ZK60基儲氫合金之原子針尖斷層影像分析研究

Abstract

這份研究利用原子探針斷層掃描技術（Atom Probe Tomography, APT）探討ZK60鎂合金系統中的氫吸收行為，特別聚焦於原子尺度下界面動力學與催化增效的影響。ZK60合金的基體為鎂(Mg)，其中包含約6 wt.%的鋅（Zn）與0.5 wt.%的鋯（Zr）。鋅可提供固溶強化作用，並可能形成影響氫動力學的金屬間化合物；而鋯則作為晶粒細化劑。儘管ZK60因其優異的強度與耐腐蝕性廣泛應用於結構材料領域，但其作為氫儲存材料的潛力仍少有探討，特別是在原子氫交互機制方面。 為探究這些效應，本研究將ZK60製成霧化粉末（Atomized Powder, AP），該製程相較於傳統切削具有顯著優勢，不僅時間效率高，且能有效控制顆粒大小與化學性質。霧化粉末呈現均勻球形結構，晶粒尺寸達到次微米等級。 此霧化ZK60粉末進一步摻入5 wt.%石墨烯與0.5 wt.%鈀（Pd），兩者皆為提升氫吸收動力學的催化劑，其中Pd亦被此研究證實能透過固溶效應促進氫化物相的生成。混合後於氬氣氛下進行機械式球磨，引入差排(dislocation)與晶界(grain boundary)等結構缺陷，這些缺陷提供氫擴散路徑並成為氫化物成核位置。考量APT分析中氫的質譜峰會與背景殘留氫重疊，實驗選用氘氣（D2）作為充氣氣體以提高鑑別度。 樣品製備方面，使用聚焦離子束（Focused Ion Beam, FIB）技術進行局部納米尺度製程，經過拔取(lift-out)與環狀研磨(annular milling)後製得針狀樣品，並謹慎控制損傷以避免裂縫或孔隙產生。 APT的質荷比譜圖顯示明確的氘相關訊號，包括D⁺、HD⁺與MgD2⁺，證實材料成功吸氘。此外，APT重建出的原子分佈顯示出氫化物形成與界面行為的關鍵特徵。在25%與50%充氘樣品中均可觀察到氘富(deuterium-rich)區與鎂基(magnesium-based)區域的共存，且氘濃度在界面處出現明顯下降，支持「持續移動界面理論(continuously moving boundary)」(一種描述氫化物成長時相界面遷移行為的動力學模型)。 此研究首次在ZK60系統中，以APT直接觀察氫動力學行為於納米尺度下的表現。APT能夠定量解析局部化學組成並辨識微觀結構特徵，提供對於複雜鎂合金氫儲存機制前所未有的深入理解。

In this study, we investigate hydrogen absorption behavior in the ZK60 magnesium alloy system using Atom Probe Tomography (APT), with a focus on the influence of interface dynamics and catalytic enhancement at the atomic scale. ZK60 alloy comprises approximately 6 wt.% Zn and 0.5 wt.% Zr in a magnesium matrix. Zinc contributes to solid solution strengthening and can form intermetallic phases that may alter hydrogen kinetics. Zirconium serves as grain refiner. While ZK60 is widely applied in structural applications for its superior strength and corrosion resistance, its potential as a hydrogen storage material remains relatively unexplored, particularly in terms of atomic hydrogen interaction mechanisms. To investigate these effects, ZK60 alloy was processed as atomized power (AP), which provides significant advantages over conventional cutting and milling methods. The atomized powder exhibits uniform spherical morphology with sub-micron grain size and allows for precise control over particle size distribution and chemical properties in a less time-consuming process. The atomized ZK60 powders were further modified with 5 wt.% graphene and 0.5 wt.% palladium (Pd), both of which serve as catalysts to enhance hydrogen uptake kinetics, while Pd is also proved to enhance hydride phase formation via solid solution effect. The mixture was subjected to mechanical ball milling under an argon atmosphere, introducing structural defects such as dislocations and grain boundaries. These defects act as diffusion pathways and nucleation sites for hydride formation. Deuterium (D2) gas was used for charging instead of H2 to prevent confusion in APT, owing to overlapping mass peaks from residual hydrogen contamination. To prepare site-specific nanoscale specimens, Focused Ion Beam (FIB) techniques were employed. FIB allowed for the lift-out and annular milling of needle-shaped samples suitable for APT, with attention paid to minimizing damage and avoiding internal cracks or porosity. The mass-to-charge spectra revealed clear signatures of deuterium-related species, including D⁺, HD⁺, and MgD2⁺, confirming successful absorption in the material. Additionally, atomic distributions reconstructed from APT data highlighted key features of hydride formation and interface behavior. In both 25% and 50% loaded samples, deuterium-rich zones were identified alongside magnesium-rich matrix regions. Notably, the deuterium concentration sharply decreased at interfaces, supporting the continuously moving boundary theory, a kinetic model describing phase front migration during hydride growth. The application of APT in this context provides the first direct, nanoscale observation of hydrogen kinetics in a ZK60-based system. The ability to quantitatively resolve local chemical compositions and identify microstructural features offers unprecedented insight into the mechanisms governing hydrogen storage in complicated Mg alloys.