形成。合金的微观结构受热处理影响，热处理后共晶薄片粗化。添加Hf和Ho提高了力学性能，断裂韧性增加。总体而言，本研究深入了解了NiAl-Cr（Mo）共晶合金的微观结构和性能，并突出了添加Hf和Ho改善室温性能的潜力。具体而言，铸态NiAl-Cr（Mo）-Hf共晶合金含有相对细小的NiAl/Cr（Mo）共晶薄片，但具有粗大的共晶胞，沿胞界点缀有NiAlHf相。少量添加Ho诱导形成NiAlHo相，这导致胞间区域粗化，但有助于共晶胞细化。此外，Ho和Hf的协同作用促进了NiAlHo和NiAlHf相在胞间区域的析出，并增加了界面位错。热处理有利于NiAlHo和NiAlHf相的固溶，通过二次析出改善了它们的尺寸和分布。NiAl-Cr（Mo）-Hf共晶合金中的NiAlHf相变得细小且分布均匀，但共晶胞中的NiAl/Cr（Mo）共晶薄片变得粗大。相比之下，热处理主要优化了NiAl-Cr（Mo）-Hf-Ho共晶合金中NiAlHo和NiAlHf相的尺寸和分布。此外，热处理有助于消除大尺寸NiAl析出物和NiAl/Cr（Mo）相界面中的界面位错，这也通过降低应力集中有助于提高断裂韧性。少量添加Ho使铸态NiAl-Cr（Mo）-Hf共晶合金的断裂韧性从6.7MPa·m降至6.1MPa·m，这应归因于包括聚集的NiAlHo和NiAlHf相在内的粗化胞间区域。然而，少量Ho掺杂的NiAl-Cr（Mo）-Hf共晶合金在热处理后获得了最高的断裂韧性8.2MPa·m。这种提高的断裂韧性应主要归因于热处理后的NiAl-Cr（Mo）-Hf-Ho共晶合金中细化且分布良好的NiAlHo和NiAlHf相。
