香港城市大學Acta Materialia：擁有高強度和層錯誘導塑性的新型L12納米析出增強型多組元Co-Al-Nb 系高溫合金

【研究概述】

2006年, Co-Al-W三元合金中L1₂型 γ′-Co₃（Al，W）有序沉淀相的發現為在富鈷合金體系中開發下一代高溫結構材料提供了新的機遇。為了獲得更優的沉淀強化效果，這要求沉淀增強相在工作溫度下保持足夠優越的熱力學穩定性。然而，據報道，L1₂-Co₃（Al，W）相在高溫下是亞穩的，在900°C下長期服役后分解為脆性的B2相和D0₁₉-Co₃W相。此外，大多數添加到三元Co-Al-W合金中的四元合金也傾向于誘導脆性沉淀相的形成。在拉伸變形過程中，這些有害的脆性相可能會導致嚴重的脆化和災難性的脆性斷裂。此外，這種脆性相的形成也會耗盡基體中的耐火元素，大大降低固溶強化的有效性。L1₂沉淀相的粗化速度也會因耐火元素的消耗而大大加快。如何有效提高L1₂相的相穩定性成為了開發高承溫能力、使用壽命長的高溫結構材料。值得注意的是，除了相穩定性外，在評估其工程應用潛力時還應充分考慮材料的質量密度。為了穩定Co-Al-W基合金中的L1₂相，人們通常需要添加大量的高密度的鎢元素，導致這類合金的質量密度通常過高（例如，Co-9Al-9.8W合金的密度高達9.82 g/ cm³，將嚴重降低能量損耗和轉換效率。因此，開發高熱穩定、密度較低的無鎢Co基高溫合金最近受到了學術界的極大關注。

在本研究中，我們著重于在充分保障Co基合金系統內FCC- L1₂雙相結構穩定性的前提下設計了新型高性能的Co-Al-Nb基高溫合金，顯示出了優越的高溫熱穩定性、強韌性和較低的質量密度；并系統研究了其合金化元素對微觀組織演化、相穩定性和機械性能的影響。這些發現不僅為L1₂強化合金的合金設計與變形行為提供了基本的理解，而且還向人們展示了基于多組分富鈷合金系統開發下一代高溫結構材料的巨大潛力。相關研究成果以 “L1₂-strengthened multicomponent Co-Al-Nb-based alloys with high strength and matrix-confined stacking-fault-mediated plasticity”為標題發表在Acta Materialia期刊。論文鏈接：https://doi.org/10.1016/j.actamat.2022.117763。該論文第一作者為香港城市大學材料科學與工程系曹博軒博士(B.X. Cao), 通訊作者為香港城市大學材料科學與工程系的楊濤教授（T. Yang），主要合作者包括來自廈門大學的W.W. Xu教授，深圳大學的C.Y. Yu教授等，香港城市大學的劉錦川院士 (C.T. Liu) 為該論文的共同通訊作者。該研究工作受到了來自國家自然科學基金委（Grant 52101151）和廣東省科技部 (Grant 2020A1515110647) 以及香港研資局（CityU Grant: 11213319, 11202718, 21205621, 9610498）等多方面的支持。

【圖文摘要】

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

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

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

圖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.

圖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.

圖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.

圖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.

圖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).

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

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

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

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

圖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.

圖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.