3-6. High throughput technique for SCC susceptibility measurement based on film-induced stress

Kewei Gao*, Youpeng Song, Luchun Yan, Xiaolu Pang, Yanjing Su, Lijie Qiao

School of Materials Science and Engineering, University of Science and Technology Beijing,

Beijing 100083, China

Abstract: Stress corrosion cracking is a kind of brittle fracture process with low stress, which often causes catastrophic accidents without warning, resulting in huge economic losses and casualties. For most anodic dissolution stress corrosion, a corrosion product film is usually formed on the surface of the metal during stress corrosion. During the formation of the corrosion product film, the cations of the base metal migrate to the surface, while the anions in the solution migrate to the inside of the sample. Due to the difference in migration rate, there will be a large tensile stress or compressive stress at the interface between the film and the substrate. The superposition of this stress and the external stress can promote the plastic deformation of the local zone in the metal. When the local plastic deformation promoted by corrosion develops to a critical state, the stress concentration in the front of the dislocation pile-up group or in the dislocation free zone may be equal to the atomic bond force, resulting in the nucleation and propagation of stress corrosion cracks. Therefore, the greater film-induced stress is, the more sensitive the stress corrosion is.

The formation of the corrosion product film is closely related to the electrochemical behavior of the components in the alloy in the electrolyte, so there exists a correlation between the corrosion product film-induced stress level and the alloy component content. Generally speaking, the addition of a small amounts of alloying elements will have a greater impact on the corrosion behavior of the alloy. For example, stress corrosion sensitivity will be greatly increased if zinc is added to copper. How to quickly screen the alloy composition with excellent stress corrosion cracking resistance is an important step in the research and development of structural materials. In this study, a simple and reliable high throughput evaluation method of stress corrosion cracking susceptibility was presented based on the traditional corrosion product film-induced stress measurement method.

(1) A number of cantilevers were machined on the stainless steel sheet and resin substrate by laser cutting technology, and their size meet the requirements of the calculation formula of corrosion product film-induced stress. In view of the precision and efficiency, the cantilever arrays with different sizes were fabricated on silicon wafer by semiconductor processing technology.

(2) Cu-Cr alloy films with continuous Cr content were prepared on the surface of cantilever substrate by double target magnetron co-deposition technology. In this process, the basic problems of high diffusivity, high reactivity and low adhesion between copper alloy films and silicon substrate were solved.

(3) The deflection of the cantilever arrays immersed in 0.35wt% ammonia solution for different time was continuously measured by white light interference, and the effect of Cr content on stress corrosion sensitivity of Cu-Cr alloy was studied. The results showed that the higher the Cr content in the film, the change of the deflection more obvious, which showed that the Cr content has a significant effect on the stress corrosion sensitivity of the Cu-Cr alloy films.

(4) Due to the change of the relative thickness between the film and the cantilever beam, the basic formula of residual stress is no longer applicable. Therefore, the stoney’s formula was modified in theory, and the accurate calculation formula suitable for this experimental situation was obtained.

This high-throughput experimental method is also applicable to other alloy film systems, especially for the study of stress corrosion behavior of combinatorial material chip prepared by high-throughput technology. By using this method, the stress corrosion cracking susceptibility of a large number of samples with different alloy components can be characterized in a short time, and the effect of alloy component content on stress corrosion cracking susceptibility can be explored, so as to accelerate the selection and optimization of alloy components. Furthermore, it is helpful to find a new stress corrosion resistant material system quickly.

Keywords: Stress corrosion; Film-induced stress; High throughput; Combinatorial material chip

基于膜致应力的应力腐蚀敏感性高通量测试技术

高克玮，宋有朋，颜鲁春，庞晓露，宿彦京，乔利杰

北京科技大学材料科学与工程学院，北京，100083

摘要：应力腐蚀是一种低应力的脆断过程，往往在无征兆的情况下造成灾难性的恶性事故，导致巨大的经济损失和人员伤亡。对于大多数阳极溶解型应力腐蚀，应力腐蚀过程中通常会在金属表面形成一层腐蚀产物膜。在腐蚀产物膜形成过程中，基体金属阳离子向表面迁移，而溶液中的阴离子向试样内部迁移。由于迁移速率的不同，在膜和基体界面处会产生较大的拉应力或压应力。此应力和外应力叠加能促进金属内部局部区域的塑性变形。当这种腐蚀促进的局部塑性变形发展到临界状态，位错塞积群前端或无位错区中的应力集中就可能等于原子键合力，从而引起应力腐蚀裂纹形核扩展。因此，膜致附加应力愈大，应力腐蚀就愈敏感。

腐蚀产物膜的形成与合金中各组元在电解质中的电化学行为密切相关，故膜致应力的大小与合金的成分必然存在一定的联系。通常而言，少量合金元素的添加会对合金的腐蚀行为产生较大的影响。例如在纯铜中，加入少量的锌将大大增加其应力腐蚀的敏感性。如何快速筛选耐应力腐蚀开裂的合金成分，是结构材料研发的重要环节。基于传统的膜致应力测量方法，本研究提出了一种简单、可靠的高通量应力腐蚀敏感性评价方法：

（1） 为了满足基于膜致应力计算公式对悬臂尺寸的要求及制备连续梯度合金薄膜样品的需要，首先通过激光切割技术在不锈钢薄片和树脂基底上加工了多个悬臂。鉴于精度和效率方面的考虑，进一步通过半导体加工工艺在硅片上成功制备了不同尺寸的悬臂阵列。

（2） 利用双靶磁控溅射共沉积技术，通过调整溅射沉积靶源与悬臂基底的相对角度和距离，在悬臂基底表面制备了 Cr 含量呈连续分布的 Cu-Cr 合金薄膜样品，解决了铜合金薄膜与硅基底间高扩散性、高反应性与低附着力的基础问题。

（3） 通过白光干涉仪测量了Cr含量呈连续分布的Cu-Cr 合金薄膜样品在0.35Wt.%氨水溶液中浸泡不同时间时悬臂梁的挠度变化，研究了Cr含量对Cu-Cr 合金应力腐蚀敏感性的影响。结果表明：随浸泡时间延长，悬臂梁挠度呈连续变化趋势；薄膜中 Cr 含量越高，悬臂梁挠度变化越明显，说明Cr 含量对Cu-Cr 合金薄膜应力腐蚀敏感性有显著影响。

（4） 由于薄膜与悬臂梁厚度间相对比值的变化，已不能满足膜致应力计算公式的要求，因此从理论上对Stoney公式进行了变形，得到了适用于本实验情形的精确计算公式。

这种高通量的实验方法同样适用于其它合金薄膜体系，尤其适用于通过高通量制备技术制备的组合材料芯片应力腐蚀行为的研究，可在短时间内表征大量不同合金成分样品的应力腐蚀敏感性，探寻合金元素含量对应力腐蚀敏感性的影响，从而加速合金成分的筛选和优化，同时有利于快速寻找新的耐应力腐蚀材料体系。

关键词：应力腐蚀；膜致应力；高通量；组合材料芯片