目的 针对铸态锌锂（Zn-Li）合金在泌尿植入物服役环境中极易发生严重局部腐蚀及结壳增生的问题,通过搅拌摩擦加工（FSP）在Zn-0.5%Li（质量分数）合金表面原位构筑耐腐蚀和抗结壳功能化改性层,并系统性研究该改性层改善耐腐蚀及抑制结壳的过程机制。方法 利用电化学测试与体外浸泡实验评估其降解速率与结壳行为,结合微观组织与成分分析,揭示Zn-0.5%Li合金表面功能化改性层对合金腐蚀行为及结壳增生的影响。结果 FSP加工形成的改性层完全破碎了铸态合金中粗大的初生β-LiZn4相并促进其溶解,同时显著细化合金的微观组织,形成由再结晶Zn与Zn/β-LiZn4共晶组织组成的均匀组织。体外模拟腐蚀实验表明,FSP合金功能化改性层的腐蚀速率与表面结壳增重远低于原始铸态合金,质量分别下降了58%和54%。结论 FSP在合金表面构筑的功能化改性层引起微观组织均匀化,β-LiZn4相溶解及（0002）基面织构的形成削弱了局部腐蚀倾向,使合金耐蚀性上升,避免了合金表面局部pH值的快速升高,从而赋予了材料优异的抑制结壳功能。
Abstract
As-cast Zn-0.5wt%Li alloy encounters severe localized corrosion and excessive encrustation in urinary implant applications, which greatly restrict its clinical use. The coarse and continuously distributed primary β-LiZn4 dendrites lead to intense electrochemical heterogeneity in the microstructure. These intermetallic phases act as effective cathodes and form micro-galvanic cells with the α-Zn matrix, which significantly accelerates localized degradation and causes a rapid increase in local pH value at the alloy surface. This pH variation promotes the continuous deposition of phosphate substances and further leads to the blockage of urinary implants during clinical service.
In this work, friction stir processing (FSP) was employed to in-situ fabricate a homogeneous functionalized modified layer on the surface of Zn-0.5wt.%Li alloy to overcome these drawbacks. As a solid-state surface modification technique, FSP avoided melting and solidification defects, and achieved controllable microstructure regulation at the surface region. Single-pass FSP was performed with optimized processing parameters, including a rotation speed of 800 rpm, a traveling speed of 100 mm/min, a tool tilt angle of 2.5° and a plunge depth of 0.3 mm. The microstructural evolution of the modified layer was characterized by X-ray diffraction and scanning electron microscopy. The corrosion resistance and anti-encrustation properties were evaluated by potentiodynamic polarization, electrochemical impedance spectroscopy and 28-day in vitro immersion tests in artificial urine at 37 ℃. The composition and morphology of corrosion products were analyzed by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy.
Microstructural results confirmed that FSP effectively eliminated the microstructure inhomogeneity of the as-cast alloy. The coarse primary β-LiZn4 dendrites and lamellar Zn/β-LiZn4 eutectic structures were thoroughly broken and refined. Dynamic recrystallization induced by FSP promoted the formation of uniform equiaxed fine grains. The high-density dislocations introduced by severe plastic deformation accelerated the dissolution of β-LiZn4 phase, reducing its volume fraction from 30.8% to 25.4%. Meanwhile, a strong (0002) basal plane texture was formed in the α-Zn matrix, which was the most densely packed crystal plane in hexagonal close-packed Zn.
Benefiting from the optimized microstructure, the FSP-modified alloy presented significantly enhanced corrosion resistance and anti-encrustation performance. The corrosion current density was reduced by 54.7%, from 59.2 μA/cm2 to 26.8 μA/cm2. The charge transfer resistance increased from 59 Ω·cm2 to 156 Ω·cm2, and the film resistance rose from 80 Ω·cm2 to 172 Ω·cm2. After 28-day immersion, the stable degradation rate of the FSP sample was 0.05 mm/a, which was much lower than 0.12 mm/y of the as-cast sample. The surface encrustation weight was reduced by 54%, from 8.2 mg/cm2 to 3.8 mg/cm2, and the corrosion products presented a thinner and more compact layered structure on the modified surface.
The enhancement mechanism revealed that phase homogenization and β-LiZn4 dissolution suppressed micro-galvanic corrosion by reducing potential differences. The (0002) basal texture acted as a physical barrier to inhibit ion dissolution. The uniform degradation process avoided sharp local pH increase, thus restraining the precipitation of Zn3(PO4)2 and the formation of CaZn2(PO4)2·2H2O. This work provides a feasible surface modification method for the development of anti-encrustation biodegradable Zn-Li alloys for urinary implants, and also offers a reliable design strategy for high-performance biodegradable metallic implants.
关键词
锌锂合金 /
搅拌摩擦加工 /
人工尿液 /
生物可降解 /
腐蚀行为
Key words
Zn-Li alloy /
friction stir processing /
artificial urine /
biodegradable /
corrosion behavior
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基金
国家自然科学基金（52305385,52405384）
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