香港城市大学Acta Materialia：拥有高强度和层错诱导塑性的新型L12纳米析出增强型多组元Co-Al-Nb 系高温合金

【研究概述】

2006年, Co-Al-W三元合金中L1₂型 γ′-Co₃（Al，W）有序沉淀相的发现为在富钴合金体系中开发下一代高温结构材料提供了新的机遇。为了获得更优的沉淀强化效果，这要求沉淀增强相在工作温度下保持足够优越的热力学稳定性。然而，据报道，L1₂-Co₃（Al，W）相在高温下是亚稳的，在900°C下长期服役后分解为脆性的B2相和D0₁₉-Co₃W相。此外，大多数添加到三元Co-Al-W合金中的四元合金也倾向于诱导脆性沉淀相的形成。在拉伸变形过程中，这些有害的脆性相可能会导致严重的脆化和灾难性的脆性断裂。此外，这种脆性相的形成也会耗尽基体中的耐火元素，大大降低固溶强化的有效性。L1₂沉淀相的粗化速度也会因耐火元素的消耗而大大加快。如何有效提高L1₂相的相稳定性成为了开发高承温能力、使用寿命长的高温结构材料。值得注意的是，除了相稳定性外，在评估其工程应用潜力时还应充分考虑材料的质量密度。为了稳定Co-Al-W基合金中的L1₂相，人们通常需要添加大量的高密度的钨元素，导致这类合金的质量密度通常过高（例如，Co-9Al-9.8W合金的密度高达9.82 g/ cm³，将严重降低能量损耗和转换效率。因此，开发高热稳定、密度较低的无钨Co基高温合金最近受到了学术界的极大关注。

在本研究中，我们着重于在充分保障Co基合金系统内FCC- L1₂双相结构稳定性的前提下设计了新型高性能的Co-Al-Nb基高温合金，显示出了优越的高温热稳定性、强韧性和较低的质量密度；并系统研究了其合金化元素对微观组织演化、相稳定性和机械性能的影响。这些发现不仅为L1₂强化合金的合金设计与变形行为提供了基本的理解，而且还向人们展示了基于多组分富钴合金系统开发下一代高温结构材料的巨大潜力。相关研究成果以 “L1₂-strengthened multicomponent Co-Al-Nb-based alloys with high strength and matrix-confined stacking-fault-mediated plasticity”为标题发表在Acta Materialia期刊。论文链接：https://doi.org/10.1016/j.actamat.2022.117763。该论文第一作者为香港城市大学材料科学与工程系曹博轩博士(B.X. Cao), 通讯作者为香港城市大学材料科学与工程系的杨涛教授（T. Yang），主要合作者包括来自厦门大学的W.W. Xu教授，深圳大学的C.Y. Yu教授等，香港城市大学的刘锦川院士 (C.T. Liu) 为该论文的共同通讯作者。该研究工作受到了来自国家自然科学基金委（Grant 52101151）和广东省科技部 (Grant 2020A1515110647) 以及香港研资局（CityU Grant: 11213319, 11202718, 21205621, 9610498）等多方面的支持。

【图文摘要】

图1.  Microstructures of the Co-10Al-3Nb alloy after aging at 700°C for 168 h.

图2. The X-ray diffraction patterns of the Co-10Al-3Nb alloy aged at 700°C for 168 h.

图3. Temporal microhardness evolutions of the ternary Co-10Al-3Nb alloy aging at 700°C and 800°C, respectively.

图4. Representative SEM micrographs of grain boundary triple junction region of the (a) Co-15Ni-10Al-3Nb (15Ni) and (b) Co-30Ni-10Al-3Nb (30Ni) alloys after aging at 700°C for 168 h. Representative SEM micrographs of grain boundary triple junction region of the (c) Co-10Al-3Nb-30Ni (30Ni), (d) Co-10Al-3Nb-30Ni-2Ti (30Ni2Ti), (e) Co-10Al-3Nb-30Ni-2Ta (30Ni2Ta), and (f) Co-10Al-3Nb-30Ni-2Ti-2Ta (30Ni2Ti2Ta) alloys after aging at 800°C for 168 h.

图5. (a) The γ′-solvus and γ-solidus temperatures of the base, 15Ni, and 30Ni alloys. (b) The γ′-solvus, γ-solidus, and liquidus temperatures of the 30Ni, 30Ni2Ti, 30Ni2Ta, and 30Ni2Ti2Ta alloys. (c) The volume fraction of the γ′ phase among the Co-Al-Nb-based alloys at 800°C. (d) Microhardness evolutions of the Co-Al-Nb-based alloys after aging at 700 and 800°C for 168 h.

图6. Elemental partitioning coefficients of the multicomponent Co-rich alloys, showing Co partitioned to the γ matrix phase, whereas Ni, Al, Nb, Ti, and Ta partitioned to the γ′ precipitates.

图7. (a) SEM micrograph of the 30Ni2Ti2Ta alloy and (b) corresponding schematic diagram, showing that high-density γ′ precipitates divide the γ phase into nanoscale channels. (c) Ion maps of reconstructed nanotips by APT. (d) Proximity histograms across the γ/γ′ interfaces, showing distinctly different elemental compositions between the γ and γ′ phases.

图8. The plots between the yield strength and deformation temperature of the 30Ni2Ti2Ta alloy, together with other L12-strengthened Co-based alloys (Co-11Ti-15Cr, Co-9Al-9W, Co-30Ni-12Al-4Ta-12Cr, and Co-30Ni-10Al-5V-4Ta-2Ti alloys), a conventional carbide-hardened Co-based alloy (Haynes 188), and a commercial Ni-based superalloy (Waspaloy).

图9. Deformation mechanism of the 30Ni2Ti2Ta alloy after ∼2% plastic deformation at room temperature.

图10. Deformation mechanism of the 30Ni2Ti2Ta alloy deformed at 700°C with a plastic strain of ∼2%.

图11. Schematic diagrams of the deformed substructures at 25 and 700°C. Abbreviations: SF, stacking fault; SSF, superlattice stacking fault.

图12. Enthalpy formation energies of the L12-type Co3(Al, Nb) phase as a function of Nb concentrations.

图13. (a) Phase fraction of the L12 structure and the B2 structure as a function of Ni concentration in the Co-10Al-3Nb-xNi alloy. (b) L12 phase fraction as a function of temperature among Ti- and Ta-alloyed Co-10Al-3Nb-30Ni-based alloys.

图14. The critical resolved shear stress required for perfect dislocations gliding through the narrow matrix channels and dissociation into partial dislocations is plotted as a function of the stacking fault energy of the matrix phase. Various matrix spacings are considered in this plot to reveal the extra resistance from the geometric constraint to the movement of dislocations.