样品温度和脉冲分数对7085铝合金APT实验数据的影响

Influence of specimen temperature and pulse fraction on APT data of 7085 Al alloy

基于原子探针层析技术(APT)优异的空间和成分分辨率,针对析出强化型7085铝合金,研究了进行APT测试分析时样品温度和脉冲分数对质谱的背底噪声、化学成分、第二相分布特征的影响。结果表明:该铝合金中存在细小的MgZn2和较大尺寸的Al3(Sc,Zr)两种析出相;随着样品温度的升高,质谱的背底噪声强度不断增大,Zn、Mg、Cu和Si等元素的含量几乎不变,而Zr含量显著持续快速增加,MgZn2析出相的尺寸几乎不变,但数量密度先减小后增加。此外,随着脉冲分数的增加,背底噪声强度不断降低,并在≥25%时趋于平稳,主要合金元素的含量仍保持不变,但Zr含量不断降低,第二相析出特征的变化不明显。因此对于7085铝合金,样品温度应选择在20 K左右,脉冲分数在15%~20%之间,以提高APT的数据质量。

[Objective]Atom probe tomography(APT)is an effective technique that can provide spatial and compositional information on atoms at the nanoscale with exceptional resolution.It reconstructs three-dimensional spatial distribution maps of atoms,allowing for a detailed quantitative analysis of a material’s internal microstructure and nanostructure.However,due to the varying field evaporation voltages or ionization field strengths of different elements,some atoms,particularly those in second-phase precipitates,may exhibit positional lag in the reconstructions,leading to a“local magnification effect.”This effect can distort the apparent morphology of the precipitates.In addition,flying ions composed of multiple elements may have mass-to-charge ratios that overlap with single atomic ions in the mass spectrum,causing compositional deviations.The sample temperature and pulse fraction during APT analysis can significantly impact spatial and compositional resolution,potentially leading to inaccuracies in the final data.This study investigates the influence of these parameters on the alloy matrix composition,with a particular focus on second-phase components susceptible to local magnification effects,aiming to identify the key factors affecting mass spectrum noise and compositional accuracy,thereby enhancing the reliability of APT reconstruction data.[Methods]In this study,a precipitation-strengthened 7085 aluminum alloy was selected,and needle-shaped specimens with a curvature radius of less than 100 nm were prepared using standard two-step electropolishing.A LEAP 4000HR atomic probe was employed for analysis.The maximum separation algorithm was used to identify clusters or second-phase particles,with Mg,Si,and Cu selected as clustering solutes,requiring a minimum of 10 atoms.The cluster analysis parameters included a separation distance of 0.7 nm and an envelope distance of 0.5 nm.Two sets of parameters were tested:(1)a fixed pulse fraction of 15%while varying the sample temperature,and(2)a fixed sample temperature of 20 K while varying the pulse fraction.The detection rate was maintained at 1%,and the pulse frequency was set at 200 kHz to study the effects of sample temperature and pulse fraction on background noise,chemical composition,and second-phase distribution characteristics in the mass spectrum.[Results]The experimental results revealed the presence of two precipitated phases within the aluminum alloy:small MgZn2 particles and larger Al3(Sc,Zr)particles.As the sample temperature increased,the background noise intensity in the mass spectrum consistently increased,while the concentrations of Zn,Mg,Cu,and Si remained largely unchanged.However,the Zr content increased significantly and continuously with increasing temperature.The size of the MgZn2 precipitates remained almost constant,although their number density initially decreased before increasing again.In contrast,as the pulse fraction increased,the background noise intensity decreased and stabilized at values≥25%.The concentrations of the primary alloying elements remained stable,but the Zr content decreased steadily,with no significant changes observed in the second-phase precipitation characteristics.[Conclusions]For optimal APT analysis of 7085 aluminum alloy,a sample temperature around 20 K and a pulse fraction between 15%and 20%are recommended.These conditions minimize background noise intensity and improve the accuracy of chemical composition measurements and second-phase distribution characteristics,thereby enhancing the overall reliability of the APT data.