Recently, the research group led by Assistant Professor Zhang Zhenbo at the Center for Adaptive System Engineering (CASE), School of Creativity and Art (SCA), ShanghaiTech University, published a research paper in Acta Materialia. The study revealed the hidden driver behind crack initiation in 3D-printed high-strength titanium alloys under extreme conditions, such as the deep sea, providing new design insights for more robust exploration equipment.
Titanium alloys are celebrated for being lightweight, strong, and corrosion-resistant. They are essential structural materials for manufacturing submersibles and spacecraft. Today, scientists frequently use additive manufacturing (3D printing) to create complex parts. However, in a specific type known as metastable βtitanium alloy, a continuous “wall” (known as the grain boundary α phase, or αGB) tends to form between the small metal crystals during the process. Despite its microscopic scale, this wall is a critical factor affecting the material’s service life.
To understand how this wall compromises the material, Prof. Zhang’s team utilized in-situ high-resolution digital image correlation (HRDIC), capturing the metal’s deformation process in extreme detail.
The study found that adjacent to this wall, there exists an extremely narrow “soft zone” called the precipitate-free zone (PFZ). When the metal is under tension, this PFZ acts like the thinnest part of a rubber band. Experimental data shows that the microscopic localized strain in this zone can be 100 times (two orders of magnitude) greater than the strain inside the crystals! This extreme imbalance causes the material to approach failure locally long before it reaches its overall design strength.
In deep-sea environments, hydrogen from seawater can penetrate the metal. The study found that hydrogen causes the material to embrittle (hydrogen-induced crack initiation), and the high stress at the soft zone causes the crack to unzip rapidly. This explains why some 3D-printed parts might experience sudden brittle fracture underwater.
By precisely tuning the 3D printing process, scientists can alter the morphology of the wall and eliminate these vulnerable soft zones. This is of great practical significance for developing exploration equipment capable of reaching significant depths, ensuring they remain durable and safe.
Doctoral student Chang Jiaqiang is the first author. Prof. Zhang and Research Fellow Ma Yingjie from the Institute of Metal Research, Chinese Academy of Sciences, are the co-corresponding authors.